1 The LTTng Documentation
2 =======================
3 Philippe Proulx <pproulx@efficios.com>
7 include::../common/copyright.txt[]
10 include::../common/welcome.txt[]
13 include::../common/audience.txt[]
17 === What's in this documentation?
19 The LTTng Documentation is divided into the following sections:
21 * **<<nuts-and-bolts,Nuts and bolts>>** explains the
22 rudiments of software tracing and the rationale behind the
25 You can skip this section if you’re familiar with software tracing and
26 with the LTTng project.
28 * **<<installing-lttng,Installation>>** describes the steps to
29 install the LTTng packages on common Linux distributions and from
32 You can skip this section if you already properly installed LTTng on
35 * **<<getting-started,Quick start>>** is a concise guide to
36 getting started quickly with LTTng kernel and user space tracing.
38 We recommend this section if you're new to LTTng or to software tracing
41 You can skip this section if you're not new to LTTng.
43 * **<<core-concepts,Core concepts>>** explains the concepts at
46 It's a good idea to become familiar with the core concepts
47 before attempting to use the toolkit.
49 * **<<plumbing,Components of LTTng>>** describes the various components
50 of the LTTng machinery, like the daemons, the libraries, and the
51 command-line interface.
52 * **<<instrumenting,Instrumentation>>** shows different ways to
53 instrument user applications and the Linux kernel.
55 Instrumenting source code is essential to provide a meaningful
58 You can skip this section if you do not have a programming background.
60 * **<<controlling-tracing,Tracing control>>** is divided into topics
61 which demonstrate how to use the vast array of features that
62 LTTng{nbsp}{revision} offers.
63 * **<<reference,Reference>>** contains reference tables.
64 * **<<glossary,Glossary>>** is a specialized dictionary of terms related
65 to LTTng or to the field of software tracing.
68 include::../common/convention.txt[]
71 include::../common/acknowledgements.txt[]
75 == What's new in LTTng {revision}?
77 LTTng{nbsp}{revision} bears the name _Isseki Nicho_. The result of a
78 collaboration between http://www.dieuduciel.com/[Dieu du Ciel!] and
79 Nagano-based Shiga Kogen,
80 https://www.beeradvocate.com/beer/profile/1141/53111/[_**Isseki
81 Nicho**_] is a strong Imperial Dark Saison offering a rich roasted malt
82 flavor combined with a complex fruity finish typical of Saison yeasts.
84 New features and changes in LTTng{nbsp}{revision}:
86 * **Tracing control**:
87 ** You can attach <<java-application-context,Java application-specific
88 context fields>> to a <<channel,channel>> with the
89 man:lttng-add-context(1) command:
94 lttng add-context --jul --type='$app.retriever:cur_msg_id'
98 Here, `$app` is the prefix of all application-specific context fields,
99 `retriever` names a _context information retriever_ defined at the
100 application level, and `cur_msg_id` names a context field read from this
103 Both the `java.util.logging` and Apache log4j <<domain,tracing domains>>
106 ** You can use Java application-specific <<adding-context,context>>
107 fields in the <<enabling-disabling-events,filter expression>> of an
108 <<event,event rule>>:
113 lttng enable-event --log4j my_logger \
114 --filter='$app.retriever:cur_msg_id == 23'
118 ** New `lttng status` command which is the equivalent of +lttng list
119 __CUR__+, where +__CUR__+ is the name of the current
120 <<tracing-session,tracing session>>.
122 See man:lttng-status(1).
124 ** New `lttng metadata regenerate` command to
125 <<metadata-regenerate,regenerate the metadata file of an LTTng
126 trace>> at any moment. This command is meant to be used to resample
127 the wall time following a major
128 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
129 so that a system which boots with an incorrect wall time can be
130 traced before its wall time is NTP-corrected.
132 See man:lttng-metadata(1).
134 ** New command-line interface warnings when <<event,event records>> or
135 whole sub-buffers are
136 <<channel-overwrite-mode-vs-discard-mode,lost>>. The warning messages
137 are printed when a <<tracing-session,tracing session>> is
138 <<basic-tracing-session-control,stopped>> (man:lttng-stop(1)
141 * **User space tracing**:
142 ** Shared object base address dump in order to map <<event,event
143 records>> to original source location (file and line number).
145 If you attach the `ip` and `vpid` <<adding-context,context fields>> to a
146 user space <<channel,channel>> and if you use the
147 <<liblttng-ust-dl,path:{liblttng-ust-dl.so} helper>>, you can retrieve
148 the source location where a given event record was generated.
150 The http://diamon.org/babeltrace/[Babeltrace] trace viewer supports this
151 state dump and those context fields since version 1.4 to print the
152 source location of a given event record. http://tracecompass.org/[Trace
153 Compass] also supports this since version 2.0.
155 ** A <<java-application,Java application>> which uses
156 `java.util.logging` now adds an LTTng-UST log handler to the desired
159 The previous workflow was to initialize the LTTng-UST Java agent
160 by calling `LTTngAgent.getLTTngAgent()`. This had the effect of adding
161 an LTTng-UST log handler to the root loggers.
163 ** A <<java-application,Java application>> which uses Apache log4j now
164 adds an LTTng-UST log appender to the desired log4j loggers.
166 The previous workflow was to initialize the LTTng-UST Java agent
167 by calling `LTTngAgent.getLTTngAgent()`. This had the effect of adding
168 an LTTng-UST appender to the root loggers.
170 ** Any <<java-application,Java application>> can provide
171 <<java-application-context,dynamic context fields>> while running
172 thanks to a new API provided by the <<lttng-ust-agents,LTTng-UST Java
173 agent>>. You can require LTTng to record specific context fields in
174 event records, and you can use them in the filter expression of
175 <<event,event rules>>.
177 * **Linux kernel tracing**:
178 ** The LTTng kernel modules can now be built into a Linux kernel image,
179 that is, not as loadable modules.
182 https://github.com/lttng/lttng-modules/blob/stable-{revision}/README.md#kernel-built-in-support[`README.md`]
185 ** New instrumentation:
186 *** ARM64 architecture support.
188 *** x86 `irq_vectors`.
189 ** New <<adding-context,context fields>>:
192 *** `need_reschedule`
193 *** `migratable` (specific to RT-Preempt)
194 ** Clock source plugin support for advanced cases where a custom source
195 of time is needed to timestamp LTTng event records.
197 See https://github.com/lttng/lttng-modules/blob/stable-{revision}/lttng-clock.h[`lttng-clock.h`]
198 for an overview of the small API.
201 ** The link:/man[man pages] of the man:lttng(1) command-line tool are
202 split into one man page per command (à la Git), for example:
207 man lttng-enable-event
211 You can also use the `--help` option of any man:lttng(1) command to
214 The content and formatting of all the LTTng man pages has improved
221 What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
222 generation_ is a modern toolkit for tracing Linux systems and
223 applications. So your first question might be:
230 As the history of software engineering progressed and led to what
231 we now take for granted--complex, numerous and
232 interdependent software applications running in parallel on
233 sophisticated operating systems like Linux--the authors of such
234 components, software developers, began feeling a natural
235 urge to have tools that would ensure the robustness and good performance
236 of their masterpieces.
238 One major achievement in this field is, inarguably, the
239 https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
240 an essential tool for developers to find and fix bugs. But even the best
241 debugger won't help make your software run faster, and nowadays, faster
242 software means either more work done by the same hardware, or cheaper
243 hardware for the same work.
245 A _profiler_ is often the tool of choice to identify performance
246 bottlenecks. Profiling is suitable to identify _where_ performance is
247 lost in a given software. The profiler outputs a profile, a statistical
248 summary of observed events, which you may use to discover which
249 functions took the most time to execute. However, a profiler won't
250 report _why_ some identified functions are the bottleneck. Bottlenecks
251 might only occur when specific conditions are met, conditions that are
252 sometimes impossible to capture by a statistical profiler, or impossible
253 to reproduce with an application altered by the overhead of an
254 event-based profiler. For a thorough investigation of software
255 performance issues, a history of execution is essential, with the
256 recorded values of variables and context fields you choose, and
257 with as little influence as possible on the instrumented software. This
258 is where tracing comes in handy.
260 _Tracing_ is a technique used to understand what goes on in a running
261 software system. The software used for tracing is called a _tracer_,
262 which is conceptually similar to a tape recorder. When recording,
263 specific instrumentation points placed in the software source code
264 generate events that are saved on a giant tape: a _trace_ file. You
265 can trace user applications and the operating system at the same time,
266 opening the possibility of resolving a wide range of problems that would
267 otherwise be extremely challenging.
269 Tracing is often compared to _logging_. However, tracers and loggers are
270 two different tools, serving two different purposes. Tracers are
271 designed to record much lower-level events that occur much more
272 frequently than log messages, often in the range of thousands per
273 second, with very little execution overhead. Logging is more appropriate
274 for a very high-level analysis of less frequent events: user accesses,
275 exceptional conditions (errors and warnings, for example), database
276 transactions, instant messaging communications, and such. Simply put,
277 logging is one of the many use cases that can be satisfied with tracing.
279 The list of recorded events inside a trace file can be read manually
280 like a log file for the maximum level of detail, but it is generally
281 much more interesting to perform application-specific analyses to
282 produce reduced statistics and graphs that are useful to resolve a
283 given problem. Trace viewers and analyzers are specialized tools
286 In the end, this is what LTTng is: a powerful, open source set of
287 tools to trace the Linux kernel and user applications at the same time.
288 LTTng is composed of several components actively maintained and
289 developed by its link:/community/#where[community].
292 [[lttng-alternatives]]
293 === Alternatives to noch:{LTTng}
295 Excluding proprietary solutions, a few competing software tracers
298 * https://github.com/dtrace4linux/linux[dtrace4linux] is a port of
299 Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets
300 user scripts and is responsible for loading code into the
301 Linux kernel for further execution and collecting the outputted data.
302 * https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a
303 subsystem in the Linux kernel in which a virtual machine can execute
304 programs passed from the user space to the kernel. You can attach
305 such programs to tracepoints and KProbes thanks to a system call, and
306 they can output data to the user space when executed thanks to
307 different mechanisms (pipe, VM register values, and eBPF maps, to name
309 * https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]
310 is the de facto function tracer of the Linux kernel. Its user
311 interface is a set of special files in sysfs.
312 * https://perf.wiki.kernel.org/[perf] is
313 a performance analyzing tool for Linux which supports hardware
314 performance counters, tracepoints, as well as other counters and
315 types of probes. perf's controlling utility is the cmd:perf command
317 * http://linux.die.net/man/1/strace[strace]
318 is a command-line utility which records system calls made by a
319 user process, as well as signal deliveries and changes of process
320 state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace]
321 to fulfill its function.
322 * http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
323 analyze Linux kernel events. You write scripts, or _chisels_ in
324 sysdig's jargon, in Lua and sysdig executes them while the system is
325 being traced or afterwards. sysdig's interface is the cmd:sysdig
326 command-line tool as well as the curses-based cmd:csysdig tool.
327 * https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and
328 user space tracer which uses custom user scripts to produce plain text
329 traces. SystemTap converts the scripts to the C language, and then
330 compiles them as Linux kernel modules which are loaded to produce
331 trace data. SystemTap's primary user interface is the cmd:stap
334 The main distinctive features of LTTng is that it produces correlated
335 kernel and user space traces, as well as doing so with the lowest
336 overhead amongst other solutions. It produces trace files in the
337 http://diamon.org/ctf[CTF] format, a file format optimized
338 for the production and analyses of multi-gigabyte data.
340 LTTng is the result of more than 10 years of active open source
341 development by a community of passionate developers.
342 LTTng{nbsp}{revision} is currently available on major desktop and server
345 The main interface for tracing control is a single command-line tool
346 named cmd:lttng. The latter can create several tracing sessions, enable
347 and disable events on the fly, filter events efficiently with custom
348 user expressions, start and stop tracing, and much more. LTTng can
349 record the traces on the file system or send them over the network, and
350 keep them totally or partially. You can view the traces once tracing
351 becomes inactive or in real-time.
353 <<installing-lttng,Install LTTng now>> and
354 <<getting-started,start tracing>>!
360 **LTTng** is a set of software <<plumbing,components>> which interact to
361 <<instrumenting,instrument>> the Linux kernel and user applications, and
362 to <<controlling-tracing,control tracing>> (start and stop
363 tracing, enable and disable event rules, and the rest). Those
364 components are bundled into the following packages:
366 * **LTTng-tools**: Libraries and command-line interface to
368 * **LTTng-modules**: Linux kernel modules to instrument and
370 * **LTTng-UST**: Libraries and Java/Python packages to instrument and
371 trace user applications.
373 Most distributions mark the LTTng-modules and LTTng-UST packages as
374 optional when installing LTTng-tools (which is always required). In the
375 following sections, we always provide the steps to install all three,
378 * You only need to install LTTng-modules if you intend to trace the
380 * You only need to install LTTng-UST if you intend to trace user
384 .Availability of LTTng{nbsp}{revision} for major Linux distributions as of 2 December 2016.
386 |Distribution |Available in releases |Alternatives
388 |https://www.ubuntu.com/[Ubuntu]
389 |<<ubuntu,Ubuntu{nbsp}16.10 _Yakkety Yak_>>.
390 |LTTng{nbsp}{revision} for Ubuntu{nbsp}14.04 _Trusty Tahr_
391 and Ubuntu{nbsp}16.04 _Xenial Xerus_:
392 <<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
394 LTTng{nbsp}2.9 for Ubuntu{nbsp}14.04 _Trusty Tahr_
395 and Ubuntu{nbsp}16.04 _Xenial Xerus_:
396 link:/docs/v2.9#doc-ubuntu-ppa[use the LTTng Stable{nbsp}2.9 PPA].
398 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
399 other Ubuntu releases.
401 |https://getfedora.org/[Fedora]
402 |<<fedora,Fedora{nbsp}25>>.
403 |<<building-from-source,Build LTTng{nbsp}{revision} from source>> for
404 other Fedora releases.
406 |https://www.debian.org/[Debian]
407 |<<debian,Debian "stretch" (testing)>>.
408 |<<building-from-source,Build LTTng{nbsp}{revision} from source>> for
409 previous Debian releases.
411 |https://www.opensuse.org/[openSUSE]
413 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
415 |https://www.archlinux.org/[Arch Linux]
417 |link:/docs/v2.9#doc-arch-linux[LTTng{nbsp}2.9 from the AUR].
419 |https://alpinelinux.org/[Alpine Linux]
420 |<<alpine-linux,Alpine Linux "edge">>.
421 |LTTng{nbsp}{revision} for Alpine Linux{nbsp}3.5 (not released yet).
423 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
424 other Alpine Linux releases.
426 |https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
427 |See http://packages.efficios.com/[EfficiOS Enterprise Packages].
430 |https://buildroot.org/[Buildroot]
431 |<<buildroot,Buildroot 2016.11>>.
432 |<<building-from-source,Build LTTng{nbsp}{revision} from source>> for
433 other Buildroot releases.
435 |http://www.openembedded.org/wiki/Main_Page[OpenEmbedded] and
436 https://www.yoctoproject.org/[Yocto]
437 |<<oe-yocto,Yocto Project{nbsp}2.2 _Morty_>> (`openembedded-core` layer).
438 |<<building-from-source,Build LTTng{nbsp}{revision} from source>> for
439 other OpenEmbedded releases.
444 === [[ubuntu-official-repositories]]Ubuntu
446 LTTng{nbsp}{revision} is available on Ubuntu{nbsp}16.10 _Yakkety Yak_.
447 For previous releases of Ubuntu, <<ubuntu-ppa,use the LTTng
448 Stable{nbsp}{revision} PPA>>.
450 To install LTTng{nbsp}{revision} on Ubuntu{nbsp}16.10 _Yakkety Yak_:
452 . Install the main LTTng{nbsp}{revision} packages:
457 sudo apt-get install lttng-tools
458 sudo apt-get install lttng-modules-dkms
459 sudo apt-get install liblttng-ust-dev
463 . **If you need to instrument and trace
464 <<java-application,Java applications>>**, install the LTTng-UST
470 sudo apt-get install liblttng-ust-agent-java
474 . **If you need to instrument and trace
475 <<python-application,Python{nbsp}3 applications>>**, install the
476 LTTng-UST Python agent:
481 sudo apt-get install python3-lttngust
487 ==== noch:{LTTng} Stable {revision} PPA
489 The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng
490 Stable{nbsp}{revision} PPA] offers the latest stable
491 LTTng{nbsp}{revision} packages for:
493 * Ubuntu{nbsp}14.04 _Trusty Tahr_
494 * Ubuntu{nbsp}16.04 _Xenial Xerus_
496 To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA:
498 . Add the LTTng Stable{nbsp}{revision} PPA repository and update the
504 sudo apt-add-repository ppa:lttng/stable-2.8
509 . Install the main LTTng{nbsp}{revision} packages:
514 sudo apt-get install lttng-tools
515 sudo apt-get install lttng-modules-dkms
516 sudo apt-get install liblttng-ust-dev
520 . **If you need to instrument and trace
521 <<java-application,Java applications>>**, install the LTTng-UST
527 sudo apt-get install liblttng-ust-agent-java
531 . **If you need to instrument and trace
532 <<python-application,Python{nbsp}3 applications>>**, install the
533 LTTng-UST Python agent:
538 sudo apt-get install python3-lttngust
546 To install LTTng{nbsp}{revision} on Fedora{nbsp}25:
548 . Install the LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision}
554 sudo yum install lttng-tools
555 sudo yum install lttng-ust
559 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
565 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
566 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
567 cd lttng-modules-2.8.* &&
569 sudo make modules_install &&
575 .Java and Python application instrumentation and tracing
577 If you need to instrument and trace <<java-application,Java
578 applications>> on openSUSE, you need to build and install
579 LTTng-UST{nbsp}{revision} <<building-from-source,from source>> and pass
580 the `--enable-java-agent-jul`, `--enable-java-agent-log4j`, or
581 `--enable-java-agent-all` options to the `configure` script, depending
582 on which Java logging framework you use.
584 If you need to instrument and trace <<python-application,Python
585 applications>> on openSUSE, you need to build and install
586 LTTng-UST{nbsp}{revision} from source and pass the
587 `--enable-python-agent` option to the `configure` script.
594 To install LTTng{nbsp}{revision} on Debian "stretch" (testing):
596 . Install the main LTTng{nbsp}{revision} packages:
601 sudo apt-get install lttng-modules-dkms
602 sudo apt-get install liblttng-ust-dev
603 sudo apt-get install lttng-tools
607 . **If you need to instrument and trace <<java-application,Java
608 applications>>**, install the LTTng-UST Java agent:
613 sudo apt-get install liblttng-ust-agent-java
617 . **If you need to instrument and trace <<python-application,Python
618 applications>>**, install the LTTng-UST Python agent:
623 sudo apt-get install python3-lttngust
631 To install LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision} on
634 . Make sure your system is
635 https://wiki.alpinelinux.org/wiki/Edge[configured for "edge"].
636 . Enable the _testing_ repository by uncommenting the corresponding
637 line in path:{/etc/apk/repositories}.
638 . Add the LTTng packages:
643 sudo apk add lttng-tools
644 sudo apk add lttng-ust-dev
648 To install LTTng-modules{nbsp}{revision} (Linux kernel tracing support)
649 on Alpine Linux "edge":
651 . Add the vanilla Linux kernel:
656 apk add linux-vanilla linux-vanilla-dev
660 . Reboot with the vanilla Linux kernel.
661 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
667 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
668 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
669 cd lttng-modules-2.8.* &&
671 sudo make modules_install &&
677 [[enterprise-distributions]]
678 === RHEL, SUSE, and other enterprise distributions
680 To install LTTng on enterprise Linux distributions, such as Red Hat
681 Enterprise Linux (RHEL) and SUSE Linux Enterprise Server (SUSE), please
682 see http://packages.efficios.com/[EfficiOS Enterprise Packages].
688 To install LTTng{nbsp}{revision} on Buildroot{nbsp}2016.11:
690 . Launch the Buildroot configuration tool:
699 . In **Kernel**, check **Linux kernel**.
700 . In **Toolchain**, check **Enable WCHAR support**.
701 . In **Target packages**{nbsp}→ **Debugging, profiling and benchmark**,
702 check **lttng-modules** and **lttng-tools**.
703 . In **Target packages**{nbsp}→ **Libraries**{nbsp}→
704 **Other**, check **lttng-libust**.
708 === OpenEmbedded and Yocto
710 LTTng{nbsp}{revision} recipes are available in the
711 http://layers.openembedded.org/layerindex/branch/master/layer/openembedded-core/[`openembedded-core`]
712 layer for Yocto Project{nbsp}2.2 _Morty_ under the following names:
718 With BitBake, the simplest way to include LTTng recipes in your target
719 image is to add them to `IMAGE_INSTALL_append` in path:{conf/local.conf}:
722 IMAGE_INSTALL_append = " lttng-tools lttng-modules lttng-ust"
727 . Select a machine and an image recipe.
728 . Click **Edit image recipe**.
729 . Under the **All recipes** tab, search for **lttng**.
730 . Check the desired LTTng recipes.
733 .Java and Python application instrumentation and tracing
735 If you need to instrument and trace <<java-application,Java
736 applications>> on openSUSE, you need to build and install
737 LTTng-UST{nbsp}{revision} <<building-from-source,from source>> and pass
738 the `--enable-java-agent-jul`, `--enable-java-agent-log4j`, or
739 `--enable-java-agent-all` options to the `configure` script, depending
740 on which Java logging framework you use.
742 If you need to instrument and trace <<python-application,Python
743 applications>> on openSUSE, you need to build and install
744 LTTng-UST{nbsp}{revision} from source and pass the
745 `--enable-python-agent` option to the `configure` script.
749 [[building-from-source]]
750 === Build from source
752 To build and install LTTng{nbsp}{revision} from source:
754 . Using your distribution's package manager, or from source, install
755 the following dependencies of LTTng-tools and LTTng-UST:
758 * https://sourceforge.net/projects/libuuid/[libuuid]
759 * http://directory.fsf.org/wiki/Popt[popt]
760 * http://liburcu.org/[Userspace RCU]
761 * http://www.xmlsoft.org/[libxml2]
764 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
770 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
771 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
772 cd lttng-modules-2.8.* &&
774 sudo make modules_install &&
779 . Download, build, and install the latest LTTng-UST{nbsp}{revision}:
785 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.8.tar.bz2 &&
786 tar -xf lttng-ust-latest-2.8.tar.bz2 &&
787 cd lttng-ust-2.8.* &&
797 .Java and Python application tracing
799 If you need to instrument and trace <<java-application,Java
800 applications>>, pass the `--enable-java-agent-jul`,
801 `--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
802 `configure` script, depending on which Java logging framework you use.
804 If you need to instrument and trace <<python-application,Python
805 applications>>, pass the `--enable-python-agent` option to the
806 `configure` script. You can set the `PYTHON` environment variable to the
807 path to the Python interpreter for which to install the LTTng-UST Python
815 By default, LTTng-UST libraries are installed to
816 dir:{/usr/local/lib}, which is the de facto directory in which to
817 keep self-compiled and third-party libraries.
819 When <<building-tracepoint-providers-and-user-application,linking an
820 instrumented user application with `liblttng-ust`>>:
822 * Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
824 * Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
825 man:gcc(1), man:g++(1), or man:clang(1).
829 . Download, build, and install the latest LTTng-tools{nbsp}{revision}:
835 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.8.tar.bz2 &&
836 tar -xf lttng-tools-latest-2.8.tar.bz2 &&
837 cd lttng-tools-2.8.* &&
845 TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
846 previous steps automatically for a given version of LTTng and confine
847 the installed files in a specific directory. This can be useful to test
848 LTTng without installing it on your system.
854 This is a short guide to get started quickly with LTTng kernel and user
857 Before you follow this guide, make sure to <<installing-lttng,install>>
860 This tutorial walks you through the steps to:
862 . <<tracing-the-linux-kernel,Trace the Linux kernel>>.
863 . <<tracing-your-own-user-application,Trace a user application>> written
865 . <<viewing-and-analyzing-your-traces,View and analyze the
869 [[tracing-the-linux-kernel]]
870 === Trace the Linux kernel
872 The following command lines start with cmd:sudo because you need root
873 privileges to trace the Linux kernel. You can avoid using cmd:sudo if
874 your Unix user is a member of the <<tracing-group,tracing group>>.
876 . Create a <<tracing-session,tracing session>> which writes its traces
877 to dir:{/tmp/my-kernel-trace}:
882 sudo lttng create my-kernel-session --output=/tmp/my-kernel-trace
886 . List the available kernel tracepoints and system calls:
891 sudo lttng list --kernel
892 sudo lttng list --kernel --syscall
896 . Create <<event,event rules>> which match the desired instrumentation
897 point names, for example the `sched_switch` and `sched_process_fork`
898 tracepoints, and the man:open(2) and man:close(2) system calls:
903 sudo lttng enable-event --kernel sched_switch,sched_process_fork
904 sudo lttng enable-event --kernel --syscall open,close
908 You can also create an event rule which matches _all_ the Linux kernel
909 tracepoints (this will generate a lot of data when tracing):
914 sudo lttng enable-event --kernel --all
918 . <<basic-tracing-session-control,Start tracing>>:
927 . Do some operation on your system for a few seconds. For example,
928 load a website, or list the files of a directory.
929 . <<basic-tracing-session-control,Stop tracing>> and destroy the
940 The man:lttng-destroy(1) command does not destroy the trace data; it
941 only destroys the state of the tracing session.
943 . For the sake of this example, make the recorded trace accessible to
949 sudo chown -R $(whoami) /tmp/my-kernel-trace
953 See <<viewing-and-analyzing-your-traces,View and analyze the
954 recorded events>> to view the recorded events.
957 [[tracing-your-own-user-application]]
958 === Trace a user application
960 This section steps you through a simple example to trace a
961 _Hello world_ program written in C.
963 To create the traceable user application:
965 . Create the tracepoint provider header file, which defines the
966 tracepoints and the events they can generate:
972 #undef TRACEPOINT_PROVIDER
973 #define TRACEPOINT_PROVIDER hello_world
975 #undef TRACEPOINT_INCLUDE
976 #define TRACEPOINT_INCLUDE "./hello-tp.h"
978 #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
981 #include <lttng/tracepoint.h>
991 ctf_string(my_string_field, my_string_arg)
992 ctf_integer(int, my_integer_field, my_integer_arg)
996 #endif /* _HELLO_TP_H */
998 #include <lttng/tracepoint-event.h>
1002 . Create the tracepoint provider package source file:
1008 #define TRACEPOINT_CREATE_PROBES
1009 #define TRACEPOINT_DEFINE
1011 #include "hello-tp.h"
1015 . Build the tracepoint provider package:
1020 gcc -c -I. hello-tp.c
1024 . Create the _Hello World_ application source file:
1031 #include "hello-tp.h"
1033 int main(int argc, char *argv[])
1037 puts("Hello, World!\nPress Enter to continue...");
1040 * The following getchar() call is only placed here for the purpose
1041 * of this demonstration, to pause the application in order for
1042 * you to have time to list its tracepoints. It is not
1048 * A tracepoint() call.
1050 * Arguments, as defined in hello-tp.h:
1052 * 1. Tracepoint provider name (required)
1053 * 2. Tracepoint name (required)
1054 * 3. my_integer_arg (first user-defined argument)
1055 * 4. my_string_arg (second user-defined argument)
1057 * Notice the tracepoint provider and tracepoint names are
1058 * NOT strings: they are in fact parts of variables that the
1059 * macros in hello-tp.h create.
1061 tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
1063 for (x = 0; x < argc; ++x) {
1064 tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
1067 puts("Quitting now!");
1068 tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
1075 . Build the application:
1084 . Link the application with the tracepoint provider package,
1085 `liblttng-ust`, and `libdl`:
1090 gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
1094 Here's the whole build process:
1097 .User space tracing tutorial's build steps.
1098 image::ust-flow.png[]
1100 To trace the user application:
1102 . Run the application with a few arguments:
1107 ./hello world and beyond
1116 Press Enter to continue...
1120 . Start an LTTng <<lttng-sessiond,session daemon>>:
1125 lttng-sessiond --daemonize
1129 Note that a session daemon might already be running, for example as
1130 a service that the distribution's service manager started.
1132 . List the available user space tracepoints:
1137 lttng list --userspace
1141 You see the `hello_world:my_first_tracepoint` tracepoint listed
1142 under the `./hello` process.
1144 . Create a <<tracing-session,tracing session>>:
1149 lttng create my-user-space-session
1153 . Create an <<event,event rule>> which matches the
1154 `hello_world:my_first_tracepoint` event name:
1159 lttng enable-event --userspace hello_world:my_first_tracepoint
1163 . <<basic-tracing-session-control,Start tracing>>:
1172 . Go back to the running `hello` application and press Enter. The
1173 program executes all `tracepoint()` instrumentation points and exits.
1174 . <<basic-tracing-session-control,Stop tracing>> and destroy the
1185 The man:lttng-destroy(1) command does not destroy the trace data; it
1186 only destroys the state of the tracing session.
1188 By default, LTTng saves the traces in
1189 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
1190 where +__name__+ is the tracing session name. The
1191 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
1193 See <<viewing-and-analyzing-your-traces,View and analyze the
1194 recorded events>> to view the recorded events.
1197 [[viewing-and-analyzing-your-traces]]
1198 === View and analyze the recorded events
1200 Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
1201 kernel>> and <<tracing-your-own-user-application,Trace a user
1202 application>> tutorials, you can inspect the recorded events.
1204 Many tools are available to read LTTng traces:
1206 * **cmd:babeltrace** is a command-line utility which converts trace
1207 formats; it supports the format that LTTng produces, CTF, as well as a
1208 basic text output which can be ++grep++ed. The cmd:babeltrace command
1209 is part of the http://diamon.org/babeltrace[Babeltrace] project.
1210 * Babeltrace also includes
1211 **https://www.python.org/[Python] bindings** so
1212 that you can easily open and read an LTTng trace with your own script,
1213 benefiting from the power of Python.
1214 * http://tracecompass.org/[**Trace Compass**]
1215 is a graphical user interface for viewing and analyzing any type of
1216 logs or traces, including LTTng's.
1217 * https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
1218 project which includes many high-level analyses of LTTng kernel
1219 traces, like scheduling statistics, interrupt frequency distribution,
1220 top CPU usage, and more.
1222 NOTE: This section assumes that the traces recorded during the previous
1223 tutorials were saved to their default location, in the
1224 dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
1225 environment variable defaults to `$HOME` if not set.
1228 [[viewing-and-analyzing-your-traces-bt]]
1229 ==== Use the cmd:babeltrace command-line tool
1231 The simplest way to list all the recorded events of a trace is to pass
1232 its path to cmd:babeltrace with no options:
1236 babeltrace ~/lttng-traces/my-user-space-session*
1239 cmd:babeltrace finds all traces recursively within the given path and
1240 prints all their events, merging them in chronological order.
1242 You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
1247 babeltrace /tmp/my-kernel-trace | grep _switch
1250 You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
1251 count the recorded events:
1255 babeltrace /tmp/my-kernel-trace | grep _open | wc --lines
1259 [[viewing-and-analyzing-your-traces-bt-python]]
1260 ==== Use the Babeltrace Python bindings
1262 The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
1263 is useful to isolate events by simple matching using man:grep(1) and
1264 similar utilities. However, more elaborate filters, such as keeping only
1265 event records with a field value falling within a specific range, are
1266 not trivial to write using a shell. Moreover, reductions and even the
1267 most basic computations involving multiple event records are virtually
1268 impossible to implement.
1270 Fortunately, Babeltrace ships with Python 3 bindings which makes it easy
1271 to read the event records of an LTTng trace sequentially and compute the
1272 desired information.
1274 The following script accepts an LTTng Linux kernel trace path as its
1275 first argument and prints the short names of the top 5 running processes
1276 on CPU 0 during the whole trace:
1281 from collections import Counter
1287 if len(sys.argv) != 2:
1288 msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
1289 print(msg, file=sys.stderr)
1292 # A trace collection contains one or more traces
1293 col = babeltrace.TraceCollection()
1295 # Add the trace provided by the user (LTTng traces always have
1297 if col.add_trace(sys.argv[1], 'ctf') is None:
1298 raise RuntimeError('Cannot add trace')
1300 # This counter dict contains execution times:
1302 # task command name -> total execution time (ns)
1303 exec_times = Counter()
1305 # This contains the last `sched_switch` timestamp
1309 for event in col.events:
1310 # Keep only `sched_switch` events
1311 if event.name != 'sched_switch':
1314 # Keep only events which happened on CPU 0
1315 if event['cpu_id'] != 0:
1319 cur_ts = event.timestamp
1325 # Previous task command (short) name
1326 prev_comm = event['prev_comm']
1328 # Initialize entry in our dict if not yet done
1329 if prev_comm not in exec_times:
1330 exec_times[prev_comm] = 0
1332 # Compute previous command execution time
1333 diff = cur_ts - last_ts
1335 # Update execution time of this command
1336 exec_times[prev_comm] += diff
1338 # Update last timestamp
1342 for name, ns in exec_times.most_common(5):
1344 print('{:20}{} s'.format(name, s))
1349 if __name__ == '__main__':
1350 sys.exit(0 if top5proc() else 1)
1357 python3 top5proc.py /tmp/my-kernel-trace/kernel
1363 swapper/0 48.607245889 s
1364 chromium 7.192738188 s
1365 pavucontrol 0.709894415 s
1366 Compositor 0.660867933 s
1367 Xorg.bin 0.616753786 s
1370 Note that `swapper/0` is the "idle" process of CPU 0 on Linux; since we
1371 weren't using the CPU that much when tracing, its first position in the
1376 == [[understanding-lttng]]Core concepts
1378 From a user's perspective, the LTTng system is built on a few concepts,
1379 or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
1380 operates by sending commands to the <<lttng-sessiond,session daemon>>.
1381 Understanding how those objects relate to eachother is key in mastering
1384 The core concepts are:
1386 * <<tracing-session,Tracing session>>
1387 * <<domain,Tracing domain>>
1388 * <<channel,Channel and ring buffer>>
1389 * <<"event","Instrumentation point, event rule, event, and event record">>
1395 A _tracing session_ is a stateful dialogue between you and
1396 a <<lttng-sessiond,session daemon>>. You can
1397 <<creating-destroying-tracing-sessions,create a new tracing
1398 session>> with the `lttng create` command.
1400 Anything that you do when you control LTTng tracers happens within a
1401 tracing session. In particular, a tracing session:
1404 * Has its own set of trace files.
1405 * Has its own state of activity (started or stopped).
1406 * Has its own <<tracing-session-mode,mode>> (local, network streaming,
1408 * Has its own <<channel,channels>> which have their own
1409 <<event,event rules>>.
1412 .A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
1413 image::concepts.png[]
1415 Those attributes and objects are completely isolated between different
1418 A tracing session is analogous to a cash machine session:
1419 the operations you do on the banking system through the cash machine do
1420 not alter the data of other users of the same system. In the case of
1421 the cash machine, a session lasts as long as your bank card is inside.
1422 In the case of LTTng, a tracing session lasts from the `lttng create`
1423 command to the `lttng destroy` command.
1426 .Each Unix user has its own set of tracing sessions.
1427 image::many-sessions.png[]
1430 [[tracing-session-mode]]
1431 ==== Tracing session mode
1433 LTTng can send the generated trace data to different locations. The
1434 _tracing session mode_ dictates where to send it. The following modes
1435 are available in LTTng{nbsp}{revision}:
1438 LTTng writes the traces to the file system of the machine being traced
1441 Network streaming mode::
1442 LTTng sends the traces over the network to a
1443 <<lttng-relayd,relay daemon>> running on a remote system.
1446 LTTng does not write the traces by default. Instead, you can request
1447 LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
1448 current tracing buffers, and to write it to the target's file system
1449 or to send it over the network to a <<lttng-relayd,relay daemon>>
1450 running on a remote system.
1453 This mode is similar to the network streaming mode, but a live
1454 trace viewer can connect to the distant relay daemon to
1455 <<lttng-live,view event records as LTTng generates them>> by
1462 A _tracing domain_ is a namespace for event sources. A tracing domain
1463 has its own properties and features.
1465 There are currently five available tracing domains:
1469 * `java.util.logging` (JUL)
1473 You must specify a tracing domain when using some commands to avoid
1474 ambiguity. For example, since all the domains support named tracepoints
1475 as event sources (instrumentation points that you manually insert in the
1476 source code), you need to specify a tracing domain when
1477 <<enabling-disabling-events,creating an event rule>> because all the
1478 tracing domains could have tracepoints with the same names.
1480 Some features are reserved to specific tracing domains. Dynamic function
1481 entry and return instrumentation points, for example, are currently only
1482 supported in the Linux kernel tracing domain, but support for other
1483 tracing domains could be added in the future.
1485 You can create <<channel,channels>> in the Linux kernel and user space
1486 tracing domains. The other tracing domains have a single default
1491 === Channel and ring buffer
1493 A _channel_ is an object which is responsible for a set of ring buffers.
1494 Each ring buffer is divided into multiple sub-buffers. When an LTTng
1495 tracer emits an event, it can record it to one or more
1496 sub-buffers. The attributes of a channel determine what to do when
1497 there's no space left for a new event record because all sub-buffers
1498 are full, where to send a full sub-buffer, and other behaviours.
1500 A channel is always associated to a <<domain,tracing domain>>. The
1501 `java.util.logging` (JUL), log4j, and Python tracing domains each have
1502 a default channel which you cannot configure.
1504 A channel also owns <<event,event rules>>. When an LTTng tracer emits
1505 an event, it records it to the sub-buffers of all
1506 the enabled channels with a satisfied event rule, as long as those
1507 channels are part of active <<tracing-session,tracing sessions>>.
1510 [[channel-buffering-schemes]]
1511 ==== Per-user vs. per-process buffering schemes
1513 A channel has at least one ring buffer _per CPU_. LTTng always
1514 records an event to the ring buffer associated to the CPU on which it
1517 Two _buffering schemes_ are available when you
1518 <<enabling-disabling-channels,create a channel>> in the
1519 user space <<domain,tracing domain>>:
1521 Per-user buffering::
1522 Allocate one set of ring buffers--one per CPU--shared by all the
1523 instrumented processes of each Unix user.
1527 .Per-user buffering scheme.
1528 image::per-user-buffering.png[]
1531 Per-process buffering::
1532 Allocate one set of ring buffers--one per CPU--for each
1533 instrumented process.
1537 .Per-process buffering scheme.
1538 image::per-process-buffering.png[]
1541 The per-process buffering scheme tends to consume more memory than the
1542 per-user option because systems generally have more instrumented
1543 processes than Unix users running instrumented processes. However, the
1544 per-process buffering scheme ensures that one process having a high
1545 event throughput won't fill all the shared sub-buffers of the same
1548 The Linux kernel tracing domain has only one available buffering scheme
1549 which is to allocate a single set of ring buffers for the whole system.
1550 This scheme is similar to the per-user option, but with a single, global
1551 user "running" the kernel.
1554 [[channel-overwrite-mode-vs-discard-mode]]
1555 ==== Overwrite vs. discard event loss modes
1557 When an event occurs, LTTng records it to a specific sub-buffer (yellow
1558 arc in the following animation) of a specific channel's ring buffer.
1559 When there's no space left in a sub-buffer, the tracer marks it as
1560 consumable (red) and another, empty sub-buffer starts receiving the
1561 following event records. A <<lttng-consumerd,consumer daemon>>
1562 eventually consumes the marked sub-buffer (returns to white).
1565 [role="docsvg-channel-subbuf-anim"]
1570 In an ideal world, sub-buffers are consumed faster than they are filled,
1571 as is the case in the previous animation. In the real world,
1572 however, all sub-buffers can be full at some point, leaving no space to
1573 record the following events.
1575 By design, LTTng is a _non-blocking_ tracer: when no empty sub-buffer is
1576 available, it is acceptable to lose event records when the alternative
1577 would be to cause substantial delays in the instrumented application's
1578 execution. LTTng privileges performance over integrity; it aims at
1579 perturbing the traced system as little as possible in order to make
1580 tracing of subtle race conditions and rare interrupt cascades possible.
1582 When it comes to losing event records because no empty sub-buffer is
1583 available, the channel's _event loss mode_ determines what to do. The
1584 available event loss modes are:
1587 Drop the newest event records until a the tracer
1588 releases a sub-buffer.
1591 Clear the sub-buffer containing the oldest event records and start
1592 writing the newest event records there.
1594 This mode is sometimes called _flight recorder mode_ because it's
1596 https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
1597 always keep a fixed amount of the latest data.
1599 Which mechanism you should choose depends on your context: prioritize
1600 the newest or the oldest event records in the ring buffer?
1602 Beware that, in overwrite mode, the tracer abandons a whole sub-buffer
1603 as soon as a there's no space left for a new event record, whereas in
1604 discard mode, the tracer only discards the event record that doesn't
1607 In discard mode, LTTng increments a count of lost event records when
1608 an event record is lost and saves this count to the trace. In
1609 overwrite mode, LTTng keeps no information when it overwrites a
1610 sub-buffer before consuming it.
1612 There are a few ways to decrease your probability of losing event
1614 <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
1615 how you can fine-une the sub-buffer count and size of a channel to
1616 virtually stop losing event records, though at the cost of greater
1620 [[channel-subbuf-size-vs-subbuf-count]]
1621 ==== Sub-buffer count and size
1623 When you <<enabling-disabling-channels,create a channel>>, you can
1624 set its number of sub-buffers and their size.
1626 Note that there is noticeable CPU overhead introduced when
1627 switching sub-buffers (marking a full one as consumable and switching
1628 to an empty one for the following events to be recorded). Knowing this,
1629 the following list presents a few practical situations along with how
1630 to configure the sub-buffer count and size for them:
1632 * **High event throughput**: In general, prefer bigger sub-buffers to
1633 lower the risk of losing event records.
1635 Having bigger sub-buffers also ensures a lower sub-buffer switching
1638 The number of sub-buffers is only meaningful if you create the channel
1639 in overwrite mode: in this case, if a sub-buffer overwrite happens, the
1640 other sub-buffers are left unaltered.
1642 * **Low event throughput**: In general, prefer smaller sub-buffers
1643 since the risk of losing event records is low.
1645 Because events occur less frequently, the sub-buffer switching frequency
1646 should remain low and thus the tracer's overhead should not be a
1649 * **Low memory system**: If your target system has a low memory
1650 limit, prefer fewer first, then smaller sub-buffers.
1652 Even if the system is limited in memory, you want to keep the
1653 sub-buffers as big as possible to avoid a high sub-buffer switching
1656 Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
1657 which means event data is very compact. For example, the average
1658 LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
1659 sub-buffer size of 1{nbsp}MiB is considered big.
1661 The previous situations highlight the major trade-off between a few big
1662 sub-buffers and more, smaller sub-buffers: sub-buffer switching
1663 frequency vs. how much data is lost in overwrite mode. Assuming a
1664 constant event throughput and using the overwrite mode, the two
1665 following configurations have the same ring buffer total size:
1668 [role="docsvg-channel-subbuf-size-vs-count-anim"]
1673 * **2 sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
1674 switching frequency, but if a sub-buffer overwrite happens, half of
1675 the event records so far (4{nbsp}MiB) are definitely lost.
1676 * **8 sub-buffers of 1{nbsp}MiB each**: Expect 4{nbsp}times the tracer's
1677 overhead as the previous configuration, but if a sub-buffer
1678 overwrite happens, only the eighth of event records so far are
1681 In discard mode, the sub-buffers count parameter is pointless: use two
1682 sub-buffers and set their size according to the requirements of your
1686 [[channel-switch-timer]]
1687 ==== Switch timer period
1689 The _switch timer period_ is an important configurable attribute of
1690 a channel to ensure periodic sub-buffer flushing.
1692 When the _switch timer_ expires, a sub-buffer switch happens. You can
1693 set the switch timer period attribute when you
1694 <<enabling-disabling-channels,create a channel>> to ensure that event
1695 data is consumed and committed to trace files or to a distant relay
1696 daemon periodically in case of a low event throughput.
1699 [role="docsvg-channel-switch-timer"]
1704 This attribute is also convenient when you use big sub-buffers to cope
1705 with a sporadic high event throughput, even if the throughput is
1709 [[channel-read-timer]]
1710 ==== Read timer period
1712 By default, the LTTng tracers use a notification mechanism to signal a
1713 full sub-buffer so that a consumer daemon can consume it. When such
1714 notifications must be avoided, for example in real-time applications,
1715 you can use the channel's _read timer_ instead. When the read timer
1716 fires, the <<lttng-consumerd,consumer daemon>> checks for full,
1717 consumable sub-buffers.
1720 [[tracefile-rotation]]
1721 ==== Trace file count and size
1723 By default, trace files can grow as large as needed. You can set the
1724 maximum size of each trace file that a channel writes when you
1725 <<enabling-disabling-channels,create a channel>>. When the size of
1726 a trace file reaches the channel's fixed maximum size, LTTng creates
1727 another file to contain the next event records. LTTng appends a file
1728 count to each trace file name in this case.
1730 If you set the trace file size attribute when you create a channel, the
1731 maximum number of trace files that LTTng creates is _unlimited_ by
1732 default. To limit them, you can also set a maximum number of trace
1733 files. When the number of trace files reaches the channel's fixed
1734 maximum count, the oldest trace file is overwritten. This mechanism is
1735 called _trace file rotation_.
1739 === Instrumentation point, event rule, event, and event record
1741 An _event rule_ is a set of conditions which must be **all** satisfied
1742 for LTTng to record an occuring event.
1744 You set the conditions when you <<enabling-disabling-events,create
1747 You always attach an event rule to <<channel,channel>> when you create
1750 When an event passes the conditions of an event rule, LTTng records it
1751 in one of the attached channel's sub-buffers.
1753 The available conditions, as of LTTng{nbsp}{revision}, are:
1755 * The event rule _is enabled_.
1756 * The instrumentation point's type _is{nbsp}T_.
1757 * The instrumentation point's name (sometimes called _event name_)
1758 _matches{nbsp}N_, but _is not{nbsp}E_.
1759 * The instrumentation point's log level _is as severe as{nbsp}L_, or
1760 _is exactly{nbsp}L_.
1761 * The fields of the event's payload _satisfy_ a filter
1762 expression{nbsp}__F__.
1764 As you can see, all the conditions but the dynamic filter are related to
1765 the event rule's status or to the instrumentation point, not to the
1766 occurring events. This is why, without a filter, checking if an event
1767 passes an event rule is not a dynamic task: when you create or modify an
1768 event rule, all the tracers of its tracing domain enable or disable the
1769 instrumentation points themselves once. This is possible because the
1770 attributes of an instrumentation point (type, name, and log level) are
1771 defined statically. In other words, without a dynamic filter, the tracer
1772 _does not evaluate_ the arguments of an instrumentation point unless it
1773 matches an enabled event rule.
1775 Note that, for LTTng to record an event, the <<channel,channel>> to
1776 which a matching event rule is attached must also be enabled, and the
1777 tracing session owning this channel must be active.
1780 .Logical path from an instrumentation point to an event record.
1781 image::event-rule.png[]
1783 .Event, event record, or event rule?
1785 With so many similar terms, it's easy to get confused.
1787 An **event** is the consequence of the execution of an _instrumentation
1788 point_, like a tracepoint that you manually place in some source code,
1789 or a Linux kernel KProbe. An event is said to _occur_ at a specific
1790 time. Different actions can be taken upon the occurance of an event,
1791 like record the event's payload to a buffer.
1793 An **event record** is the representation of an event in a sub-buffer. A
1794 tracer is responsible for capturing the payload of an event, current
1795 context variables, the event's ID, and the event's timestamp. LTTng
1796 can append this sub-buffer to a trace file.
1798 An **event rule** is a set of conditions which must all be satisfied for
1799 LTTng to record an occuring event. Events still occur without
1800 satisfying event rules, but LTTng does not record them.
1805 == Components of noch:{LTTng}
1807 The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
1808 to call LTTng a simple _tool_ since it is composed of multiple
1809 interacting components. This section describes those components,
1810 explains their respective roles, and shows how they connect together to
1811 form the LTTng ecosystem.
1813 The following diagram shows how the most important components of LTTng
1814 interact with user applications, the Linux kernel, and you:
1817 .Control and trace data paths between LTTng components.
1818 image::plumbing.png[]
1820 The LTTng project incorporates:
1822 * **LTTng-tools**: Libraries and command-line interface to
1823 control tracing sessions.
1824 ** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
1825 ** <<lttng-consumerd,Consumer daemon>> (man:lttng-consumerd(8)).
1826 ** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
1827 ** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
1828 ** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
1829 * **LTTng-UST**: Libraries and Java/Python packages to trace user
1831 ** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
1832 headers to instrument and trace any native user application.
1833 ** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
1834 *** `liblttng-ust-libc-wrapper`
1835 *** `liblttng-ust-pthread-wrapper`
1836 *** `liblttng-ust-cyg-profile`
1837 *** `liblttng-ust-cyg-profile-fast`
1838 *** `liblttng-ust-dl`
1839 ** User space tracepoint provider source files generator command-line
1840 tool (man:lttng-gen-tp(1)).
1841 ** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
1842 Java applications using `java.util.logging` or
1843 Apache log4j 1.2 logging.
1844 ** <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
1845 Python applications using the standard `logging` package.
1846 * **LTTng-modules**: <<lttng-modules,Linux kernel modules>> to trace
1848 ** LTTng kernel tracer module.
1849 ** Tracing ring buffer kernel modules.
1850 ** Probe kernel modules.
1851 ** LTTng logger kernel module.
1855 === Tracing control command-line interface
1858 .The tracing control command-line interface.
1859 image::plumbing-lttng-cli.png[]
1861 The _man:lttng(1) command-line tool_ is the standard user interface to
1862 control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
1863 is part of LTTng-tools.
1865 The cmd:lttng tool is linked with
1866 <<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
1867 one or more <<lttng-sessiond,session daemons>> behind the scenes.
1869 The cmd:lttng tool has a Git-like interface:
1873 lttng <general options> <command> <command options>
1876 The <<controlling-tracing,Tracing control>> section explores the
1877 available features of LTTng using the cmd:lttng tool.
1880 [[liblttng-ctl-lttng]]
1881 === Tracing control library
1884 .The tracing control library.
1885 image::plumbing-liblttng-ctl.png[]
1887 The _LTTng control library_, `liblttng-ctl`, is used to communicate
1888 with a <<lttng-sessiond,session daemon>> using a C API that hides the
1889 underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.
1891 The <<lttng-cli,cmd:lttng command-line tool>>
1892 is linked with `liblttng-ctl`.
1894 You can use `liblttng-ctl` in C or $$C++$$ source code by including its
1899 #include <lttng/lttng.h>
1902 Some objects are referenced by name (C string), such as tracing
1903 sessions, but most of them require to create a handle first using
1904 `lttng_create_handle()`.
1906 The best available developer documentation for `liblttng-ctl` is, as of
1907 LTTng{nbsp}{revision}, its installed header files. Every function and
1908 structure is thoroughly documented.
1912 === User space tracing library
1915 .The user space tracing library.
1916 image::plumbing-liblttng-ust.png[]
1918 The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
1919 is the LTTng user space tracer. It receives commands from a
1920 <<lttng-sessiond,session daemon>>, for example to
1921 enable and disable specific instrumentation points, and writes event
1922 records to ring buffers shared with a
1923 <<lttng-consumerd,consumer daemon>>.
1924 `liblttng-ust` is part of LTTng-UST.
1926 Public C header files are installed beside `liblttng-ust` to
1927 instrument any <<c-application,C or $$C++$$ application>>.
1929 <<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
1930 packages, use their own library providing tracepoints which is
1931 linked with `liblttng-ust`.
1933 An application or library does not have to initialize `liblttng-ust`
1934 manually: its constructor does the necessary tasks to properly register
1935 to a session daemon. The initialization phase also enables the
1936 instrumentation points matching the <<event,event rules>> that you
1940 [[lttng-ust-agents]]
1941 === User space tracing agents
1944 .The user space tracing agents.
1945 image::plumbing-lttng-ust-agents.png[]
1947 The _LTTng-UST Java and Python agents_ are regular Java and Python
1948 packages which add LTTng tracing capabilities to the
1949 native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
1951 In the case of Java, the
1952 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
1953 core logging facilities] and
1954 https://logging.apache.org/log4j/1.2/[Apache log4j 1.2] are supported.
1955 Note that Apache Log4{nbsp}2 is not supported.
1957 In the case of Python, the standard
1958 https://docs.python.org/3/library/logging.html[`logging`] package
1959 is supported. Both Python 2 and Python 3 modules can import the
1960 LTTng-UST Python agent package.
1962 The applications using the LTTng-UST agents are in the
1963 `java.util.logging` (JUL),
1964 log4j, and Python <<domain,tracing domains>>.
1966 Both agents use the same mechanism to trace the log statements. When an
1967 agent is initialized, it creates a log handler that attaches to the root
1968 logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
1969 When the application executes a log statement, it is passed to the
1970 agent's log handler by the root logger. The agent's log handler calls a
1971 native function in a tracepoint provider package shared library linked
1972 with <<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
1973 other fields, like its logger name and its log level. This native
1974 function contains a user space instrumentation point, hence tracing the
1977 The log level condition of an
1978 <<event,event rule>> is considered when tracing
1979 a Java or a Python application, and it's compatible with the standard
1980 JUL, log4j, and Python log levels.
1984 === LTTng kernel modules
1987 .The LTTng kernel modules.
1988 image::plumbing-lttng-modules.png[]
1990 The _LTTng kernel modules_ are a set of Linux kernel modules
1991 which implement the kernel tracer of the LTTng project. The LTTng
1992 kernel modules are part of LTTng-modules.
1994 The LTTng kernel modules include:
1996 * A set of _probe_ modules.
1998 Each module attaches to a specific subsystem
1999 of the Linux kernel using its tracepoint instrument points. There are
2000 also modules to attach to the entry and return points of the Linux
2001 system call functions.
2003 * _Ring buffer_ modules.
2005 A ring buffer implementation is provided as kernel modules. The LTTng
2006 kernel tracer writes to the ring buffer; a
2007 <<lttng-consumerd,consumer daemon>> reads from the ring buffer.
2009 * The _LTTng kernel tracer_ module.
2010 * The _LTTng logger_ module.
2012 The LTTng logger module implements the special path:{/proc/lttng-logger}
2013 file so that any executable can generate LTTng events by opening and
2014 writing to this file.
2016 See <<proc-lttng-logger-abi,LTTng logger>>.
2018 Generally, you do not have to load the LTTng kernel modules manually
2019 (using man:modprobe(8), for example): a root <<lttng-sessiond,session
2020 daemon>> loads the necessary modules when starting. If you have extra
2021 probe modules, you can specify to load them to the session daemon on
2024 The LTTng kernel modules are installed in
2025 +/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
2026 the kernel release (see `uname --kernel-release`).
2033 .The session daemon.
2034 image::plumbing-sessiond.png[]
2036 The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
2037 managing tracing sessions and for controlling the various components of
2038 LTTng. The session daemon is part of LTTng-tools.
2040 The session daemon sends control requests to and receives control
2043 * The <<lttng-ust,user space tracing library>>.
2045 Any instance of the user space tracing library first registers to
2046 a session daemon. Then, the session daemon can send requests to
2047 this instance, such as:
2050 ** Get the list of tracepoints.
2051 ** Share an <<event,event rule>> so that the user space tracing library
2052 can enable or disable tracepoints. Amongst the possible conditions
2053 of an event rule is a filter expression which `liblttng-ust` evalutes
2054 when an event occurs.
2055 ** Share <<channel,channel>> attributes and ring buffer locations.
2058 The session daemon and the user space tracing library use a Unix
2059 domain socket for their communication.
2061 * The <<lttng-ust-agents,user space tracing agents>>.
2063 Any instance of a user space tracing agent first registers to
2064 a session daemon. Then, the session daemon can send requests to
2065 this instance, such as:
2068 ** Get the list of loggers.
2069 ** Enable or disable a specific logger.
2072 The session daemon and the user space tracing agent use a TCP connection
2073 for their communication.
2075 * The <<lttng-modules,LTTng kernel tracer>>.
2076 * The <<lttng-consumerd,consumer daemon>>.
2078 The session daemon sends requests to the consumer daemon to instruct
2079 it where to send the trace data streams, amongst other information.
2081 * The <<lttng-relayd,relay daemon>>.
2083 The session daemon receives commands from the
2084 <<liblttng-ctl-lttng,tracing control library>>.
2086 The root session daemon loads the appropriate
2087 <<lttng-modules,LTTng kernel modules>> on startup. It also spawns
2088 a <<lttng-consumerd,consumer daemon>> as soon as you create
2089 an <<event,event rule>>.
2091 The session daemon does not send and receive trace data: this is the
2092 role of the <<lttng-consumerd,consumer daemon>> and
2093 <<lttng-relayd,relay daemon>>. It does, however, generate the
2094 http://diamon.org/ctf/[CTF] metadata stream.
2096 Each Unix user can have its own session daemon instance. The
2097 tracing sessions managed by different session daemons are completely
2100 The root user's session daemon is the only one which is
2101 allowed to control the LTTng kernel tracer, and its spawned consumer
2102 daemon is the only one which is allowed to consume trace data from the
2103 LTTng kernel tracer. Note, however, that any Unix user which is a member
2104 of the <<tracing-group,tracing group>> is allowed
2105 to create <<channel,channels>> in the
2106 Linux kernel <<domain,tracing domain>>, and thus to trace the Linux
2109 The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
2110 session daemon when using its `create` command if none is currently
2111 running. You can also start the session daemon manually.
2118 .The consumer daemon.
2119 image::plumbing-consumerd.png[]
2121 The _consumer daemon_, man:lttng-consumerd(8), is a daemon which shares
2122 ring buffers with user applications or with the LTTng kernel modules to
2123 collect trace data and send it to some location (on disk or to a
2124 <<lttng-relayd,relay daemon>> over the network). The consumer daemon
2125 is part of LTTng-tools.
2127 You do not start a consumer daemon manually: a consumer daemon is always
2128 spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
2129 <<event,event rule>>, that is, before you start tracing. When you kill
2130 its owner session daemon, the consumer daemon also exits because it is
2131 the session daemon's child process. Command-line options of
2132 man:lttng-sessiond(8) target the consumer daemon process.
2134 There are up to two running consumer daemons per Unix user, whereas only
2135 one session daemon can run per user. This is because each process can be
2136 either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
2137 and 64-bit processes, it is more efficient to have separate
2138 corresponding 32-bit and 64-bit consumer daemons. The root user is an
2139 exception: it can have up to _three_ running consumer daemons: 32-bit
2140 and 64-bit instances for its user applications, and one more
2141 reserved for collecting kernel trace data.
2149 image::plumbing-relayd.png[]
2151 The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
2152 between remote session and consumer daemons, local trace files, and a
2153 remote live trace viewer. The relay daemon is part of LTTng-tools.
2155 The main purpose of the relay daemon is to implement a receiver of
2156 <<sending-trace-data-over-the-network,trace data over the network>>.
2157 This is useful when the target system does not have much file system
2158 space to record trace files locally.
2160 The relay daemon is also a server to which a
2161 <<lttng-live,live trace viewer>> can
2162 connect. The live trace viewer sends requests to the relay daemon to
2163 receive trace data as the target system emits events. The
2164 communication protocol is named _LTTng live_; it is used over TCP
2167 Note that you can start the relay daemon on the target system directly.
2168 This is the setup of choice when the use case is to view events as
2169 the target system emits them without the need of a remote system.
2173 == [[using-lttng]]Instrumentation
2175 There are many examples of tracing and monitoring in our everyday life:
2177 * You have access to real-time and historical weather reports and
2178 forecasts thanks to weather stations installed around the country.
2179 * You know your heart is safe thanks to an electrocardiogram.
2180 * You make sure not to drive your car too fast and to have enough fuel
2181 to reach your destination thanks to gauges visible on your dashboard.
2183 All the previous examples have something in common: they rely on
2184 **instruments**. Without the electrodes attached to the surface of your
2185 body's skin, cardiac monitoring is futile.
2187 LTTng, as a tracer, is no different from those real life examples. If
2188 you're about to trace a software system or, in other words, record its
2189 history of execution, you better have **instrumentation points** in the
2190 subject you're tracing, that is, the actual software.
2192 Various ways were developed to instrument a piece of software for LTTng
2193 tracing. The most straightforward one is to manually place
2194 instrumentation points, called _tracepoints_, in the software's source
2195 code. It is also possible to add instrumentation points dynamically in
2196 the Linux kernel <<domain,tracing domain>>.
2198 If you're only interested in tracing the Linux kernel, your
2199 instrumentation needs are probably already covered by LTTng's built-in
2200 <<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
2201 user application which is already instrumented for LTTng tracing.
2202 In such cases, you can skip this whole section and read the topics of
2203 the <<controlling-tracing,Tracing control>> section.
2205 Many methods are available to instrument a piece of software for LTTng
2208 * <<c-application,User space instrumentation for C and $$C++$$
2210 * <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
2211 * <<java-application,User space Java agent>>.
2212 * <<python-application,User space Python agent>>.
2213 * <<proc-lttng-logger-abi,LTTng logger>>.
2214 * <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
2218 === [[cxx-application]]User space instrumentation for C and $$C++$$ applications
2220 The procedure to instrument a C or $$C++$$ user application with
2221 the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
2223 . <<tracepoint-provider,Create the source files of a tracepoint provider
2225 . <<probing-the-application-source-code,Add tracepoints to
2226 the application's source code>>.
2227 . <<building-tracepoint-providers-and-user-application,Build and link
2228 a tracepoint provider package and the user application>>.
2230 If you need quick, man:printf(3)-like instrumentation, you can skip
2231 those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
2234 IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
2235 instrument a user application with `liblttng-ust`.
2238 [[tracepoint-provider]]
2239 ==== Create the source files of a tracepoint provider package
2241 A _tracepoint provider_ is a set of compiled functions which provide
2242 **tracepoints** to an application, the type of instrumentation point
2243 supported by LTTng-UST. Those functions can emit events with
2244 user-defined fields and serialize those events as event records to one
2245 or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
2246 macro, which you <<probing-the-application-source-code,insert in a user
2247 application's source code>>, calls those functions.
2249 A _tracepoint provider package_ is an object file (`.o`) or a shared
2250 library (`.so`) which contains one or more tracepoint providers.
2251 Its source files are:
2253 * One or more <<tpp-header,tracepoint provider header>> (`.h`).
2254 * A <<tpp-source,tracepoint provider package source>> (`.c`).
2256 A tracepoint provider package is dynamically linked with `liblttng-ust`,
2257 the LTTng user space tracer, at run time.
2260 .User application linked with `liblttng-ust` and containing a tracepoint provider.
2261 image::ust-app.png[]
2263 NOTE: If you need quick, man:printf(3)-like instrumentation, you can
2264 skip creating and using a tracepoint provider and use
2265 <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
2269 ===== Create a tracepoint provider header file template
2271 A _tracepoint provider header file_ contains the tracepoint
2272 definitions of a tracepoint provider.
2274 To create a tracepoint provider header file:
2276 . Start from this template:
2280 .Tracepoint provider header file template (`.h` file extension).
2282 #undef TRACEPOINT_PROVIDER
2283 #define TRACEPOINT_PROVIDER provider_name
2285 #undef TRACEPOINT_INCLUDE
2286 #define TRACEPOINT_INCLUDE "./tp.h"
2288 #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
2291 #include <lttng/tracepoint.h>
2294 * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
2295 * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
2300 #include <lttng/tracepoint-event.h>
2306 * `provider_name` with the name of your tracepoint provider.
2307 * `"tp.h"` with the name of your tracepoint provider header file.
2309 . Below the `#include <lttng/tracepoint.h>` line, put your
2310 <<defining-tracepoints,tracepoint definitions>>.
2312 Your tracepoint provider name must be unique amongst all the possible
2313 tracepoint provider names used on the same target system. We
2314 suggest to include the name of your project or company in the name,
2315 for example, `org_lttng_my_project_tpp`.
2317 TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
2318 this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
2319 write are the <<defining-tracepoints,tracepoint definitions>>.
2322 [[defining-tracepoints]]
2323 ===== Create a tracepoint definition
2325 A _tracepoint definition_ defines, for a given tracepoint:
2327 * Its **input arguments**. They are the macro parameters that the
2328 `tracepoint()` macro accepts for this particular tracepoint
2329 in the user application's source code.
2330 * Its **output event fields**. They are the sources of event fields
2331 that form the payload of any event that the execution of the
2332 `tracepoint()` macro emits for this particular tracepoint.
2334 You can create a tracepoint definition by using the
2335 `TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
2337 <<tpp-header,tracepoint provider header file template>>.
2339 The syntax of the `TRACEPOINT_EVENT()` macro is:
2342 .`TRACEPOINT_EVENT()` macro syntax.
2345 /* Tracepoint provider name */
2348 /* Tracepoint name */
2351 /* Input arguments */
2356 /* Output event fields */
2365 * `provider_name` with your tracepoint provider name.
2366 * `tracepoint_name` with your tracepoint name.
2367 * `arguments` with the <<tpp-def-input-args,input arguments>>.
2368 * `fields` with the <<tpp-def-output-fields,output event field>>
2371 This tracepoint emits events named `provider_name:tracepoint_name`.
2374 .Event name's length limitation
2376 The concatenation of the tracepoint provider name and the
2377 tracepoint name must not exceed **254 characters**. If it does, the
2378 instrumented application compiles and runs, but LTTng throws multiple
2379 warnings and you could experience serious issues.
2382 [[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is:
2385 .`TP_ARGS()` macro syntax.
2394 * `type` with the C type of the argument.
2395 * `arg_name` with the argument name.
2397 You can repeat `type` and `arg_name` up to 10 times to have
2398 more than one argument.
2400 .`TP_ARGS()` usage with three arguments.
2412 The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
2413 tracepoint definition with no input arguments.
2415 [[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
2416 `ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
2417 man:lttng-ust(3) for a complete description of the available `ctf_*()`
2418 macros. A `ctf_*()` macro specifies the type, size, and byte order of
2421 Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
2422 C expression that the tracer evalutes at the `tracepoint()` macro site
2423 in the application's source code. This expression provides a field's
2424 source of data. The argument expression can include input argument names
2425 listed in the `TP_ARGS()` macro.
2427 Each `ctf_*()` macro also takes a _field name_ parameter. Field names
2428 must be unique within a given tracepoint definition.
2430 Here's a complete tracepoint definition example:
2432 .Tracepoint definition.
2434 The following tracepoint definition defines a tracepoint which takes
2435 three input arguments and has four output event fields.
2439 #include "my-custom-structure.h"
2445 const struct my_custom_structure*, my_custom_structure,
2450 ctf_string(query_field, query)
2451 ctf_float(double, ratio_field, ratio)
2452 ctf_integer(int, recv_size, my_custom_structure->recv_size)
2453 ctf_integer(int, send_size, my_custom_structure->send_size)
2458 You can refer to this tracepoint definition with the `tracepoint()`
2459 macro in your application's source code like this:
2463 tracepoint(my_provider, my_tracepoint,
2464 my_structure, some_ratio, the_query);
2468 NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
2469 if they satisfy an enabled <<event,event rule>>.
2472 [[using-tracepoint-classes]]
2473 ===== Use a tracepoint class
2475 A _tracepoint class_ is a class of tracepoints which share the same
2476 output event field definitions. A _tracepoint instance_ is one
2477 instance of such a defined tracepoint class, with its own tracepoint
2480 The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
2481 shorthand which defines both a tracepoint class and a tracepoint
2482 instance at the same time.
2484 When you build a tracepoint provider package, the C or $$C++$$ compiler
2485 creates one serialization function for each **tracepoint class**. A
2486 serialization function is responsible for serializing the event fields
2487 of a tracepoint to a sub-buffer when tracing.
2489 For various performance reasons, when your situation requires multiple
2490 tracepoint definitions with different names, but with the same event
2491 fields, we recommend that you manually create a tracepoint class
2492 and instantiate as many tracepoint instances as needed. One positive
2493 effect of such a design, amongst other advantages, is that all
2494 tracepoint instances of the same tracepoint class reuse the same
2495 serialization function, thus reducing
2496 https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
2498 .Use a tracepoint class and tracepoint instances.
2500 Consider the following three tracepoint definitions:
2512 ctf_integer(int, userid, userid)
2513 ctf_integer(size_t, len, len)
2525 ctf_integer(int, userid, userid)
2526 ctf_integer(size_t, len, len)
2538 ctf_integer(int, userid, userid)
2539 ctf_integer(size_t, len, len)
2544 In this case, we create three tracepoint classes, with one implicit
2545 tracepoint instance for each of them: `get_account`, `get_settings`, and
2546 `get_transaction`. However, they all share the same event field names
2547 and types. Hence three identical, yet independent serialization
2548 functions are created when you build the tracepoint provider package.
2550 A better design choice is to define a single tracepoint class and three
2551 tracepoint instances:
2555 /* The tracepoint class */
2556 TRACEPOINT_EVENT_CLASS(
2557 /* Tracepoint provider name */
2560 /* Tracepoint class name */
2563 /* Input arguments */
2569 /* Output event fields */
2571 ctf_integer(int, userid, userid)
2572 ctf_integer(size_t, len, len)
2576 /* The tracepoint instances */
2577 TRACEPOINT_EVENT_INSTANCE(
2578 /* Tracepoint provider name */
2581 /* Tracepoint class name */
2584 /* Tracepoint name */
2587 /* Input arguments */
2593 TRACEPOINT_EVENT_INSTANCE(
2602 TRACEPOINT_EVENT_INSTANCE(
2615 [[assigning-log-levels]]
2616 ===== Assign a log level to a tracepoint definition
2618 You can assign an optional _log level_ to a
2619 <<defining-tracepoints,tracepoint definition>>.
2621 Assigning different levels of severity to tracepoint definitions can
2622 be useful: when you <<enabling-disabling-events,create an event rule>>,
2623 you can target tracepoints having a log level as severe as a specific
2626 The concept of LTTng-UST log levels is similar to the levels found
2627 in typical logging frameworks:
2629 * In a logging framework, the log level is given by the function
2630 or method name you use at the log statement site: `debug()`,
2631 `info()`, `warn()`, `error()`, and so on.
2632 * In LTTng-UST, you statically assign the log level to a tracepoint
2633 definition; any `tracepoint()` macro invocation which refers to
2634 this definition has this log level.
2636 You can assign a log level to a tracepoint definition with the
2637 `TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
2638 <<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
2639 <<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
2642 The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
2645 .`TRACEPOINT_LOGLEVEL()` macro syntax.
2647 TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
2652 * `provider_name` with the tracepoint provider name.
2653 * `tracepoint_name` with the tracepoint name.
2654 * `log_level` with the log level to assign to the tracepoint
2655 definition named `tracepoint_name` in the `provider_name`
2656 tracepoint provider.
2658 See man:lttng-ust(3) for a list of available log level names.
2660 .Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
2664 /* Tracepoint definition */
2673 ctf_integer(int, userid, userid)
2674 ctf_integer(size_t, len, len)
2678 /* Log level assignment */
2679 TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
2685 ===== Create a tracepoint provider package source file
2687 A _tracepoint provider package source file_ is a C source file which
2688 includes a <<tpp-header,tracepoint provider header file>> to expand its
2689 macros into event serialization and other functions.
2691 You can always use the following tracepoint provider package source
2695 .Tracepoint provider package source file template.
2697 #define TRACEPOINT_CREATE_PROBES
2702 Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
2703 header file>> name. You may also include more than one tracepoint
2704 provider header file here to create a tracepoint provider package
2705 holding more than one tracepoint providers.
2708 [[probing-the-application-source-code]]
2709 ==== Add tracepoints to an application's source code
2711 Once you <<tpp-header,create a tracepoint provider header file>>, you
2712 can use the `tracepoint()` macro in your application's
2713 source code to insert the tracepoints that this header
2714 <<defining-tracepoints,defines>>.
2716 The `tracepoint()` macro takes at least two parameters: the tracepoint
2717 provider name and the tracepoint name. The corresponding tracepoint
2718 definition defines the other parameters.
2720 .`tracepoint()` usage.
2722 The following <<defining-tracepoints,tracepoint definition>> defines a
2723 tracepoint which takes two input arguments and has two output event
2727 .Tracepoint provider header file.
2729 #include "my-custom-structure.h"
2736 const char*, cmd_name
2739 ctf_string(cmd_name, cmd_name)
2740 ctf_integer(int, number_of_args, argc)
2745 You can refer to this tracepoint definition with the `tracepoint()`
2746 macro in your application's source code like this:
2749 .Application's source file.
2753 int main(int argc, char* argv[])
2755 tracepoint(my_provider, my_tracepoint, argc, argv[0]);
2761 Note how the application's source code includes
2762 the tracepoint provider header file containing the tracepoint
2763 definitions to use, path:{tp.h}.
2766 .`tracepoint()` usage with a complex tracepoint definition.
2768 Consider this complex tracepoint definition, where multiple event
2769 fields refer to the same input arguments in their argument expression
2773 .Tracepoint provider header file.
2775 /* For `struct stat` */
2776 #include <sys/types.h>
2777 #include <sys/stat.h>
2789 ctf_integer(int, my_constant_field, 23 + 17)
2790 ctf_integer(int, my_int_arg_field, my_int_arg)
2791 ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
2792 ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
2793 my_str_arg[2] + my_str_arg[3])
2794 ctf_string(my_str_arg_field, my_str_arg)
2795 ctf_integer_hex(off_t, size_field, st->st_size)
2796 ctf_float(double, size_dbl_field, (double) st->st_size)
2797 ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
2798 size_t, strlen(my_str_arg) / 2)
2803 You can refer to this tracepoint definition with the `tracepoint()`
2804 macro in your application's source code like this:
2807 .Application's source file.
2809 #define TRACEPOINT_DEFINE
2816 stat("/etc/fstab", &s);
2817 tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
2823 If you look at the event record that LTTng writes when tracing this
2824 program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
2825 it should look like this:
2827 .Event record fields
2829 |Field's name |Field's value
2830 |`my_constant_field` |40
2831 |`my_int_arg_field` |23
2832 |`my_int_arg_field2` |529
2834 |`my_str_arg_field` |`Hello, World!`
2835 |`size_field` |0x12d
2836 |`size_dbl_field` |301.0
2837 |`half_my_str_arg_field` |`Hello,`
2841 Sometimes, the arguments you pass to `tracepoint()` are expensive to
2842 compute--they use the call stack, for example. To avoid this
2843 computation when the tracepoint is disabled, you can use the
2844 `tracepoint_enabled()` and `do_tracepoint()` macros.
2846 The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
2850 .`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
2852 tracepoint_enabled(provider_name, tracepoint_name)
2853 do_tracepoint(provider_name, tracepoint_name, ...)
2858 * `provider_name` with the tracepoint provider name.
2859 * `tracepoint_name` with the tracepoint name.
2861 `tracepoint_enabled()` returns a non-zero value if the tracepoint named
2862 `tracepoint_name` from the provider named `provider_name` is enabled
2865 `do_tracepoint()` is like `tracepoint()`, except that it doesn't check
2866 if the tracepoint is enabled. Using `tracepoint()` with
2867 `tracepoint_enabled()` is dangerous since `tracepoint()` also contains
2868 the `tracepoint_enabled()` check, thus a race condition is
2869 possible in this situation:
2872 .Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
2874 if (tracepoint_enabled(my_provider, my_tracepoint)) {
2875 stuff = prepare_stuff();
2878 tracepoint(my_provider, my_tracepoint, stuff);
2881 If the tracepoint is enabled after the condition, then `stuff` is not
2882 prepared: the emitted event will either contain wrong data, or the whole
2883 application could crash (segmentation fault, for example).
2885 NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
2886 `STAP_PROBEV()` call. If you need it, you must emit
2890 [[building-tracepoint-providers-and-user-application]]
2891 ==== Build and link a tracepoint provider package and an application
2893 Once you have one or more <<tpp-header,tracepoint provider header
2894 files>> and a <<tpp-source,tracepoint provider package source file>>,
2895 you can create the tracepoint provider package by compiling its source
2896 file. From here, multiple build and run scenarios are possible. The
2897 following table shows common application and library configurations
2898 along with the required command lines to achieve them.
2900 In the following diagrams, we use the following file names:
2903 Executable application.
2906 Application's object file.
2909 Tracepoint provider package object file.
2912 Tracepoint provider package archive file.
2915 Tracepoint provider package shared object file.
2918 User library object file.
2921 User library shared object file.
2923 We use the following symbols in the diagrams of table below:
2926 .Symbols used in the build scenario diagrams.
2927 image::ust-sit-symbols.png[]
2929 We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
2930 variable in the following instructions.
2932 [role="growable ust-scenarios",cols="asciidoc,asciidoc"]
2933 .Common tracepoint provider package scenarios.
2935 |Scenario |Instructions
2938 The instrumented application is statically linked with
2939 the tracepoint provider package object.
2941 image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
2944 include::../common/ust-sit-step-tp-o.txt[]
2946 To build the instrumented application:
2948 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2953 #define TRACEPOINT_DEFINE
2957 . Compile the application source file:
2966 . Build the application:
2971 gcc -o app app.o tpp.o -llttng-ust -ldl
2975 To run the instrumented application:
2977 * Start the application:
2987 The instrumented application is statically linked with the
2988 tracepoint provider package archive file.
2990 image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
2993 To create the tracepoint provider package archive file:
2995 . Compile the <<tpp-source,tracepoint provider package source file>>:
3004 . Create the tracepoint provider package archive file:
3013 To build the instrumented application:
3015 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3020 #define TRACEPOINT_DEFINE
3024 . Compile the application source file:
3033 . Build the application:
3038 gcc -o app app.o tpp.a -llttng-ust -ldl
3042 To run the instrumented application:
3044 * Start the application:
3054 The instrumented application is linked with the tracepoint provider
3055 package shared object.
3057 image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
3060 include::../common/ust-sit-step-tp-so.txt[]
3062 To build the instrumented application:
3064 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3069 #define TRACEPOINT_DEFINE
3073 . Compile the application source file:
3082 . Build the application:
3087 gcc -o app app.o -ldl -L. -ltpp
3091 To run the instrumented application:
3093 * Start the application:
3103 The tracepoint provider package shared object is preloaded before the
3104 instrumented application starts.
3106 image::ust-sit+tp-so-preloaded+app-instrumented.png[]
3109 include::../common/ust-sit-step-tp-so.txt[]
3111 To build the instrumented application:
3113 . In path:{app.c}, before including path:{tpp.h}, add the
3119 #define TRACEPOINT_DEFINE
3120 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3124 . Compile the application source file:
3133 . Build the application:
3138 gcc -o app app.o -ldl
3142 To run the instrumented application with tracing support:
3144 * Preload the tracepoint provider package shared object and
3145 start the application:
3150 LD_PRELOAD=./libtpp.so ./app
3154 To run the instrumented application without tracing support:
3156 * Start the application:
3166 The instrumented application dynamically loads the tracepoint provider
3167 package shared object.
3169 See the <<dlclose-warning,warning about `dlclose()`>>.
3171 image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
3174 include::../common/ust-sit-step-tp-so.txt[]
3176 To build the instrumented application:
3178 . In path:{app.c}, before including path:{tpp.h}, add the
3184 #define TRACEPOINT_DEFINE
3185 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3189 . Compile the application source file:
3198 . Build the application:
3203 gcc -o app app.o -ldl
3207 To run the instrumented application:
3209 * Start the application:
3219 The application is linked with the instrumented user library.
3221 The instrumented user library is statically linked with the tracepoint
3222 provider package object file.
3224 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
3227 include::../common/ust-sit-step-tp-o-fpic.txt[]
3229 To build the instrumented user library:
3231 . In path:{emon.c}, before including path:{tpp.h}, add the
3237 #define TRACEPOINT_DEFINE
3241 . Compile the user library source file:
3246 gcc -I. -fpic -c emon.c
3250 . Build the user library shared object:
3255 gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
3259 To build the application:
3261 . Compile the application source file:
3270 . Build the application:
3275 gcc -o app app.o -L. -lemon
3279 To run the application:
3281 * Start the application:
3291 The application is linked with the instrumented user library.
3293 The instrumented user library is linked with the tracepoint provider
3294 package shared object.
3296 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3299 include::../common/ust-sit-step-tp-so.txt[]
3301 To build the instrumented user library:
3303 . In path:{emon.c}, before including path:{tpp.h}, add the
3309 #define TRACEPOINT_DEFINE
3313 . Compile the user library source file:
3318 gcc -I. -fpic -c emon.c
3322 . Build the user library shared object:
3327 gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3331 To build the application:
3333 . Compile the application source file:
3342 . Build the application:
3347 gcc -o app app.o -L. -lemon
3351 To run the application:
3353 * Start the application:
3363 The tracepoint provider package shared object is preloaded before the
3366 The application is linked with the instrumented user library.
3368 image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
3371 include::../common/ust-sit-step-tp-so.txt[]
3373 To build the instrumented user library:
3375 . In path:{emon.c}, before including path:{tpp.h}, add the
3381 #define TRACEPOINT_DEFINE
3382 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3386 . Compile the user library source file:
3391 gcc -I. -fpic -c emon.c
3395 . Build the user library shared object:
3400 gcc -shared -o libemon.so emon.o -ldl
3404 To build the application:
3406 . Compile the application source file:
3415 . Build the application:
3420 gcc -o app app.o -L. -lemon
3424 To run the application with tracing support:
3426 * Preload the tracepoint provider package shared object and
3427 start the application:
3432 LD_PRELOAD=./libtpp.so ./app
3436 To run the application without tracing support:
3438 * Start the application:
3448 The application is linked with the instrumented user library.
3450 The instrumented user library dynamically loads the tracepoint provider
3451 package shared object.
3453 See the <<dlclose-warning,warning about `dlclose()`>>.
3455 image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3458 include::../common/ust-sit-step-tp-so.txt[]
3460 To build the instrumented user library:
3462 . In path:{emon.c}, before including path:{tpp.h}, add the
3468 #define TRACEPOINT_DEFINE
3469 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3473 . Compile the user library source file:
3478 gcc -I. -fpic -c emon.c
3482 . Build the user library shared object:
3487 gcc -shared -o libemon.so emon.o -ldl
3491 To build the application:
3493 . Compile the application source file:
3502 . Build the application:
3507 gcc -o app app.o -L. -lemon
3511 To run the application:
3513 * Start the application:
3523 The application dynamically loads the instrumented user library.
3525 The instrumented user library is linked with the tracepoint provider
3526 package shared object.
3528 See the <<dlclose-warning,warning about `dlclose()`>>.
3530 image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3533 include::../common/ust-sit-step-tp-so.txt[]
3535 To build the instrumented user library:
3537 . In path:{emon.c}, before including path:{tpp.h}, add the
3543 #define TRACEPOINT_DEFINE
3547 . Compile the user library source file:
3552 gcc -I. -fpic -c emon.c
3556 . Build the user library shared object:
3561 gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3565 To build the application:
3567 . Compile the application source file:
3576 . Build the application:
3581 gcc -o app app.o -ldl -L. -lemon
3585 To run the application:
3587 * Start the application:
3597 The application dynamically loads the instrumented user library.
3599 The instrumented user library dynamically loads the tracepoint provider
3600 package shared object.
3602 See the <<dlclose-warning,warning about `dlclose()`>>.
3604 image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3607 include::../common/ust-sit-step-tp-so.txt[]
3609 To build the instrumented user library:
3611 . In path:{emon.c}, before including path:{tpp.h}, add the
3617 #define TRACEPOINT_DEFINE
3618 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3622 . Compile the user library source file:
3627 gcc -I. -fpic -c emon.c
3631 . Build the user library shared object:
3636 gcc -shared -o libemon.so emon.o -ldl
3640 To build the application:
3642 . Compile the application source file:
3651 . Build the application:
3656 gcc -o app app.o -ldl -L. -lemon
3660 To run the application:
3662 * Start the application:
3672 The tracepoint provider package shared object is preloaded before the
3675 The application dynamically loads the instrumented user library.
3677 image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
3680 include::../common/ust-sit-step-tp-so.txt[]
3682 To build the instrumented user library:
3684 . In path:{emon.c}, before including path:{tpp.h}, add the
3690 #define TRACEPOINT_DEFINE
3691 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3695 . Compile the user library source file:
3700 gcc -I. -fpic -c emon.c
3704 . Build the user library shared object:
3709 gcc -shared -o libemon.so emon.o -ldl
3713 To build the application:
3715 . Compile the application source file:
3724 . Build the application:
3729 gcc -o app app.o -L. -lemon
3733 To run the application with tracing support:
3735 * Preload the tracepoint provider package shared object and
3736 start the application:
3741 LD_PRELOAD=./libtpp.so ./app
3745 To run the application without tracing support:
3747 * Start the application:
3757 The application is statically linked with the tracepoint provider
3758 package object file.
3760 The application is linked with the instrumented user library.
3762 image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
3765 include::../common/ust-sit-step-tp-o.txt[]
3767 To build the instrumented user library:
3769 . In path:{emon.c}, before including path:{tpp.h}, add the
3775 #define TRACEPOINT_DEFINE
3779 . Compile the user library source file:
3784 gcc -I. -fpic -c emon.c
3788 . Build the user library shared object:
3793 gcc -shared -o libemon.so emon.o
3797 To build the application:
3799 . Compile the application source file:
3808 . Build the application:
3813 gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
3817 To run the instrumented application:
3819 * Start the application:
3829 The application is statically linked with the tracepoint provider
3830 package object file.
3832 The application dynamically loads the instrumented user library.
3834 image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
3837 include::../common/ust-sit-step-tp-o.txt[]
3839 To build the application:
3841 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3846 #define TRACEPOINT_DEFINE
3850 . Compile the application source file:
3859 . Build the application:
3864 gcc -Wl,--export-dynamic -o app app.o tpp.o \
3869 The `--export-dynamic` option passed to the linker is necessary for the
3870 dynamically loaded library to ``see'' the tracepoint symbols defined in
3873 To build the instrumented user library:
3875 . Compile the user library source file:
3880 gcc -I. -fpic -c emon.c
3884 . Build the user library shared object:
3889 gcc -shared -o libemon.so emon.o
3893 To run the application:
3895 * Start the application:
3907 .Do not use man:dlclose(3) on a tracepoint provider package
3909 Never use man:dlclose(3) on any shared object which:
3911 * Is linked with, statically or dynamically, a tracepoint provider
3913 * Calls man:dlopen(3) itself to dynamically open a tracepoint provider
3914 package shared object.
3916 This is currently considered **unsafe** due to a lack of reference
3917 counting from LTTng-UST to the shared object.
3919 A known workaround (available since glibc 2.2) is to use the
3920 `RTLD_NODELETE` flag when calling man:dlopen(3) initially. This has the
3921 effect of not unloading the loaded shared object, even if man:dlclose(3)
3924 You can also preload the tracepoint provider package shared object with
3925 the env:LD_PRELOAD environment variable to overcome this limitation.
3929 [[using-lttng-ust-with-daemons]]
3930 ===== Use noch:{LTTng-UST} with daemons
3932 If your instrumented application calls man:fork(2), man:clone(2),
3933 or BSD's man:rfork(2), without a following man:exec(3)-family
3934 system call, you must preload the path:{liblttng-ust-fork.so} shared
3935 object when starting the application.
3939 LD_PRELOAD=liblttng-ust-fork.so ./my-app
3942 If your tracepoint provider package is
3943 a shared library which you also preload, you must put both
3944 shared objects in env:LD_PRELOAD:
3948 LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
3952 [[lttng-ust-pkg-config]]
3953 ===== Use noch:{pkg-config}
3955 On some distributions, LTTng-UST ships with a
3956 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
3957 metadata file. If this is your case, then you can use cmd:pkg-config to
3958 build an application on the command line:
3962 gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
3966 [[instrumenting-32-bit-app-on-64-bit-system]]
3967 ===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
3969 In order to trace a 32-bit application running on a 64-bit system,
3970 LTTng must use a dedicated 32-bit
3971 <<lttng-consumerd,consumer daemon>>.
3973 The following steps show how to build and install a 32-bit consumer
3974 daemon, which is _not_ part of the default 64-bit LTTng build, how to
3975 build and install the 32-bit LTTng-UST libraries, and how to build and
3976 link an instrumented 32-bit application in that context.
3978 To build a 32-bit instrumented application for a 64-bit target system,
3979 assuming you have a fresh target system with no installed Userspace RCU
3982 . Download, build, and install a 32-bit version of Userspace RCU:
3988 wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
3989 tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
3990 cd userspace-rcu-0.9.* &&
3991 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
3993 sudo make install &&
3998 . Using your distribution's package manager, or from source, install
3999 the following 32-bit versions of the following dependencies of
4000 LTTng-tools and LTTng-UST:
4003 * https://sourceforge.net/projects/libuuid/[libuuid]
4004 * http://directory.fsf.org/wiki/Popt[popt]
4005 * http://www.xmlsoft.org/[libxml2]
4008 . Download, build, and install a 32-bit version of the latest
4009 LTTng-UST{nbsp}{revision}:
4015 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.8.tar.bz2 &&
4016 tar -xf lttng-ust-latest-2.8.tar.bz2 &&
4017 cd lttng-ust-2.8.* &&
4018 ./configure --libdir=/usr/local/lib32 \
4019 CFLAGS=-m32 CXXFLAGS=-m32 \
4020 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
4022 sudo make install &&
4029 Depending on your distribution,
4030 32-bit libraries could be installed at a different location than
4031 `/usr/lib32`. For example, Debian is known to install
4032 some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
4034 In this case, make sure to set `LDFLAGS` to all the
4035 relevant 32-bit library paths, for example:
4039 LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
4043 . Download the latest LTTng-tools{nbsp}{revision}, build, and install
4044 the 32-bit consumer daemon:
4050 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.8.tar.bz2 &&
4051 tar -xf lttng-tools-latest-2.8.tar.bz2 &&
4052 cd lttng-tools-2.8.* &&
4053 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
4054 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
4055 --disable-bin-lttng --disable-bin-lttng-crash \
4056 --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
4058 cd src/bin/lttng-consumerd &&
4059 sudo make install &&
4064 . From your distribution or from source,
4065 <<installing-lttng,install>> the 64-bit versions of
4066 LTTng-UST and Userspace RCU.
4067 . Download, build, and install the 64-bit version of the
4068 latest LTTng-tools{nbsp}{revision}:
4074 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.8.tar.bz2 &&
4075 tar -xf lttng-tools-latest-2.8.tar.bz2 &&
4076 cd lttng-tools-2.8.* &&
4077 ./configure --with-consumerd32-libdir=/usr/local/lib32 \
4078 --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
4080 sudo make install &&
4085 . Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
4086 when linking your 32-bit application:
4089 -m32 -L/usr/lib32 -L/usr/local/lib32 \
4090 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
4093 For example, let's rebuild the quick start example in
4094 <<tracing-your-own-user-application,Trace a user application>> as an
4095 instrumented 32-bit application:
4100 gcc -m32 -c -I. hello-tp.c
4102 gcc -m32 -o hello hello.o hello-tp.o \
4103 -L/usr/lib32 -L/usr/local/lib32 \
4104 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
4109 No special action is required to execute the 32-bit application and
4110 to trace it: use the command-line man:lttng(1) tool as usual.
4117 man:tracef(3) is a small LTTng-UST API designed for quick,
4118 man:printf(3)-like instrumentation without the burden of
4119 <<tracepoint-provider,creating>> and
4120 <<building-tracepoint-providers-and-user-application,building>>
4121 a tracepoint provider package.
4123 To use `tracef()` in your application:
4125 . In the C or C++ source files where you need to use `tracef()`,
4126 include `<lttng/tracef.h>`:
4131 #include <lttng/tracef.h>
4135 . In the application's source code, use `tracef()` like you would use
4143 tracef("my message: %d (%s)", my_integer, my_string);
4149 . Link your application with `liblttng-ust`:
4154 gcc -o app app.c -llttng-ust
4158 To trace the events that `tracef()` calls emit:
4160 * <<enabling-disabling-events,Create an event rule>> which matches the
4161 `lttng_ust_tracef:*` event name:
4166 lttng enable-event --userspace 'lttng_ust_tracef:*'
4171 .Limitations of `tracef()`
4173 The `tracef()` utility function was developed to make user space tracing
4174 super simple, albeit with notable disadvantages compared to
4175 <<defining-tracepoints,user-defined tracepoints>>:
4177 * All the emitted events have the same tracepoint provider and
4178 tracepoint names, respectively `lttng_ust_tracef` and `event`.
4179 * There is no static type checking.
4180 * The only event record field you actually get, named `msg`, is a string
4181 potentially containing the values you passed to `tracef()`
4182 using your own format string. This also means that you cannot filter
4183 events with a custom expression at run time because there are no
4185 * Since `tracef()` uses the C standard library's man:vasprintf(3)
4186 function behind the scenes to format the strings at run time, its
4187 expected performance is lower than with user-defined tracepoints,
4188 which do not require a conversion to a string.
4190 Taking this into consideration, `tracef()` is useful for some quick
4191 prototyping and debugging, but you should not consider it for any
4192 permanent and serious applicative instrumentation.
4198 ==== Use `tracelog()`
4200 The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
4201 the difference that it accepts an additional log level parameter.
4203 The goal of `tracelog()` is to ease the migration from logging to
4206 To use `tracelog()` in your application:
4208 . In the C or C++ source files where you need to use `tracelog()`,
4209 include `<lttng/tracelog.h>`:
4214 #include <lttng/tracelog.h>
4218 . In the application's source code, use `tracelog()` like you would use
4219 man:printf(3), except for the first parameter which is the log
4227 tracelog(TRACE_WARNING, "my message: %d (%s)",
4228 my_integer, my_string);
4234 See man:lttng-ust(3) for a list of available log level names.
4236 . Link your application with `liblttng-ust`:
4241 gcc -o app app.c -llttng-ust
4245 To trace the events that `tracelog()` calls emit with a log level
4246 _as severe as_ a specific log level:
4248 * <<enabling-disabling-events,Create an event rule>> which matches the
4249 `lttng_ust_tracelog:*` event name and a minimum level
4255 lttng enable-event --userspace 'lttng_ust_tracelog:*'
4256 --loglevel=TRACE_WARNING
4260 To trace the events that `tracelog()` calls emit with a
4261 _specific log level_:
4263 * Create an event rule which matches the `lttng_ust_tracelog:*`
4264 event name and a specific log level:
4269 lttng enable-event --userspace 'lttng_ust_tracelog:*'
4270 --loglevel-only=TRACE_INFO
4275 [[prebuilt-ust-helpers]]
4276 === Prebuilt user space tracing helpers
4278 The LTTng-UST package provides a few helpers in the form or preloadable
4279 shared objects which automatically instrument system functions and
4282 The helper shared objects are normally found in dir:{/usr/lib}. If you
4283 built LTTng-UST <<building-from-source,from source>>, they are probably
4284 located in dir:{/usr/local/lib}.
4286 The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
4289 path:{liblttng-ust-libc-wrapper.so}::
4290 path:{liblttng-ust-pthread-wrapper.so}::
4291 <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
4292 memory and POSIX threads function tracing>>.
4294 path:{liblttng-ust-cyg-profile.so}::
4295 path:{liblttng-ust-cyg-profile-fast.so}::
4296 <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
4298 path:{liblttng-ust-dl.so}::
4299 <<liblttng-ust-dl,Dynamic linker tracing>>.
4301 To use a user space tracing helper with any user application:
4303 * Preload the helper shared object when you start the application:
4308 LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
4312 You can preload more than one helper:
4317 LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
4323 [[liblttng-ust-libc-pthread-wrapper]]
4324 ==== Instrument C standard library memory and POSIX threads functions
4326 The path:{liblttng-ust-libc-wrapper.so} and
4327 path:{liblttng-ust-pthread-wrapper.so} helpers
4328 add instrumentation to some C standard library and POSIX
4332 .Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
4334 |TP provider name |TP name |Instrumented function
4336 .6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
4337 |`calloc` |man:calloc(3)
4338 |`realloc` |man:realloc(3)
4339 |`free` |man:free(3)
4340 |`memalign` |man:memalign(3)
4341 |`posix_memalign` |man:posix_memalign(3)
4345 .Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
4347 |TP provider name |TP name |Instrumented function
4349 .4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
4350 |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
4351 |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
4352 |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
4355 When you preload the shared object, it replaces the functions listed
4356 in the previous tables by wrappers which contain tracepoints and call
4357 the replaced functions.
4360 [[liblttng-ust-cyg-profile]]
4361 ==== Instrument function entry and exit
4363 The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
4364 to the entry and exit points of functions.
4366 man:gcc(1) and man:clang(1) have an option named
4367 https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
4368 which generates instrumentation calls for entry and exit to functions.
4369 The LTTng-UST function tracing helpers,
4370 path:{liblttng-ust-cyg-profile.so} and
4371 path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
4372 to add tracepoints to the two generated functions (which contain
4373 `cyg_profile` in their names, hence the helper's name).
4375 To use the LTTng-UST function tracing helper, the source files to
4376 instrument must be built using the `-finstrument-functions` compiler
4379 There are two versions of the LTTng-UST function tracing helper:
4381 * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
4382 that you should only use when it can be _guaranteed_ that the
4383 complete event stream is recorded without any lost event record.
4384 Any kind of duplicate information is left out.
4386 Assuming no event record is lost, having only the function addresses on
4387 entry is enough to create a call graph, since an event record always
4388 contains the ID of the CPU that generated it.
4390 You can use a tool like man:addr2line(1) to convert function addresses
4391 back to source file names and line numbers.
4393 * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
4394 which also works in use cases where event records might get discarded or
4395 not recorded from application startup.
4396 In these cases, the trace analyzer needs more information to be
4397 able to reconstruct the program flow.
4399 See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
4400 points of this helper.
4402 All the tracepoints that this helper provides have the
4403 log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
4405 TIP: It's sometimes a good idea to limit the number of source files that
4406 you compile with the `-finstrument-functions` option to prevent LTTng
4407 from writing an excessive amount of trace data at run time. When using
4408 man:gcc(1), you can use the
4409 `-finstrument-functions-exclude-function-list` option to avoid
4410 instrument entries and exits of specific function names.
4415 ==== Instrument the dynamic linker
4417 The path:{liblttng-ust-dl.so} helper adds instrumentation to the
4418 man:dlopen(3) and man:dlclose(3) function calls.
4420 See man:lttng-ust-dl(3) to learn more about the instrumentation points
4425 [[java-application]]
4426 === User space Java agent
4428 You can instrument any Java application which uses one of the following
4431 * The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
4432 (JUL) core logging facilities.
4433 * http://logging.apache.org/log4j/1.2/[**Apache log4j 1.2**], since
4434 LTTng 2.6. Note that Apache Log4j{nbsp}2 is not supported.
4437 .LTTng-UST Java agent imported by a Java application.
4438 image::java-app.png[]
4440 Note that the methods described below are new in LTTng{nbsp}{revision}.
4441 Previous LTTng versions use another technique.
4443 NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
4444 and https://ci.lttng.org/[continuous integration], thus this version is
4445 directly supported. However, the LTTng-UST Java agent is also tested
4446 with OpenJDK{nbsp}7.
4451 ==== Use the LTTng-UST Java agent for `java.util.logging`
4453 To use the LTTng-UST Java agent in a Java application which uses
4454 `java.util.logging` (JUL):
4456 . In the Java application's source code, import the LTTng-UST
4457 log handler package for `java.util.logging`:
4462 import org.lttng.ust.agent.jul.LttngLogHandler;
4466 . Create an LTTng-UST JUL log handler:
4471 Handler lttngUstLogHandler = new LttngLogHandler();
4475 . Add this handler to the JUL loggers which should emit LTTng events:
4480 Logger myLogger = Logger.getLogger("some-logger");
4482 myLogger.addHandler(lttngUstLogHandler);
4486 . Use `java.util.logging` log statements and configuration as usual.
4487 The loggers with an attached LTTng-UST log handler can emit
4490 . Before exiting the application, remove the LTTng-UST log handler from
4491 the loggers attached to it and call its `close()` method:
4496 myLogger.removeHandler(lttngUstLogHandler);
4497 lttngUstLogHandler.close();
4501 This is not strictly necessary, but it is recommended for a clean
4502 disposal of the handler's resources.
4504 . Include the LTTng-UST Java agent's common and JUL-specific JAR files,
4505 path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
4507 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4508 path] when you build the Java application.
4510 The JAR files are typically located in dir:{/usr/share/java}.
4512 IMPORTANT: The LTTng-UST Java agent must be
4513 <<installing-lttng,installed>> for the logging framework your
4516 .Use the LTTng-UST Java agent for `java.util.logging`.
4521 import java.io.IOException;
4522 import java.util.logging.Handler;
4523 import java.util.logging.Logger;
4524 import org.lttng.ust.agent.jul.LttngLogHandler;
4528 private static final int answer = 42;
4530 public static void main(String[] argv) throws Exception
4533 Logger logger = Logger.getLogger("jello");
4535 // Create an LTTng-UST log handler
4536 Handler lttngUstLogHandler = new LttngLogHandler();
4538 // Add the LTTng-UST log handler to our logger
4539 logger.addHandler(lttngUstLogHandler);
4542 logger.info("some info");
4543 logger.warning("some warning");
4545 logger.finer("finer information; the answer is " + answer);
4547 logger.severe("error!");
4549 // Not mandatory, but cleaner
4550 logger.removeHandler(lttngUstLogHandler);
4551 lttngUstLogHandler.close();
4560 javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4563 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4564 <<enabling-disabling-events,create an event rule>> matching the
4565 `jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
4570 lttng enable-event --jul jello
4574 Run the compiled class:
4578 java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4581 <<basic-tracing-session-control,Stop tracing>> and inspect the
4591 In the resulting trace, an <<event,event record>> generated by a Java
4592 application using `java.util.logging` is named `lttng_jul:event` and
4593 has the following fields:
4596 Log record's message.
4602 Name of the class in which the log statement was executed.
4605 Name of the method in which the log statement was executed.
4608 Logging time (timestamp in milliseconds).
4611 Log level integer value.
4614 ID of the thread in which the log statement was executed.
4616 You can use the opt:lttng-enable-event(1):--loglevel or
4617 opt:lttng-enable-event(1):--loglevel-only option of the
4618 man:lttng-enable-event(1) command to target a range of JUL log levels
4619 or a specific JUL log level.
4624 ==== Use the LTTng-UST Java agent for Apache log4j
4626 To use the LTTng-UST Java agent in a Java application which uses
4629 . In the Java application's source code, import the LTTng-UST
4630 log appender package for Apache log4j:
4635 import org.lttng.ust.agent.log4j.LttngLogAppender;
4639 . Create an LTTng-UST log4j log appender:
4644 Appender lttngUstLogAppender = new LttngLogAppender();
4648 . Add this appender to the log4j loggers which should emit LTTng events:
4653 Logger myLogger = Logger.getLogger("some-logger");
4655 myLogger.addAppender(lttngUstLogAppender);
4659 . Use Apache log4j log statements and configuration as usual. The
4660 loggers with an attached LTTng-UST log appender can emit LTTng events.
4662 . Before exiting the application, remove the LTTng-UST log appender from
4663 the loggers attached to it and call its `close()` method:
4668 myLogger.removeAppender(lttngUstLogAppender);
4669 lttngUstLogAppender.close();
4673 This is not strictly necessary, but it is recommended for a clean
4674 disposal of the appender's resources.
4676 . Include the LTTng-UST Java agent's common and log4j-specific JAR
4677 files, path:{lttng-ust-agent-common.jar} and
4678 path:{lttng-ust-agent-log4j.jar}, in the
4679 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4680 path] when you build the Java application.
4682 The JAR files are typically located in dir:{/usr/share/java}.
4684 IMPORTANT: The LTTng-UST Java agent must be
4685 <<installing-lttng,installed>> for the logging framework your
4688 .Use the LTTng-UST Java agent for Apache log4j.
4693 import org.apache.log4j.Appender;
4694 import org.apache.log4j.Logger;
4695 import org.lttng.ust.agent.log4j.LttngLogAppender;
4699 private static final int answer = 42;
4701 public static void main(String[] argv) throws Exception
4704 Logger logger = Logger.getLogger("jello");
4706 // Create an LTTng-UST log appender
4707 Appender lttngUstLogAppender = new LttngLogAppender();
4709 // Add the LTTng-UST log appender to our logger
4710 logger.addAppender(lttngUstLogAppender);
4713 logger.info("some info");
4714 logger.warn("some warning");
4716 logger.debug("debug information; the answer is " + answer);
4718 logger.fatal("error!");
4720 // Not mandatory, but cleaner
4721 logger.removeAppender(lttngUstLogAppender);
4722 lttngUstLogAppender.close();
4728 Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
4733 javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
4736 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4737 <<enabling-disabling-events,create an event rule>> matching the
4738 `jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
4743 lttng enable-event --log4j jello
4747 Run the compiled class:
4751 java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
4754 <<basic-tracing-session-control,Stop tracing>> and inspect the
4764 In the resulting trace, an <<event,event record>> generated by a Java
4765 application using log4j is named `lttng_log4j:event` and
4766 has the following fields:
4769 Log record's message.
4775 Name of the class in which the log statement was executed.
4778 Name of the method in which the log statement was executed.
4781 Name of the file in which the executed log statement is located.
4784 Line number at which the log statement was executed.
4790 Log level integer value.
4793 Name of the Java thread in which the log statement was executed.
4795 You can use the opt:lttng-enable-event(1):--loglevel or
4796 opt:lttng-enable-event(1):--loglevel-only option of the
4797 man:lttng-enable-event(1) command to target a range of Apache log4j log levels
4798 or a specific log4j log level.
4802 [[java-application-context]]
4803 ==== Provide application-specific context fields in a Java application
4805 A Java application-specific context field is a piece of state provided
4806 by the application which <<adding-context,you can add>>, using the
4807 man:lttng-add-context(1) command, to each <<event,event record>>
4808 produced by the log statements of this application.
4810 For example, a given object might have a current request ID variable.
4811 You can create a context information retriever for this object and
4812 assign a name to this current request ID. You can then, using the
4813 man:lttng-add-context(1) command, add this context field by name to
4814 the JUL or log4j <<channel,channel>>.
4816 To provide application-specific context fields in a Java application:
4818 . In the Java application's source code, import the LTTng-UST
4819 Java agent context classes and interfaces:
4824 import org.lttng.ust.agent.context.ContextInfoManager;
4825 import org.lttng.ust.agent.context.IContextInfoRetriever;
4829 . Create a context information retriever class, that is, a class which
4830 implements the `IContextInfoRetriever` interface:
4835 class MyContextInfoRetriever implements IContextInfoRetriever
4838 public Object retrieveContextInfo(String key)
4840 if (key.equals("intCtx")) {
4842 } else if (key.equals("strContext")) {
4843 return "context value!";
4852 This `retrieveContextInfo()` method is the only member of the
4853 `IContextInfoRetriever` interface. Its role is to return the current
4854 value of a state by name to create a context field. The names of the
4855 context fields and which state variables they return depends on your
4858 All primitive types and objects are supported as context fields.
4859 When `retrieveContextInfo()` returns an object, the context field
4860 serializer calls its `toString()` method to add a string field to
4861 event records. The method can also return `null`, which means that
4862 no context field is available for the required name.
4864 . Register an instance of your context information retriever class to
4865 the context information manager singleton:
4870 IContextInfoRetriever cir = new MyContextInfoRetriever();
4871 ContextInfoManager cim = ContextInfoManager.getInstance();
4872 cim.registerContextInfoRetriever("retrieverName", cir);
4876 . Before exiting the application, remove your context information
4877 retriever from the context information manager singleton:
4882 ContextInfoManager cim = ContextInfoManager.getInstance();
4883 cim.unregisterContextInfoRetriever("retrieverName");
4887 This is not strictly necessary, but it is recommended for a clean
4888 disposal of some manager's resources.
4890 . Build your Java application with LTTng-UST Java agent support as
4891 usual, following the procedure for either the <<jul,JUL>> or
4892 <<log4j,Apache log4j>> framework.
4895 .Provide application-specific context fields in a Java application.
4900 import java.util.logging.Handler;
4901 import java.util.logging.Logger;
4902 import org.lttng.ust.agent.jul.LttngLogHandler;
4903 import org.lttng.ust.agent.context.ContextInfoManager;
4904 import org.lttng.ust.agent.context.IContextInfoRetriever;
4908 // Our context information retriever class
4909 private static class MyContextInfoRetriever
4910 implements IContextInfoRetriever
4913 public Object retrieveContextInfo(String key) {
4914 if (key.equals("intCtx")) {
4916 } else if (key.equals("strContext")) {
4917 return "context value!";
4924 private static final int answer = 42;
4926 public static void main(String args[]) throws Exception
4928 // Get the context information manager instance
4929 ContextInfoManager cim = ContextInfoManager.getInstance();
4931 // Create and register our context information retriever
4932 IContextInfoRetriever cir = new MyContextInfoRetriever();
4933 cim.registerContextInfoRetriever("myRetriever", cir);
4936 Logger logger = Logger.getLogger("jello");
4938 // Create an LTTng-UST log handler
4939 Handler lttngUstLogHandler = new LttngLogHandler();
4941 // Add the LTTng-UST log handler to our logger
4942 logger.addHandler(lttngUstLogHandler);
4945 logger.info("some info");
4946 logger.warning("some warning");
4948 logger.finer("finer information; the answer is " + answer);
4950 logger.severe("error!");
4952 // Not mandatory, but cleaner
4953 logger.removeHandler(lttngUstLogHandler);
4954 lttngUstLogHandler.close();
4955 cim.unregisterContextInfoRetriever("myRetriever");
4964 javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4967 <<creating-destroying-tracing-sessions,Create a tracing session>>
4968 and <<enabling-disabling-events,create an event rule>> matching the
4974 lttng enable-event --jul jello
4977 <<adding-context,Add the application-specific context fields>> to the
4982 lttng add-context --jul --type='$app.myRetriever:intCtx'
4983 lttng add-context --jul --type='$app.myRetriever:strContext'
4986 <<basic-tracing-session-control,Start tracing>>:
4993 Run the compiled class:
4997 java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
5000 <<basic-tracing-session-control,Stop tracing>> and inspect the
5012 [[python-application]]
5013 === User space Python agent
5015 You can instrument a Python 2 or Python 3 application which uses the
5016 standard https://docs.python.org/3/library/logging.html[`logging`]
5019 Each log statement emits an LTTng event once the
5020 application module imports the
5021 <<lttng-ust-agents,LTTng-UST Python agent>> package.
5024 .A Python application importing the LTTng-UST Python agent.
5025 image::python-app.png[]
5027 To use the LTTng-UST Python agent:
5029 . In the Python application's source code, import the LTTng-UST Python
5039 The LTTng-UST Python agent automatically adds its logging handler to the
5040 root logger at import time.
5042 Any log statement that the application executes before this import does
5043 not emit an LTTng event.
5045 IMPORTANT: The LTTng-UST Python agent must be
5046 <<installing-lttng,installed>>.
5048 . Use log statements and logging configuration as usual.
5049 Since the LTTng-UST Python agent adds a handler to the _root_
5050 logger, you can trace any log statement from any logger.
5052 .Use the LTTng-UST Python agent.
5063 logging.basicConfig()
5064 logger = logging.getLogger('my-logger')
5067 logger.debug('debug message')
5068 logger.info('info message')
5069 logger.warn('warn message')
5070 logger.error('error message')
5071 logger.critical('critical message')
5075 if __name__ == '__main__':
5079 NOTE: `logging.basicConfig()`, which adds to the root logger a basic
5080 logging handler which prints to the standard error stream, is not
5081 strictly required for LTTng-UST tracing to work, but in versions of
5082 Python preceding 3.2, you could see a warning message which indicates
5083 that no handler exists for the logger `my-logger`.
5085 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5086 <<enabling-disabling-events,create an event rule>> matching the
5087 `my-logger` Python logger, and <<basic-tracing-session-control,start
5093 lttng enable-event --python my-logger
5097 Run the Python script:
5104 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5114 In the resulting trace, an <<event,event record>> generated by a Python
5115 application is named `lttng_python:event` and has the following fields:
5118 Logging time (string).
5121 Log record's message.
5127 Name of the function in which the log statement was executed.
5130 Line number at which the log statement was executed.
5133 Log level integer value.
5136 ID of the Python thread in which the log statement was executed.
5139 Name of the Python thread in which the log statement was executed.
5141 You can use the opt:lttng-enable-event(1):--loglevel or
5142 opt:lttng-enable-event(1):--loglevel-only option of the
5143 man:lttng-enable-event(1) command to target a range of Python log levels
5144 or a specific Python log level.
5146 When an application imports the LTTng-UST Python agent, the agent tries
5147 to register to a <<lttng-sessiond,session daemon>>. Note that you must
5148 <<start-sessiond,start the session daemon>> _before_ you run the Python
5149 application. If a session daemon is found, the agent tries to register
5150 to it during 5{nbsp}seconds, after which the application continues
5151 without LTTng tracing support. You can override this timeout value with
5152 the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
5155 If the session daemon stops while a Python application with an imported
5156 LTTng-UST Python agent runs, the agent retries to connect and to
5157 register to a session daemon every 3{nbsp}seconds. You can override this
5158 delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
5163 [[proc-lttng-logger-abi]]
5166 The `lttng-tracer` Linux kernel module, part of
5167 <<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
5168 path:{/proc/lttng-logger} when it's loaded. Any application can write
5169 text data to this file to emit an LTTng event.
5172 .An application writes to the LTTng logger file to emit an LTTng event.
5173 image::lttng-logger.png[]
5175 The LTTng logger is the quickest method--not the most efficient,
5176 however--to add instrumentation to an application. It is designed
5177 mostly to instrument shell scripts:
5181 echo "Some message, some $variable" > /proc/lttng-logger
5184 Any event that the LTTng logger emits is named `lttng_logger` and
5185 belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
5186 other instrumentation points in the kernel tracing domain, **any Unix
5187 user** can <<enabling-disabling-events,create an event rule>> which
5188 matches its event name, not only the root user or users in the
5189 <<tracing-group,tracing group>>.
5191 To use the LTTng logger:
5193 * From any application, write text data to the path:{/proc/lttng-logger}
5196 The `msg` field of `lttng_logger` event records contains the
5199 NOTE: The maximum message length of an LTTng logger event is
5200 1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
5201 than one event to contain the remaining data.
5203 You should not use the LTTng logger to trace a user application which
5204 can be instrumented in a more efficient way, namely:
5206 * <<c-application,C and $$C++$$ applications>>.
5207 * <<java-application,Java applications>>.
5208 * <<python-application,Python applications>>.
5210 .Use the LTTng logger.
5215 echo 'Hello, World!' > /proc/lttng-logger
5217 df --human-readable --print-type / > /proc/lttng-logger
5220 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5221 <<enabling-disabling-events,create an event rule>> matching the
5222 `lttng_logger` Linux kernel tracepoint, and
5223 <<basic-tracing-session-control,start tracing>>:
5228 lttng enable-event --kernel lttng_logger
5232 Run the Bash script:
5239 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5250 [[instrumenting-linux-kernel]]
5251 === LTTng kernel tracepoints
5253 NOTE: This section shows how to _add_ instrumentation points to the
5254 Linux kernel. The kernel's subsystems are already thoroughly
5255 instrumented at strategic places for LTTng when you
5256 <<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
5260 There are two methods to instrument the Linux kernel:
5262 . <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
5263 tracepoint which uses the `TRACE_EVENT()` API.
5265 Choose this if you want to instrumentation a Linux kernel tree with an
5266 instrumentation point compatible with ftrace, perf, and SystemTap.
5268 . Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
5269 instrument an out-of-tree kernel module.
5271 Choose this if you don't need ftrace, perf, or SystemTap support.
5275 [[linux-add-lttng-layer]]
5276 ==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
5278 This section shows how to add an LTTng layer to existing ftrace
5279 instrumentation using the `TRACE_EVENT()` API.
5281 This section does not document the `TRACE_EVENT()` macro. You can
5282 read the following articles to learn more about this API:
5284 * http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part 1)]
5285 * http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part 2)]
5286 * http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part 3)]
5288 The following procedure assumes that your ftrace tracepoints are
5289 correctly defined in their own header and that they are created in
5290 one source file using the `CREATE_TRACE_POINTS` definition.
5292 To add an LTTng layer over an existing ftrace tracepoint:
5294 . Make sure the following kernel configuration options are
5300 * `CONFIG_HIGH_RES_TIMERS`
5301 * `CONFIG_TRACEPOINTS`
5304 . Build the Linux source tree with your custom ftrace tracepoints.
5305 . Boot the resulting Linux image on your target system.
5307 Confirm that the tracepoints exist by looking for their names in the
5308 dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
5309 is your subsystem's name.
5311 . Get a copy of the latest LTTng-modules{nbsp}{revision}:
5317 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
5318 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
5319 cd lttng-modules-2.8.*
5323 . In dir:{instrumentation/events/lttng-module}, relative to the root
5324 of the LTTng-modules source tree, create a header file named
5325 +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
5326 LTTng-modules tracepoint definitions using the LTTng-modules
5329 Start with this template:
5333 .path:{instrumentation/events/lttng-module/my_subsys.h}
5336 #define TRACE_SYSTEM my_subsys
5338 #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
5339 #define _LTTNG_MY_SUBSYS_H
5341 #include "../../../probes/lttng-tracepoint-event.h"
5342 #include <linux/tracepoint.h>
5344 LTTNG_TRACEPOINT_EVENT(
5346 * Format is identical to TRACE_EVENT()'s version for the three
5347 * following macro parameters:
5350 TP_PROTO(int my_int, const char *my_string),
5351 TP_ARGS(my_int, my_string),
5353 /* LTTng-modules specific macros */
5355 ctf_integer(int, my_int_field, my_int)
5356 ctf_string(my_bar_field, my_bar)
5360 #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
5362 #include "../../../probes/define_trace.h"
5366 The entries in the `TP_FIELDS()` section are the list of fields for the
5367 LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
5368 ftrace's `TRACE_EVENT()` macro.
5370 See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
5371 complete description of the available `ctf_*()` macros.
5373 . Create the LTTng-modules probe's kernel module C source file,
5374 +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
5379 .path:{probes/lttng-probe-my-subsys.c}
5381 #include <linux/module.h>
5382 #include "../lttng-tracer.h"
5385 * Build-time verification of mismatch between mainline
5386 * TRACE_EVENT() arguments and the LTTng-modules adaptation
5387 * layer LTTNG_TRACEPOINT_EVENT() arguments.
5389 #include <trace/events/my_subsys.h>
5391 /* Create LTTng tracepoint probes */
5392 #define LTTNG_PACKAGE_BUILD
5393 #define CREATE_TRACE_POINTS
5394 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
5396 #include "../instrumentation/events/lttng-module/my_subsys.h"
5398 MODULE_LICENSE("GPL and additional rights");
5399 MODULE_AUTHOR("Your name <your-email>");
5400 MODULE_DESCRIPTION("LTTng my_subsys probes");
5401 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
5402 __stringify(LTTNG_MODULES_MINOR_VERSION) "."
5403 __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
5404 LTTNG_MODULES_EXTRAVERSION);
5408 . Edit path:{probes/Makefile} and add your new kernel module object
5409 next to the existing ones:
5413 .path:{probes/Makefile}
5417 obj-m += lttng-probe-module.o
5418 obj-m += lttng-probe-power.o
5420 obj-m += lttng-probe-my-subsys.o
5426 . Build and install the LTTng kernel modules:
5431 make KERNELDIR=/path/to/linux
5432 sudo make modules_install
5436 Replace `/path/to/linux` with the path to the Linux source tree where
5437 you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro.
5439 Note that you can also use the
5440 <<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
5441 instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
5442 C code that need to be executed before the event fields are recorded.
5444 The best way to learn how to use the previous LTTng-modules macros is to
5445 inspect the existing LTTng-modules tracepoint definitions in the
5446 dir:{instrumentation/events/lttng-module} header files. Compare them
5447 with the Linux kernel mainline versions in the
5448 dir:{include/trace/events} directory of the Linux source tree.
5452 [[lttng-tracepoint-event-code]]
5453 ===== Use custom C code to access the data for tracepoint fields
5455 Although we recommended to always use the
5456 <<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
5457 the arguments and fields of an LTTng-modules tracepoint when possible,
5458 sometimes you need a more complex process to access the data that the
5459 tracer records as event record fields. In other words, you need local
5460 variables and multiple C{nbsp}statements instead of simple
5461 argument-based expressions that you pass to the
5462 <<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
5464 You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
5465 `LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
5466 a block of C{nbsp}code to be executed before LTTng records the fields.
5467 The structure of this macro is:
5470 .`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
5472 LTTNG_TRACEPOINT_EVENT_CODE(
5474 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5475 * version for the following three macro parameters:
5478 TP_PROTO(int my_int, const char *my_string),
5479 TP_ARGS(my_int, my_string),
5481 /* Declarations of custom local variables */
5484 unsigned long b = 0;
5485 const char *name = "(undefined)";
5486 struct my_struct *my_struct;
5490 * Custom code which uses both tracepoint arguments
5491 * (in TP_ARGS()) and local variables (in TP_locvar()).
5493 * Local variables are actually members of a structure pointed
5494 * to by the special variable tp_locvar.
5498 tp_locvar->a = my_int + 17;
5499 tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
5500 tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
5501 tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
5502 put_my_struct(tp_locvar->my_struct);
5511 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5512 * version for this, except that tp_locvar members can be
5513 * used in the argument expression parameters of
5514 * the ctf_*() macros.
5517 ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
5518 ctf_integer(int, my_struct_a, tp_locvar->a)
5519 ctf_string(my_string_field, my_string)
5520 ctf_string(my_struct_name, tp_locvar->name)
5525 IMPORTANT: The C code defined in `TP_code()` must not have any side
5526 effects when executed. In particular, the code must not allocate
5527 memory or get resources without deallocating this memory or putting
5528 those resources afterwards.
5531 [[instrumenting-linux-kernel-tracing]]
5532 ==== Load and unload a custom probe kernel module
5534 You must load a <<lttng-adaptation-layer,created LTTng-modules probe
5535 kernel module>> in the kernel before it can emit LTTng events.
5537 To load the default probe kernel modules and a custom probe kernel
5540 * Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
5541 probe modules to load when starting a root <<lttng-sessiond,session
5545 .Load the `my_subsys`, `usb`, and the default probe modules.
5549 sudo lttng-sessiond --extra-kmod-probes=my_subsys,usb
5554 You only need to pass the subsystem name, not the whole kernel module
5557 To load _only_ a given custom probe kernel module:
5559 * Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
5560 modules to load when starting a root session daemon:
5563 .Load only the `my_subsys` and `usb` probe modules.
5567 sudo lttng-sessiond --kmod-probes=my_subsys,usb
5572 To confirm that a probe module is loaded:
5579 lsmod | grep lttng_probe_usb
5583 To unload the loaded probe modules:
5585 * Kill the session daemon with `SIGTERM`:
5590 sudo pkill lttng-sessiond
5594 You can also use man:modprobe(8)'s `--remove` option if the session
5595 daemon terminates abnormally.
5598 [[controlling-tracing]]
5601 Once an application or a Linux kernel is
5602 <<instrumenting,instrumented>> for LTTng tracing,
5605 This section is divided in topics on how to use the various
5606 <<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
5607 command-line tool>>, to _control_ the LTTng daemons and tracers.
5609 NOTE: In the following subsections, we refer to an man:lttng(1) command
5610 using its man page name. For example, instead of _Run the `create`
5611 command to..._, we use _Run the man:lttng-create(1) command to..._.
5615 === Start a session daemon
5617 In some situations, you need to run a <<lttng-sessiond,session daemon>>
5618 (man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
5621 You will see the following error when you run a command while no session
5625 Error: No session daemon is available
5628 The only command that automatically runs a session daemon is
5629 man:lttng-create(1), which you use to
5630 <<creating-destroying-tracing-sessions,create a tracing session>>. While
5631 this is most of the time the first operation that you do, sometimes it's
5632 not. Some examples are:
5634 * <<list-instrumentation-points,List the available instrumentation points>>.
5635 * <<saving-loading-tracing-session,Load a tracing session configuration>>.
5637 [[tracing-group]] Each Unix user must have its own running session
5638 daemon to trace user applications. The session daemon that the root user
5639 starts is the only one allowed to control the LTTng kernel tracer. Users
5640 that are part of the _tracing group_ can control the root session
5641 daemon. The default tracing group name is `tracing`; you can set it to
5642 something else with the opt:lttng-sessiond(8):--group option when you
5643 start the root session daemon.
5645 To start a user session daemon:
5647 * Run man:lttng-sessiond(8):
5652 lttng-sessiond --daemonize
5656 To start the root session daemon:
5658 * Run man:lttng-sessiond(8) as the root user:
5663 sudo lttng-sessiond --daemonize
5667 In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
5668 start the session daemon in foreground.
5670 To stop a session daemon, use man:kill(1) on its process ID (standard
5673 Note that some Linux distributions could manage the LTTng session daemon
5674 as a service. In this case, you should use the service manager to
5675 start, restart, and stop session daemons.
5678 [[creating-destroying-tracing-sessions]]
5679 === Create and destroy a tracing session
5681 Almost all the LTTng control operations happen in the scope of
5682 a <<tracing-session,tracing session>>, which is the dialogue between the
5683 <<lttng-sessiond,session daemon>> and you.
5685 To create a tracing session with a generated name:
5687 * Use the man:lttng-create(1) command:
5696 The created tracing session's name is `auto` followed by the
5699 To create a tracing session with a specific name:
5701 * Use the optional argument of the man:lttng-create(1) command:
5706 lttng create my-session
5710 Replace `my-session` with the specific tracing session name.
5712 LTTng appends the creation date to the created tracing session's name.
5714 LTTng writes the traces of a tracing session in
5715 +$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
5716 name of the tracing session. Note that the env:LTTNG_HOME environment
5717 variable defaults to `$HOME` if not set.
5719 To output LTTng traces to a non-default location:
5721 * Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
5726 lttng create my-session --output=/tmp/some-directory
5730 You may create as many tracing sessions as you wish.
5732 To list all the existing tracing sessions for your Unix user:
5734 * Use the man:lttng-list(1) command:
5743 When you create a tracing session, it is set as the _current tracing
5744 session_. The following man:lttng(1) commands operate on the current
5745 tracing session when you don't specify one:
5747 [role="list-3-cols"]
5763 To change the current tracing session:
5765 * Use the man:lttng-set-session(1) command:
5770 lttng set-session new-session
5774 Replace `new-session` by the name of the new current tracing session.
5776 When you are done tracing in a given tracing session, you can destroy
5777 it. This operation frees the resources taken by the tracing session
5778 to destroy; it does not destroy the trace data that LTTng wrote for
5779 this tracing session.
5781 To destroy the current tracing session:
5783 * Use the man:lttng-destroy(1) command:
5793 [[list-instrumentation-points]]
5794 === List the available instrumentation points
5796 The <<lttng-sessiond,session daemon>> can query the running instrumented
5797 user applications and the Linux kernel to get a list of available
5798 instrumentation points. For the Linux kernel <<domain,tracing domain>>,
5799 they are tracepoints and system calls. For the user space tracing
5800 domain, they are tracepoints. For the other tracing domains, they are
5803 To list the available instrumentation points:
5805 * Use the man:lttng-list(1) command with the requested tracing domain's
5809 * opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
5810 must be a root user, or it must be a member of the
5811 <<tracing-group,tracing group>>).
5812 * opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
5813 kernel system calls (your Unix user must be a root user, or it must be
5814 a member of the tracing group).
5815 * opt:lttng-list(1):--userspace: user space tracepoints.
5816 * opt:lttng-list(1):--jul: `java.util.logging` loggers.
5817 * opt:lttng-list(1):--log4j: Apache log4j loggers.
5818 * opt:lttng-list(1):--python: Python loggers.
5821 .List the available user space tracepoints.
5825 lttng list --userspace
5829 .List the available Linux kernel system call tracepoints.
5833 lttng list --kernel --syscall
5838 [[enabling-disabling-events]]
5839 === Create and enable an event rule
5841 Once you <<creating-destroying-tracing-sessions,create a tracing
5842 session>>, you can create <<event,event rules>> with the
5843 man:lttng-enable-event(1) command.
5845 You specify each condition with a command-line option. The available
5846 condition options are shown in the following table.
5848 [role="growable",cols="asciidoc,asciidoc,default"]
5849 .Condition command-line options for the man:lttng-enable-event(1) command.
5851 |Option |Description |Applicable tracing domains
5857 . +--probe=__ADDR__+
5858 . +--function=__ADDR__+
5861 Instead of using the default _tracepoint_ instrumentation type, use:
5863 . A Linux system call.
5864 . A Linux https://lwn.net/Articles/132196/[KProbe] (symbol or address).
5865 . The entry and return points of a Linux function (symbol or address).
5869 |First positional argument.
5872 Tracepoint or system call name. In the case of a Linux KProbe or
5873 function, this is a custom name given to the event rule. With the
5874 JUL, log4j, and Python domains, this is a logger name.
5876 With a tracepoint, logger, or system call name, the last character
5877 can be `*` to match anything that remains.
5884 . +--loglevel=__LEVEL__+
5885 . +--loglevel-only=__LEVEL__+
5888 . Match only tracepoints or log statements with a logging level at
5889 least as severe as +__LEVEL__+.
5890 . Match only tracepoints or log statements with a logging level
5891 equal to +__LEVEL__+.
5893 See man:lttng-enable-event(1) for the list of available logging level
5896 |User space, JUL, log4j, and Python.
5898 |+--exclude=__EXCLUSIONS__+
5901 When you use a `*` character at the end of the tracepoint or logger
5902 name (first positional argument), exclude the specific names in the
5903 comma-delimited list +__EXCLUSIONS__+.
5906 User space, JUL, log4j, and Python.
5908 |+--filter=__EXPR__+
5911 Match only events which satisfy the expression +__EXPR__+.
5913 See man:lttng-enable-event(1) to learn more about the syntax of a
5920 You attach an event rule to a <<channel,channel>> on creation. If you do
5921 not specify the channel with the opt:lttng-enable-event(1):--channel
5922 option, and if the event rule to create is the first in its
5923 <<domain,tracing domain>> for a given tracing session, then LTTng
5924 creates a _default channel_ for you. This default channel is reused in
5925 subsequent invocations of the man:lttng-enable-event(1) command for the
5926 same tracing domain.
5928 An event rule is always enabled at creation time.
5930 The following examples show how you can combine the previous
5931 command-line options to create simple to more complex event rules.
5933 .Create an event rule targetting a Linux kernel tracepoint (default channel).
5937 lttng enable-event --kernel sched_switch
5941 .Create an event rule matching four Linux kernel system calls (default channel).
5945 lttng enable-event --kernel --syscall open,write,read,close
5949 .Create event rules matching tracepoints with filter expressions (default channel).
5953 lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
5958 lttng enable-event --kernel --all \
5959 --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
5964 lttng enable-event --jul my_logger \
5965 --filter='$app.retriever:cur_msg_id > 3'
5968 IMPORTANT: Make sure to always quote the filter string when you
5969 use man:lttng(1) from a shell.
5972 .Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
5976 lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
5979 IMPORTANT: Make sure to always quote the wildcard character when you
5980 use man:lttng(1) from a shell.
5983 .Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
5987 lttng enable-event --python my-app.'*' \
5988 --exclude='my-app.module,my-app.hello'
5992 .Create an event rule matching any Apache log4j logger with a specific log level (default channel).
5996 lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
6000 .Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
6004 lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
6008 The event rules of a given channel form a whitelist: as soon as an
6009 emitted event passes one of them, LTTng can record the event. For
6010 example, an event named `my_app:my_tracepoint` emitted from a user space
6011 tracepoint with a `TRACE_ERROR` log level passes both of the following
6016 lttng enable-event --userspace my_app:my_tracepoint
6017 lttng enable-event --userspace my_app:my_tracepoint \
6018 --loglevel=TRACE_INFO
6021 The second event rule is redundant: the first one includes
6025 [[disable-event-rule]]
6026 === Disable an event rule
6028 To disable an event rule that you <<enabling-disabling-events,created>>
6029 previously, use the man:lttng-disable-event(1) command. This command
6030 disables _all_ the event rules (of a given tracing domain and channel)
6031 which match an instrumentation point. The other conditions are not
6032 supported as of LTTng{nbsp}{revision}.
6034 The LTTng tracer does not record an emitted event which passes
6035 a _disabled_ event rule.
6037 .Disable an event rule matching a Python logger (default channel).
6041 lttng disable-event --python my-logger
6045 .Disable an event rule matching all `java.util.logging` loggers (default channel).
6049 lttng disable-event --jul '*'
6053 .Disable _all_ the event rules of the default channel.
6055 The opt:lttng-disable-event(1):--all-events option is not, like the
6056 opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
6057 equivalent of the event name `*` (wildcard): it disables _all_ the event
6058 rules of a given channel.
6062 lttng disable-event --jul --all-events
6066 NOTE: You cannot delete an event rule once you create it.
6070 === Get the status of a tracing session
6072 To get the status of the current tracing session, that is, its
6073 parameters, its channels, event rules, and their attributes:
6075 * Use the man:lttng-status(1) command:
6085 To get the status of any tracing session:
6087 * Use the man:lttng-list(1) command with the tracing session's name:
6092 lttng list my-session
6096 Replace `my-session` with the desired tracing session's name.
6099 [[basic-tracing-session-control]]
6100 === Start and stop a tracing session
6102 Once you <<creating-destroying-tracing-sessions,create a tracing
6104 <<enabling-disabling-events,create one or more event rules>>,
6105 you can start and stop the tracers for this tracing session.
6107 To start tracing in the current tracing session:
6109 * Use the man:lttng-start(1) command:
6118 LTTng is very flexible: you can launch user applications before
6119 or after the you start the tracers. The tracers only record the events
6120 if they pass enabled event rules and if they occur while the tracers are
6123 To stop tracing in the current tracing session:
6125 * Use the man:lttng-stop(1) command:
6134 If there were <<channel-overwrite-mode-vs-discard-mode,lost event
6135 records>> or lost sub-buffers since the last time you ran
6136 man:lttng-start(1), warnings are printed when you run the
6137 man:lttng-stop(1) command.
6140 [[enabling-disabling-channels]]
6141 === Create a channel
6143 Once you create a tracing session, you can create a <<channel,channel>>
6144 with the man:lttng-enable-channel(1) command.
6146 Note that LTTng automatically creates a default channel when, for a
6147 given <<domain,tracing domain>>, no channels exist and you
6148 <<enabling-disabling-events,create>> the first event rule. This default
6149 channel is named `channel0` and its attributes are set to reasonable
6150 values. Therefore, you only need to create a channel when you need
6151 non-default attributes.
6153 You specify each non-default channel attribute with a command-line
6154 option when you use the man:lttng-enable-channel(1) command. The
6155 available command-line options are:
6157 [role="growable",cols="asciidoc,asciidoc"]
6158 .Command-line options for the man:lttng-enable-channel(1) command.
6160 |Option |Description
6166 <<channel-overwrite-mode-vs-discard-mode,event loss mode>> instead of
6167 the default _discard_ mode.
6169 |`--buffers-pid` (user space tracing domain only)
6172 Use the per-process <<channel-buffering-schemes,buffering scheme>>
6173 instead of the default per-user buffering scheme.
6175 |+--subbuf-size=__SIZE__+
6178 Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
6179 either for each Unix user (default), or for each instrumented process.
6181 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6183 |+--num-subbuf=__COUNT__+
6186 Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
6187 for each Unix user (default), or for each instrumented process.
6189 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6191 |+--tracefile-size=__SIZE__+
6194 Set the maximum size of each trace file that this channel writes within
6195 a stream to +__SIZE__+ bytes instead of no maximum.
6197 See <<tracefile-rotation,Trace file count and size>>.
6199 |+--tracefile-count=__COUNT__+
6202 Limit the number of trace files that this channel creates to
6203 +__COUNT__+ channels instead of no limit.
6205 See <<tracefile-rotation,Trace file count and size>>.
6207 |+--switch-timer=__PERIODUS__+
6210 Set the <<channel-switch-timer,switch timer period>>
6211 to +__PERIODUS__+{nbsp}µs.
6213 |+--read-timer=__PERIODUS__+
6216 Set the <<channel-read-timer,read timer period>>
6217 to +__PERIODUS__+{nbsp}µs.
6219 |+--output=__TYPE__+ (Linux kernel tracing domain only)
6222 Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.
6226 You can only create a channel in the Linux kernel and user space
6227 <<domain,tracing domains>>: other tracing domains have their own channel
6228 created on the fly when <<enabling-disabling-events,creating event
6233 Because of a current LTTng limitation, you must create all channels
6234 _before_ you <<basic-tracing-session-control,start tracing>> in a given
6235 tracing session, that is, before the first time you run
6238 Since LTTng automatically creates a default channel when you use the
6239 man:lttng-enable-event(1) command with a specific tracing domain, you
6240 cannot, for example, create a Linux kernel event rule, start tracing,
6241 and then create a user space event rule, because no user space channel
6242 exists yet and it's too late to create one.
6244 For this reason, make sure to configure your channels properly
6245 before starting the tracers for the first time!
6248 The following examples show how you can combine the previous
6249 command-line options to create simple to more complex channels.
6251 .Create a Linux kernel channel with default attributes.
6255 lttng enable-channel --kernel my-channel
6259 .Create a user space channel with 4 sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
6263 lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
6264 --buffers-pid my-channel
6268 .Create a Linux kernel channel which rotates 8 trace files of 4{nbsp}MiB each for each stream
6272 lttng enable-channel --kernel --tracefile-count=8 \
6273 --tracefile-size=4194304 my-channel
6277 .Create a user space channel in overwrite (or _flight recorder_) mode.
6281 lttng enable-channel --userspace --overwrite my-channel
6285 You can <<enabling-disabling-events,create>> the same event rule in
6286 two different channels:
6290 lttng enable-event --userspace --channel=my-channel app:tp
6291 lttng enable-event --userspace --channel=other-channel app:tp
6294 If both channels are enabled, when a tracepoint named `app:tp` is
6295 reached, LTTng records two events, one for each channel.
6299 === Disable a channel
6301 To disable a specific channel that you <<enabling-disabling-channels,created>>
6302 previously, use the man:lttng-disable-channel(1) command.
6304 .Disable a specific Linux kernel channel.
6308 lttng disable-channel --kernel my-channel
6312 The state of a channel precedes the individual states of event rules
6313 attached to it: event rules which belong to a disabled channel, even if
6314 they are enabled, are also considered disabled.
6318 === Add context fields to a channel
6320 Event record fields in trace files provide important information about
6321 events that occured previously, but sometimes some external context may
6322 help you solve a problem faster. Examples of context fields are:
6324 * The **process ID**, **thread ID**, **process name**, and
6325 **process priority** of the thread in which the event occurs.
6326 * The **hostname** of the system on which the event occurs.
6327 * The current values of many possible **performance counters** using
6329 ** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
6331 ** Branch instructions, misses, and loads.
6333 * Any context defined at the application level (supported for the
6334 JUL and log4j <<domain,tracing domains>>).
6336 To get the full list of available context fields, see
6337 `lttng add-context --list`. Some context fields are reserved for a
6338 specific <<domain,tracing domain>> (Linux kernel or user space).
6340 You add context fields to <<channel,channels>>. All the events
6341 that a channel with added context fields records contain those fields.
6343 To add context fields to one or all the channels of a given tracing
6346 * Use the man:lttng-add-context(1) command.
6348 .Add context fields to all the channels of the current tracing session.
6350 The following command line adds the virtual process identifier and
6351 the per-thread CPU cycles count fields to all the user space channels
6352 of the current tracing session.
6356 lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
6360 .Add a context field to a specific channel.
6362 The following command line adds the thread identifier context field
6363 to the Linux kernel channel named `my-channel` in the current
6368 lttng add-context --kernel --channel=my-channel --type=tid
6372 .Add an application-specific context field to a specific channel.
6374 The following command line adds the `cur_msg_id` context field of the
6375 `retriever` context retriever for all the instrumented
6376 <<java-application,Java applications>> recording <<event,event records>>
6377 in the channel named `my-channel`:
6381 lttng add-context --kernel --channel=my-channel \
6382 --type='$app:retriever:cur_msg_id'
6385 IMPORTANT: Make sure to always quote the `$` character when you
6386 use man:lttng-add-context(1) from a shell.
6389 NOTE: You cannot remove context fields from a channel once you add it.
6394 === Track process IDs
6396 It's often useful to allow only specific process IDs (PIDs) to emit
6397 events. For example, you may wish to record all the system calls made by
6398 a given process (à la http://linux.die.net/man/1/strace[strace]).
6400 The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
6401 purpose. Both commands operate on a whitelist of process IDs. You _add_
6402 entries to this whitelist with the man:lttng-track(1) command and remove
6403 entries with the man:lttng-untrack(1) command. Any process which has one
6404 of the PIDs in the whitelist is allowed to emit LTTng events which pass
6405 an enabled <<event,event rule>>.
6407 NOTE: The PID tracker tracks the _numeric process IDs_. Should a
6408 process with a given tracked ID exit and another process be given this
6409 ID, then the latter would also be allowed to emit events.
6411 .Track and untrack process IDs.
6413 For the sake of the following example, assume the target system has 16
6417 <<creating-destroying-tracing-sessions,create a tracing session>>,
6418 the whitelist contains all the possible PIDs:
6421 .All PIDs are tracked.
6422 image::track-all.png[]
6424 When the whitelist is full and you use the man:lttng-track(1) command to
6425 specify some PIDs to track, LTTng first clears the whitelist, then it
6426 tracks the specific PIDs. After:
6430 lttng track --pid=3,4,7,10,13
6436 .PIDs 3, 4, 7, 10, and 13 are tracked.
6437 image::track-3-4-7-10-13.png[]
6439 You can add more PIDs to the whitelist afterwards:
6443 lttng track --pid=1,15,16
6449 .PIDs 1, 15, and 16 are added to the whitelist.
6450 image::track-1-3-4-7-10-13-15-16.png[]
6452 The man:lttng-untrack(1) command removes entries from the PID tracker's
6453 whitelist. Given the previous example, the following command:
6457 lttng untrack --pid=3,7,10,13
6460 leads to this whitelist:
6463 .PIDs 3, 7, 10, and 13 are removed from the whitelist.
6464 image::track-1-4-15-16.png[]
6466 LTTng can track all possible PIDs again using the opt:track(1):--all
6471 lttng track --pid --all
6474 The result is, again:
6477 .All PIDs are tracked.
6478 image::track-all.png[]
6481 .Track only specific PIDs
6483 A very typical use case with PID tracking is to start with an empty
6484 whitelist, then <<basic-tracing-session-control,start the tracers>>, and
6485 then add PIDs manually while tracers are active. You can accomplish this
6486 by using the opt:lttng-untrack(1):--all option of the
6487 man:lttng-untrack(1) command to clear the whitelist after you
6488 <<creating-destroying-tracing-sessions,create a tracing session>>:
6492 lttng untrack --pid --all
6498 .No PIDs are tracked.
6499 image::untrack-all.png[]
6501 If you trace with this whitelist configuration, the tracer records no
6502 events for this <<domain,tracing domain>> because no processes are
6503 tracked. You can use the man:lttng-track(1) command as usual to track
6504 specific PIDs, for example:
6508 lttng track --pid=6,11
6514 .PIDs 6 and 11 are tracked.
6515 image::track-6-11.png[]
6520 [[saving-loading-tracing-session]]
6521 === Save and load tracing session configurations
6523 Configuring a <<tracing-session,tracing session>> can be long. Some of
6524 the tasks involved are:
6526 * <<enabling-disabling-channels,Create channels>> with
6527 specific attributes.
6528 * <<adding-context,Add context fields>> to specific channels.
6529 * <<enabling-disabling-events,Create event rules>> with specific log
6530 level and filter conditions.
6532 If you use LTTng to solve real world problems, chances are you have to
6533 record events using the same tracing session setup over and over,
6534 modifying a few variables each time in your instrumented program
6535 or environment. To avoid constant tracing session reconfiguration,
6536 the man:lttng(1) command-line tool can save and load tracing session
6537 configurations to/from XML files.
6539 To save a given tracing session configuration:
6541 * Use the man:lttng-save(1) command:
6546 lttng save my-session
6550 Replace `my-session` with the name of the tracing session to save.
6552 LTTng saves tracing session configurations to
6553 dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
6554 env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
6555 the opt:lttng-save(1):--output-path option to change this destination
6558 LTTng saves all configuration parameters, for example:
6560 * The tracing session name.
6561 * The trace data output path.
6562 * The channels with their state and all their attributes.
6563 * The context fields you added to channels.
6564 * The event rules with their state, log level and filter conditions.
6566 To load a tracing session:
6568 * Use the man:lttng-load(1) command:
6573 lttng load my-session
6577 Replace `my-session` with the name of the tracing session to load.
6579 When LTTng loads a configuration, it restores your saved tracing session
6580 as if you just configured it manually.
6582 See man:lttng(1) for the complete list of command-line options. You
6583 can also save and load all many sessions at a time, and decide in which
6584 directory to output the XML files.
6587 [[sending-trace-data-over-the-network]]
6588 === Send trace data over the network
6590 LTTng can send the recorded trace data to a remote system over the
6591 network instead of writing it to the local file system.
6593 To send the trace data over the network:
6595 . On the _remote_ system (which can also be the target system),
6596 start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
6605 . On the _target_ system, create a tracing session configured to
6606 send trace data over the network:
6611 lttng create my-session --set-url=net://remote-system
6615 Replace `remote-system` by the host name or IP address of the
6616 remote system. See man:lttng-create(1) for the exact URL format.
6618 . On the target system, use the man:lttng(1) command-line tool as usual.
6619 When tracing is active, the target's consumer daemon sends sub-buffers
6620 to the relay daemon running on the remote system instead of flushing
6621 them to the local file system. The relay daemon writes the received
6622 packets to the local file system.
6624 The relay daemon writes trace files to
6625 +$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
6626 +__hostname__+ is the host name of the target system and +__session__+
6627 is the tracing session name. Note that the env:LTTNG_HOME environment
6628 variable defaults to `$HOME` if not set. Use the
6629 opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
6630 trace files to another base directory.
6635 === View events as LTTng emits them (noch:{LTTng} live)
6637 LTTng live is a network protocol implemented by the <<lttng-relayd,relay
6638 daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
6639 display events as LTTng emits them on the target system while tracing is
6642 The relay daemon creates a _tee_: it forwards the trace data to both
6643 the local file system and to connected live viewers:
6646 .The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
6651 . On the _target system_, create a <<tracing-session,tracing session>>
6657 lttng create my-session --live
6661 This spawns a local relay daemon.
6663 . Start the live viewer and configure it to connect to the relay
6664 daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
6669 babeltrace --input-format=lttng-live net://localhost/host/hostname/my-session
6676 * `hostname` with the host name of the target system.
6677 * `my-session` with the name of the tracing session to view.
6680 . Configure the tracing session as usual with the man:lttng(1)
6681 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6683 You can list the available live tracing sessions with Babeltrace:
6687 babeltrace --input-format=lttng-live net://localhost
6690 You can start the relay daemon on another system. In this case, you need
6691 to specify the relay daemon's URL when you create the tracing session
6692 with the opt:lttng-create(1):--set-url option. You also need to replace
6693 `localhost` in the procedure above with the host name of the system on
6694 which the relay daemon is running.
6696 See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
6697 command-line options.
6701 [[taking-a-snapshot]]
6702 === Take a snapshot of the current sub-buffers of a tracing session
6704 The normal behavior of LTTng is to append full sub-buffers to growing
6705 trace data files. This is ideal to keep a full history of the events
6706 that occurred on the target system, but it can
6707 represent too much data in some situations. For example, you may wish
6708 to trace your application continuously until some critical situation
6709 happens, in which case you only need the latest few recorded
6710 events to perform the desired analysis, not multi-gigabyte trace files.
6712 With the man:lttng-snapshot(1) command, you can take a snapshot of the
6713 current sub-buffers of a given <<tracing-session,tracing session>>.
6714 LTTng can write the snapshot to the local file system or send it over
6719 . Create a tracing session in _snapshot mode_:
6724 lttng create my-session --snapshot
6728 The <<channel-overwrite-mode-vs-discard-mode,event loss mode>> of
6729 <<channel,channels>> created in this mode is automatically set to
6730 _overwrite_ (flight recorder mode).
6732 . Configure the tracing session as usual with the man:lttng(1)
6733 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6735 . **Optional**: When you need to take a snapshot,
6736 <<basic-tracing-session-control,stop tracing>>.
6738 You can take a snapshot when the tracers are active, but if you stop
6739 them first, you are sure that the data in the sub-buffers does not
6740 change before you actually take the snapshot.
6747 lttng snapshot record --name=my-first-snapshot
6751 LTTng writes the current sub-buffers of all the current tracing
6752 session's channels to trace files on the local file system. Those trace
6753 files have `my-first-snapshot` in their name.
6755 There is no difference between the format of a normal trace file and the
6756 format of a snapshot: viewers of LTTng traces also support LTTng
6759 By default, LTTng writes snapshot files to the path shown by
6760 `lttng snapshot list-output`. You can change this path or decide to send
6761 snapshots over the network using either:
6763 . An output path or URL that you specify when you create the
6765 . An snapshot output path or URL that you add using
6766 `lttng snapshot add-output`
6767 . An output path or URL that you provide directly to the
6768 `lttng snapshot record` command.
6770 Method 3 overrides method 2, which overrides method 1. When you
6771 specify a URL, a relay daemon must listen on a remote system (see
6772 <<sending-trace-data-over-the-network,Send trace data over the network>>).
6777 === Use the machine interface
6779 With any command of the man:lttng(1) command-line tool, you can set the
6780 opt:lttng(1):--mi option to `xml` (before the command name) to get an
6781 XML machine interface output, for example:
6785 lttng --mi=xml enable-event --kernel --syscall open
6788 A schema definition (XSD) is
6789 https://github.com/lttng/lttng-tools/blob/stable-2.8/src/common/mi-lttng-3.0.xsd[available]
6790 to ease the integration with external tools as much as possible.
6794 [[metadata-regenerate]]
6795 === Regenerate the metadata of an LTTng trace
6797 An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
6798 data stream files and a metadata file. This metadata file contains,
6799 amongst other things, information about the offset of the clock sources
6800 used to timestamp <<event,event records>> when tracing.
6802 If, once a <<tracing-session,tracing session>> is
6803 <<basic-tracing-session-control,started>>, a major
6804 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
6805 happens, the trace's clock offset also needs to be updated. You
6806 can use the man:lttng-metadata(1) command to do so.
6808 The main use case of this command is to allow a system to boot with
6809 an incorrect wall time and trace it with LTTng before its wall time
6810 is corrected. Once the system is known to be in a state where its
6811 wall time is correct, it can run `lttng metadata regenerate`.
6813 To regenerate the metadata of an LTTng trace:
6815 * Use the `regenerate` action of the man:lttng-metadata(1) command:
6820 lttng metadata regenerate
6826 `lttng metadata regenerate` has the following limitations:
6828 * Tracing session <<creating-destroying-tracing-sessions,created>>
6830 * User space <<channel,channels>>, if any, using
6831 <<channel-buffering-schemes,per-user buffering>>.
6836 [[persistent-memory-file-systems]]
6837 === Record trace data on persistent memory file systems
6839 https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
6840 (NVRAM) is random-access memory that retains its information when power
6841 is turned off (non-volatile). Systems with such memory can store data
6842 structures in RAM and retrieve them after a reboot, without flushing
6843 to typical _storage_.
6845 Linux supports NVRAM file systems thanks to either
6846 http://pramfs.sourceforge.net/[PRAMFS] or
6847 https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
6848 (requires Linux 4.1+).
6850 This section does not describe how to operate such file systems;
6851 we assume that you have a working persistent memory file system.
6853 When you create a <<tracing-session,tracing session>>, you can specify
6854 the path of the shared memory holding the sub-buffers. If you specify a
6855 location on an NVRAM file system, then you can retrieve the latest
6856 recorded trace data when the system reboots after a crash.
6858 To record trace data on a persistent memory file system and retrieve the
6859 trace data after a system crash:
6861 . Create a tracing session with a sub-buffer shared memory path located
6862 on an NVRAM file system:
6867 lttng create my-session --shm-path=/path/to/shm
6871 . Configure the tracing session as usual with the man:lttng(1)
6872 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6874 . After a system crash, use the man:lttng-crash(1) command-line tool to
6875 view the trace data recorded on the NVRAM file system:
6880 lttng-crash /path/to/shm
6884 The binary layout of the ring buffer files is not exactly the same as
6885 the trace files layout. This is why you need to use man:lttng-crash(1)
6886 instead of your preferred trace viewer directly.
6888 To convert the ring buffer files to LTTng trace files:
6890 * Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
6895 lttng-crash --extract=/path/to/trace /path/to/shm
6903 [[lttng-modules-ref]]
6904 === noch:{LTTng-modules}
6907 [[lttng-modules-tp-fields]]
6908 ==== Tracepoint fields macros (for `TP_FIELDS()`)
6910 [[tp-fast-assign]][[tp-struct-entry]]The available macros to define
6911 tracepoint fields, which must be listed within `TP_FIELDS()` in
6912 `LTTNG_TRACEPOINT_EVENT()`, are:
6914 [role="func-desc growable",cols="asciidoc,asciidoc"]
6915 .Available macros to define LTTng-modules tracepoint fields
6917 |Macro |Description and parameters
6920 +ctf_integer(__t__, __n__, __e__)+
6922 +ctf_integer_nowrite(__t__, __n__, __e__)+
6924 +ctf_user_integer(__t__, __n__, __e__)+
6926 +ctf_user_integer_nowrite(__t__, __n__, __e__)+
6928 Standard integer, displayed in base 10.
6931 Integer C type (`int`, `long`, `size_t`, ...).
6937 Argument expression.
6940 +ctf_integer_hex(__t__, __n__, __e__)+
6942 +ctf_user_integer_hex(__t__, __n__, __e__)+
6944 Standard integer, displayed in base 16.
6953 Argument expression.
6955 |+ctf_integer_oct(__t__, __n__, __e__)+
6957 Standard integer, displayed in base 8.
6966 Argument expression.
6969 +ctf_integer_network(__t__, __n__, __e__)+
6971 +ctf_user_integer_network(__t__, __n__, __e__)+
6973 Integer in network byte order (big-endian), displayed in base 10.
6982 Argument expression.
6985 +ctf_integer_network_hex(__t__, __n__, __e__)+
6987 +ctf_user_integer_network_hex(__t__, __n__, __e__)+
6989 Integer in network byte order, displayed in base 16.
6998 Argument expression.
7001 +ctf_string(__n__, __e__)+
7003 +ctf_string_nowrite(__n__, __e__)+
7005 +ctf_user_string(__n__, __e__)+
7007 +ctf_user_string_nowrite(__n__, __e__)+
7009 Null-terminated string; undefined behavior if +__e__+ is `NULL`.
7015 Argument expression.
7018 +ctf_array(__t__, __n__, __e__, __s__)+
7020 +ctf_array_nowrite(__t__, __n__, __e__, __s__)+
7022 +ctf_user_array(__t__, __n__, __e__, __s__)+
7024 +ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
7026 Statically-sized array of integers.
7029 Array element C type.
7035 Argument expression.
7041 +ctf_array_bitfield(__t__, __n__, __e__, __s__)+
7043 +ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7045 +ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
7047 +ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7049 Statically-sized array of bits.
7051 The type of +__e__+ must be an integer type. +__s__+ is the number
7052 of elements of such type in +__e__+, not the number of bits.
7055 Array element C type.
7061 Argument expression.
7067 +ctf_array_text(__t__, __n__, __e__, __s__)+
7069 +ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
7071 +ctf_user_array_text(__t__, __n__, __e__, __s__)+
7073 +ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
7075 Statically-sized array, printed as text.
7077 The string does not need to be null-terminated.
7080 Array element C type (always `char`).
7086 Argument expression.
7092 +ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
7094 +ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7096 +ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
7098 +ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7100 Dynamically-sized array of integers.
7102 The type of +__E__+ must be unsigned.
7105 Array element C type.
7111 Argument expression.
7114 Length expression C type.
7120 +ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7122 +ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7124 Dynamically-sized array of integers, displayed in base 16.
7126 The type of +__E__+ must be unsigned.
7129 Array element C type.
7135 Argument expression.
7138 Length expression C type.
7143 |+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
7145 Dynamically-sized array of integers in network byte order (big-endian),
7146 displayed in base 10.
7148 The type of +__E__+ must be unsigned.
7151 Array element C type.
7157 Argument expression.
7160 Length expression C type.
7166 +ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7168 +ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7170 +ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7172 +ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7174 Dynamically-sized array of bits.
7176 The type of +__e__+ must be an integer type. +__s__+ is the number
7177 of elements of such type in +__e__+, not the number of bits.
7179 The type of +__E__+ must be unsigned.
7182 Array element C type.
7188 Argument expression.
7191 Length expression C type.
7197 +ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7199 +ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7201 +ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7203 +ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7205 Dynamically-sized array, displayed as text.
7207 The string does not need to be null-terminated.
7209 The type of +__E__+ must be unsigned.
7211 The behaviour is undefined if +__e__+ is `NULL`.
7214 Sequence element C type (always `char`).
7220 Argument expression.
7223 Length expression C type.
7229 Use the `_user` versions when the argument expression, `e`, is
7230 a user space address. In the cases of `ctf_user_integer*()` and
7231 `ctf_user_float*()`, `&e` must be a user space address, thus `e` must
7234 The `_nowrite` versions omit themselves from the session trace, but are
7235 otherwise identical. This means the `_nowrite` fields won't be written
7236 in the recorded trace. Their primary purpose is to make some
7237 of the event context available to the
7238 <<enabling-disabling-events,event filters>> without having to
7239 commit the data to sub-buffers.
7245 Terms related to LTTng and to tracing in general:
7248 The http://diamon.org/babeltrace[Babeltrace] project, which includes
7249 the cmd:babeltrace command, some libraries, and Python bindings.
7251 <<channel-buffering-schemes,buffering scheme>>::
7252 A layout of sub-buffers applied to a given channel.
7254 <<channel,channel>>::
7255 An entity which is responsible for a set of ring buffers.
7257 <<event,Event rules>> are always attached to a specific channel.
7260 A reference of time for a tracer.
7262 <<lttng-consumerd,consumer daemon>>::
7263 A process which is responsible for consuming the full sub-buffers
7264 and write them to a file system or send them over the network.
7266 <<channel-overwrite-mode-vs-discard-mode,discard mode>>:: The event loss
7267 mode in which the tracer _discards_ new event records when there's no
7268 sub-buffer space left to store them.
7271 The consequence of the execution of an instrumentation
7272 point, like a tracepoint that you manually place in some source code,
7273 or a Linux kernel KProbe.
7275 An event is said to _occur_ at a specific time. Different actions can
7276 be taken upon the occurance of an event, like record the event's payload
7279 <<channel-overwrite-mode-vs-discard-mode,event loss mode>>::
7280 The mechanism by which event records of a given channel are lost
7281 (not recorded) when there is no sub-buffer space left to store them.
7283 [[def-event-name]]event name::
7284 The name of an event, which is also the name of the event record.
7285 This is also called the _instrumentation point name_.
7288 A record, in a trace, of the payload of an event which occured.
7290 <<event,event rule>>::
7291 Set of conditions which must be satisfied for one or more occuring
7292 events to be recorded.
7294 `java.util.logging`::
7296 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].
7298 <<instrumenting,instrumentation>>::
7299 The use of LTTng probes to make a piece of software traceable.
7301 instrumentation point::
7302 A point in the execution path of a piece of software that, when
7303 reached by this execution, can emit an event.
7305 instrumentation point name::
7306 See _<<def-event-name,event name>>_.
7309 A http://logging.apache.org/log4j/1.2/[logging library] for Java
7310 developed by the Apache Software Foundation.
7313 Level of severity of a log statement or user space
7314 instrumentation point.
7317 The _Linux Trace Toolkit: next generation_ project.
7319 <<lttng-cli,cmd:lttng>>::
7320 A command-line tool provided by the LTTng-tools project which you
7321 can use to send and receive control messages to and from a
7325 The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
7326 which is a set of analyzing programs that are used to obtain a
7327 higher level view of an LTTng trace.
7329 cmd:lttng-consumerd::
7330 The name of the consumer daemon program.
7333 A utility provided by the LTTng-tools project which can convert
7334 ring buffer files (usually
7335 <<persistent-memory-file-systems,saved on a persistent memory file system>>)
7338 LTTng Documentation::
7341 <<lttng-live,LTTng live>>::
7342 A communication protocol between the relay daemon and live viewers
7343 which makes it possible to see events "live", as they are received by
7346 <<lttng-modules,LTTng-modules>>::
7347 The https://github.com/lttng/lttng-modules[LTTng-modules] project,
7348 which contains the Linux kernel modules to make the Linux kernel
7349 instrumentation points available for LTTng tracing.
7352 The name of the relay daemon program.
7354 cmd:lttng-sessiond::
7355 The name of the session daemon program.
7358 The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
7359 contains the various programs and libraries used to
7360 <<controlling-tracing,control tracing>>.
7362 <<lttng-ust,LTTng-UST>>::
7363 The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
7364 contains libraries to instrument user applications.
7366 <<lttng-ust-agents,LTTng-UST Java agent>>::
7367 A Java package provided by the LTTng-UST project to allow the
7368 LTTng instrumentation of `java.util.logging` and Apache log4j 1.2
7371 <<lttng-ust-agents,LTTng-UST Python agent>>::
7372 A Python package provided by the LTTng-UST project to allow the
7373 LTTng instrumentation of Python logging statements.
7375 <<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
7376 The event loss mode in which new event records overwrite older
7377 event records when there's no sub-buffer space left to store them.
7379 <<channel-buffering-schemes,per-process buffering>>::
7380 A buffering scheme in which each instrumented process has its own
7381 sub-buffers for a given user space channel.
7383 <<channel-buffering-schemes,per-user buffering>>::
7384 A buffering scheme in which all the processes of a Unix user share the
7385 same sub-buffer for a given user space channel.
7387 <<lttng-relayd,relay daemon>>::
7388 A process which is responsible for receiving the trace data sent by
7389 a distant consumer daemon.
7392 A set of sub-buffers.
7394 <<lttng-sessiond,session daemon>>::
7395 A process which receives control commands from you and orchestrates
7396 the tracers and various LTTng daemons.
7398 <<taking-a-snapshot,snapshot>>::
7399 A copy of the current data of all the sub-buffers of a given tracing
7400 session, saved as trace files.
7403 One part of an LTTng ring buffer which contains event records.
7406 The time information attached to an event when it is emitted.
7409 A set of files which are the concatenations of one or more
7410 flushed sub-buffers.
7413 The action of recording the events emitted by an application
7414 or by a system, or to initiate such recording by controlling
7418 The http://tracecompass.org[Trace Compass] project and application.
7421 An instrumentation point using the tracepoint mechanism of the Linux
7422 kernel or of LTTng-UST.
7424 tracepoint definition::
7425 The definition of a single tracepoint.
7428 The name of a tracepoint.
7430 tracepoint provider::
7431 A set of functions providing tracepoints to an instrumented user
7434 Not to be confused with a _tracepoint provider package_: many tracepoint
7435 providers can exist within a tracepoint provider package.
7437 tracepoint provider package::
7438 One or more tracepoint providers compiled as an object file or as
7442 A software which records emitted events.
7444 <<domain,tracing domain>>::
7445 A namespace for event sources.
7447 <<tracing-group,tracing group>>::
7448 The Unix group in which a Unix user can be to be allowed to trace the
7451 <<tracing-session,tracing session>>::
7452 A stateful dialogue between you and a <<lttng-sessiond,session
7456 An application running in user space, as opposed to a Linux kernel
7457 module, for example.