From: Philippe Proulx Date: Wed, 30 Nov 2016 00:15:50 +0000 (-0500) Subject: Document LTTng 2.9 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=7e35c9046df4437aecfcf05ab2178264afd461db;p=lttng-docs.git Document LTTng 2.9 Signed-off-by: Philippe Proulx --- diff --git a/2.9/images/export/concepts.png b/2.9/images/export/concepts.png new file mode 100644 index 0000000..58d2848 Binary files /dev/null and b/2.9/images/export/concepts.png differ diff --git a/2.9/images/export/event-rule.png b/2.9/images/export/event-rule.png new file mode 100644 index 0000000..bad335e Binary files /dev/null and b/2.9/images/export/event-rule.png differ diff --git a/2.9/images/export/java-app.png b/2.9/images/export/java-app.png new file mode 100644 index 0000000..bb0c210 Binary files /dev/null and b/2.9/images/export/java-app.png differ diff --git a/2.9/images/export/live.png b/2.9/images/export/live.png new file mode 100644 index 0000000..53c7d5f Binary files /dev/null and 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+ + +[[chapters]] +=== What's in this documentation? + +The LTTng Documentation is divided into the following sections: + +* **<>** explains the + rudiments of software tracing and the rationale behind the + LTTng project. ++ +You can skip this section if you’re familiar with software tracing and +with the LTTng project. + +* **<>** describes the steps to + install the LTTng packages on common Linux distributions and from + their sources. ++ +You can skip this section if you already properly installed LTTng on +your target system. + +* **<>** is a concise guide to + getting started quickly with LTTng kernel and user space tracing. ++ +We recommend this section if you're new to LTTng or to software tracing +in general. ++ +You can skip this section if you're not new to LTTng. + +* **<>** explains the concepts at + the heart of LTTng. ++ +It's a good idea to become familiar with the core concepts +before attempting to use the toolkit. + +* **<>** describes the various components + of the LTTng machinery, like the daemons, the libraries, and the + command-line interface. +* **<>** shows different ways to + instrument user applications and the Linux kernel. ++ +Instrumenting source code is essential to provide a meaningful +source of events. ++ +You can skip this section if you do not have a programming background. + +* **<>** is divided into topics + which demonstrate how to use the vast array of features that + LTTng{nbsp}{revision} offers. +* **<>** contains reference tables. +* **<>** is a specialized dictionary of terms related + to LTTng or to the field of software tracing. + + +include::../common/convention.txt[] + + +include::../common/acknowledgements.txt[] + + +[[whats-new]] +== What's new in LTTng {revision}? + +* **Tracing control**: +** You can override the name or the URL of a tracing session + configuration when you use man:lttng-load(1) thanks to the new + opt:lttng-load(1):--override-name and + opt:lttng-load(1):--override-url options. +** The new `lttng regenerate` command replaces the now deprecated + `lttng metadata` command of LTTng 2.8. man:lttng-regenerate(1) can + also <> + of a given tracing session on demand, a handy feature when + <>. +** You can add PMU counters by raw ID with man:lttng-add-context(1): ++ +-- +[role="term"] +---- +lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted +---- +-- ++ +The format of the raw ID is the same as used with man:perf-record(1). +See <> for more +examples. + +** The LTTng <> is now supported on + OS{nbsp}X and macOS for a smoother integration within a trace + analysis workflow, regardless of the platform used. + +* **User space tracing**: +** Improved performance (tested on x86-64 and ARMv7-A + (https://en.wikipedia.org/wiki/Cubieboard[Cubieboard]) + architectures). +** New helper library (`liblttng-ust-fd`) to help with + <>, for example, in a loop which closes file + descriptors after man:fork(2), or BSD's `closeall()`. +** More accurate <> and + state dump event records, especially when a dynamically loaded + library manually loads its own dependencies. +** New `ctf_*()` field definition macros (see man:lttng-ust(3)): +*** `ctf_array_hex()` +*** `ctf_array_network()` +*** `ctf_array_network_hex()` +*** `ctf_sequence_hex()` +*** `ctf_sequence_network()` +*** `ctf_sequence_network_hex()` +** New `lttng_ust_loaded` weak symbol defined by `liblttng-ust` for + an application to know if the LTTng-UST shared library is loaded + or not: ++ +-- +[source,c] +---- +#include + +int lttng_ust_loaded __attribute__((weak)); + +int main(void) +{ + if (lttng_ust_loaded) { + puts("LTTng-UST is loaded!"); + } else { + puts("LTTng-UST is not loaded!"); + } + + return 0; +} +---- +-- + +** LTTng-UST thread names have the `-ust` suffix. + +* **Linux kernel tracing**: +** Improved performance (tested on x86-64 and ARMv7-A + (https://en.wikipedia.org/wiki/Cubieboard[Cubieboard]) + architectures). +** New enumeration <>: + `ctf_enum()` and `ctf_user_enum()`. +** IPv4, IPv6, and TCP header data is recorded in the event records + produced by tracepoints starting with `net_`. +** Detailed system call event records: `select`, `pselect6`, `poll`, + `ppoll`, `epoll_wait`, `epoll_pwait`, and `epoll_ctl` on all + architectures supported by LTTng-modules, and `accept4` on x86-64. +** New I²C instrumentation: the `extract_sensitive_payload` parameter + of the new `lttng-probe-i2c` LTTng module controls whether or not + the payloads of I²C messages are recorded in I²C event records, since + they may contain sensitive data (for example, keystrokes). +** When the LTTng kernel modules are built into the Linux kernel image, + the `CONFIG_TRACEPOINTS` configuration option is automatically + selected. + + +[[nuts-and-bolts]] +== Nuts and bolts + +What is LTTng? As its name suggests, the _Linux Trace Toolkit: next +generation_ is a modern toolkit for tracing Linux systems and +applications. So your first question might be: +**what is tracing?** + + +[[what-is-tracing]] +=== What is tracing? + +As the history of software engineering progressed and led to what +we now take for granted--complex, numerous and +interdependent software applications running in parallel on +sophisticated operating systems like Linux--the authors of such +components, software developers, began feeling a natural +urge to have tools that would ensure the robustness and good performance +of their masterpieces. + +One major achievement in this field is, inarguably, the +https://www.gnu.org/software/gdb/[GNU debugger (GDB)], +an essential tool for developers to find and fix bugs. But even the best +debugger won't help make your software run faster, and nowadays, faster +software means either more work done by the same hardware, or cheaper +hardware for the same work. + +A _profiler_ is often the tool of choice to identify performance +bottlenecks. Profiling is suitable to identify _where_ performance is +lost in a given software. The profiler outputs a profile, a statistical +summary of observed events, which you may use to discover which +functions took the most time to execute. However, a profiler won't +report _why_ some identified functions are the bottleneck. Bottlenecks +might only occur when specific conditions are met, conditions that are +sometimes impossible to capture by a statistical profiler, or impossible +to reproduce with an application altered by the overhead of an +event-based profiler. For a thorough investigation of software +performance issues, a history of execution is essential, with the +recorded values of variables and context fields you choose, and +with as little influence as possible on the instrumented software. This +is where tracing comes in handy. + +_Tracing_ is a technique used to understand what goes on in a running +software system. The software used for tracing is called a _tracer_, +which is conceptually similar to a tape recorder. When recording, +specific instrumentation points placed in the software source code +generate events that are saved on a giant tape: a _trace_ file. You +can trace user applications and the operating system at the same time, +opening the possibility of resolving a wide range of problems that would +otherwise be extremely challenging. + +Tracing is often compared to _logging_. However, tracers and loggers are +two different tools, serving two different purposes. Tracers are +designed to record much lower-level events that occur much more +frequently than log messages, often in the range of thousands per +second, with very little execution overhead. Logging is more appropriate +for a very high-level analysis of less frequent events: user accesses, +exceptional conditions (errors and warnings, for example), database +transactions, instant messaging communications, and such. Simply put, +logging is one of the many use cases that can be satisfied with tracing. + +The list of recorded events inside a trace file can be read manually +like a log file for the maximum level of detail, but it is generally +much more interesting to perform application-specific analyses to +produce reduced statistics and graphs that are useful to resolve a +given problem. Trace viewers and analyzers are specialized tools +designed to do this. + +In the end, this is what LTTng is: a powerful, open source set of +tools to trace the Linux kernel and user applications at the same time. +LTTng is composed of several components actively maintained and +developed by its link:/community/#where[community]. + + +[[lttng-alternatives]] +=== Alternatives to noch:{LTTng} + +Excluding proprietary solutions, a few competing software tracers +exist for Linux: + +* https://github.com/dtrace4linux/linux[dtrace4linux] is a port of + Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets + user scripts and is responsible for loading code into the + Linux kernel for further execution and collecting the outputted data. +* https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a + subsystem in the Linux kernel in which a virtual machine can execute + programs passed from the user space to the kernel. You can attach + such programs to tracepoints and KProbes thanks to a system call, and + they can output data to the user space when executed thanks to + different mechanisms (pipe, VM register values, and eBPF maps, to name + a few). +* https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace] + is the de facto function tracer of the Linux kernel. Its user + interface is a set of special files in sysfs. +* https://perf.wiki.kernel.org/[perf] is + a performance analyzing tool for Linux which supports hardware + performance counters, tracepoints, as well as other counters and + types of probes. perf's controlling utility is the cmd:perf command + line/curses tool. +* http://linux.die.net/man/1/strace[strace] + is a command-line utility which records system calls made by a + user process, as well as signal deliveries and changes of process + state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace] + to fulfill its function. +* http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to + analyze Linux kernel events. You write scripts, or _chisels_ in + sysdig's jargon, in Lua and sysdig executes them while the system is + being traced or afterwards. sysdig's interface is the cmd:sysdig + command-line tool as well as the curses-based cmd:csysdig tool. +* https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and + user space tracer which uses custom user scripts to produce plain text + traces. SystemTap converts the scripts to the C language, and then + compiles them as Linux kernel modules which are loaded to produce + trace data. SystemTap's primary user interface is the cmd:stap + command-line tool. + +The main distinctive features of LTTng is that it produces correlated +kernel and user space traces, as well as doing so with the lowest +overhead amongst other solutions. It produces trace files in the +http://diamon.org/ctf[CTF] format, a file format optimized +for the production and analyses of multi-gigabyte data. + +LTTng is the result of more than 10 years of active open source +development by a community of passionate developers. +LTTng{nbsp}{revision} is currently available on major desktop and server +Linux distributions. + +The main interface for tracing control is a single command-line tool +named cmd:lttng. The latter can create several tracing sessions, enable +and disable events on the fly, filter events efficiently with custom +user expressions, start and stop tracing, and much more. LTTng can +record the traces on the file system or send them over the network, and +keep them totally or partially. You can view the traces once tracing +becomes inactive or in real-time. + +<> and +<>! + + +[[installing-lttng]] +== Installation + +**LTTng** is a set of software <> which interact to +<> the Linux kernel and user applications, and +to <> (start and stop +tracing, enable and disable event rules, and the rest). Those +components are bundled into the following packages: + +* **LTTng-tools**: Libraries and command-line interface to + control tracing. +* **LTTng-modules**: Linux kernel modules to instrument and + trace the kernel. +* **LTTng-UST**: Libraries and Java/Python packages to instrument and + trace user applications. + +Most distributions mark the LTTng-modules and LTTng-UST packages as +optional when installing LTTng-tools (which is always required). In the +following sections, we always provide the steps to install all three, +but note that: + +* You only need to install LTTng-modules if you intend to trace the + Linux kernel. +* You only need to install LTTng-UST if you intend to trace user + applications. + +[role="growable"] +.Availability of LTTng{nbsp}{revision} for major Linux distributions as of 29 November 2016. +|==== +|Distribution |Available in releases |Alternatives + +|Ubuntu +|Ubuntu{nbsp}14.04 _Trusty Tahr_ and Ubuntu{nbsp}16.04 _Xenial Xerus_: +<>. +|link:/docs/v2.8#doc-ubuntu[LTTng{nbsp}2.8 for Ubuntu{nbsp}16.10 _Yakkety Yak_]. + +<> for +other Ubuntu releases. + +|Fedora +|_Not available_ +|<>. + +|Debian +|_Not available_ +|<>. + +|openSUSE +|_Not available_ +|<>. + +|Arch Linux +|<>. +| + +|Alpine Linux +|_Not available_ +|link:/docs/v2.8#doc-alpine-linux[LTTng{nbsp}2.8 for Alpine Linux{nbsp}"edge"]. + +LTTng{nbsp}2.8 for Alpine Linux{nbsp}3.5 (not released yet). + +<> for +other Alpine Linux releases. + +|RHEL and SLES +|See http://packages.efficios.com/[EfficiOS Enterprise Packages]. +| + +|Buildroot +|_Not available_ +|LTTng{nbsp}2.8 for Buildroot{nbsp}2016.11 (not released yet). + +<> for +other Buildroot releases. + +|OpenEmbedded and Yocto +|_Not available_ +|link:/docs/v2.8#doc-oe-yocto[LTTng{nbsp}2.8 for Yocto Project{nbsp}2.2 _Morty_] +(`openembedded-core` layer). + +<> for +other OpenEmbedded releases. +|==== + + +[[ubuntu]] +=== [[ubuntu-official-repositories]]Ubuntu + +[[ubuntu-ppa]] +==== noch:{LTTng} Stable {revision} PPA + +The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng +Stable{nbsp}{revision} PPA] offers the latest stable +LTTng{nbsp}{revision} packages for: + +* Ubuntu{nbsp}14.04 _Trusty Tahr_ +* Ubuntu{nbsp}16.04 _Xenial Xerus_ + +To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA: + +. Add the LTTng Stable{nbsp}{revision} PPA repository and update the + list of packages: ++ +-- +[role="term"] +---- +sudo apt-add-repository ppa:lttng/stable-2.9 +sudo apt-get update +---- +-- + +. Install the main LTTng{nbsp}{revision} packages: ++ +-- +[role="term"] +---- +sudo apt-get install lttng-tools +sudo apt-get install lttng-modules-dkms +sudo apt-get install liblttng-ust-dev +---- +-- + +. **If you need to instrument and trace + <>**, install the LTTng-UST + Java agent: ++ +-- +[role="term"] +---- +sudo apt-get install liblttng-ust-agent-java +---- +-- + +. **If you need to instrument and trace + <>**, install the + LTTng-UST Python agent: ++ +-- +[role="term"] +---- +sudo apt-get install python3-lttngust +---- +-- + + +[[arch-linux]] +=== Arch Linux + +To install LTTng{nbsp}{revision} on Arch Linux using +https://archlinux.fr/yaourt-en[Yaourt]: + +. Install the main LTTng{nbsp}{revision} packages: ++ +-- +[role="term"] +---- +yaourt -S lttng-tools +yaourt -S lttng-ust +yaourt -S lttng-modules +---- +-- + +. **If you need to instrument and trace <>**, install the LTTng-UST Python agent: ++ +-- +[role="term"] +---- +yaourt -S python-lttngust +yaourt -S python2-lttngust +---- +-- + + +[[enterprise-distributions]] +=== RHEL, SUSE, and other enterprise distributions + +To install LTTng on enterprise Linux distributions, such as Red Hat +Enterprise Linux (RHEL) and SUSE Linux Enterprise Server (SUSE), please +see http://packages.efficios.com/[EfficiOS Enterprise Packages]. + + +[[building-from-source]] +=== Build from source + +To build and install LTTng{nbsp}{revision} from source: + +. Using your distribution's package manager, or from source, install + the following dependencies of LTTng-tools and LTTng-UST: ++ +-- +* https://sourceforge.net/projects/libuuid/[libuuid] +* http://directory.fsf.org/wiki/Popt[popt] +* http://liburcu.org/[Userspace RCU] +* http://www.xmlsoft.org/[libxml2] +-- + +. Download, build, and install the latest LTTng-modules{nbsp}{revision}: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.9.tar.bz2 && +tar -xf lttng-modules-latest-2.9.tar.bz2 && +cd lttng-modules-2.9.* && +make && +sudo make modules_install && +sudo depmod -a +---- +-- + +. Download, build, and install the latest LTTng-UST{nbsp}{revision}: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.9.tar.bz2 && +tar -xf lttng-ust-latest-2.9.tar.bz2 && +cd lttng-ust-2.9.* && +./configure && +make && +sudo make install && +sudo ldconfig +---- +-- ++ +-- +[IMPORTANT] +.Java and Python application tracing +==== +If you need to instrument and trace <>, pass the `--enable-java-agent-jul`, +`--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the +`configure` script, depending on which Java logging framework you use. + +If you need to instrument and trace <>, pass the `--enable-python-agent` option to the +`configure` script. You can set the `PYTHON` environment variable to the +path to the Python interpreter for which to install the LTTng-UST Python +agent package. +==== +-- ++ +-- +[NOTE] +==== +By default, LTTng-UST libraries are installed to +dir:{/usr/local/lib}, which is the de facto directory in which to +keep self-compiled and third-party libraries. + +When <>: + +* Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment + variable. +* Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to + man:gcc(1), man:g++(1), or man:clang(1). +==== +-- + +. Download, build, and install the latest LTTng-tools{nbsp}{revision}: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.9.tar.bz2 && +tar -xf lttng-tools-latest-2.9.tar.bz2 && +cd lttng-tools-2.9.* && +./configure && +make && +sudo make install && +sudo ldconfig +---- +-- + +TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the +previous steps automatically for a given version of LTTng and confine +the installed files in a specific directory. This can be useful to test +LTTng without installing it on your system. + + +[[getting-started]] +== Quick start + +This is a short guide to get started quickly with LTTng kernel and user +space tracing. + +Before you follow this guide, make sure to <> +LTTng. + +This tutorial walks you through the steps to: + +. <>. +. <> written + in C. +. <>. + + +[[tracing-the-linux-kernel]] +=== Trace the Linux kernel + +The following command lines start with cmd:sudo because you need root +privileges to trace the Linux kernel. You can avoid using cmd:sudo if +your Unix user is a member of the <>. + +. Create a <>: ++ +-- +[role="term"] +---- +sudo lttng create my-kernel-session +---- +-- + +. List the available kernel tracepoints and system calls: ++ +-- +[role="term"] +---- +lttng list --kernel +---- +-- + +. Create an <> which matches the desired event names, + for example `sched_switch` and `sched_process_fork`: ++ +-- +[role="term"] +---- +sudo lttng enable-event --kernel sched_switch,sched_process_fork +---- +-- ++ +You can also create an event rule which _matches_ all the Linux kernel +tracepoints (this will generate a lot of data when tracing): ++ +-- +[role="term"] +---- +sudo lttng enable-event --kernel --all +---- +-- + +. Start tracing: ++ +-- +[role="term"] +---- +sudo lttng start +---- +-- + +. Do some operation on your system for a few seconds. For example, + load a website, or list the files of a directory. +. Stop tracing and destroy the tracing session: ++ +-- +[role="term"] +---- +sudo lttng stop +sudo lttng destroy +---- +-- ++ +The man:lttng-destroy(1) command does not destroy the trace data; it +only destroys the state of the tracing session. + +By default, LTTng saves the traces in ++$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+, +where +__name__+ is the tracing session name. Note that the +env:LTTNG_HOME environment variable defaults to `$HOME` if not set. + +See <> to view the recorded events. + + +[[tracing-your-own-user-application]] +=== Trace a user application + +This section steps you through a simple example to trace a +_Hello world_ program written in C. + +To create the traceable user application: + +. Create the tracepoint provider header file, which defines the + tracepoints and the events they can generate: ++ +-- +[source,c] +.path:{hello-tp.h} +---- +#undef TRACEPOINT_PROVIDER +#define TRACEPOINT_PROVIDER hello_world + +#undef TRACEPOINT_INCLUDE +#define TRACEPOINT_INCLUDE "./hello-tp.h" + +#if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ) +#define _HELLO_TP_H + +#include + +TRACEPOINT_EVENT( + hello_world, + my_first_tracepoint, + TP_ARGS( + int, my_integer_arg, + char*, my_string_arg + ), + TP_FIELDS( + ctf_string(my_string_field, my_string_arg) + ctf_integer(int, my_integer_field, my_integer_arg) + ) +) + +#endif /* _HELLO_TP_H */ + +#include +---- +-- + +. Create the tracepoint provider package source file: ++ +-- +[source,c] +.path:{hello-tp.c} +---- +#define TRACEPOINT_CREATE_PROBES +#define TRACEPOINT_DEFINE + +#include "hello-tp.h" +---- +-- + +. Build the tracepoint provider package: ++ +-- +[role="term"] +---- +gcc -c -I. hello-tp.c +---- +-- + +. Create the _Hello World_ application source file: ++ +-- +[source,c] +.path:{hello.c} +---- +#include +#include "hello-tp.h" + +int main(int argc, char *argv[]) +{ + int x; + + puts("Hello, World!\nPress Enter to continue..."); + + /* + * The following getchar() call is only placed here for the purpose + * of this demonstration, to pause the application in order for + * you to have time to list its tracepoints. It is not + * needed otherwise. + */ + getchar(); + + /* + * A tracepoint() call. + * + * Arguments, as defined in hello-tp.h: + * + * 1. Tracepoint provider name (required) + * 2. Tracepoint name (required) + * 3. my_integer_arg (first user-defined argument) + * 4. my_string_arg (second user-defined argument) + * + * Notice the tracepoint provider and tracepoint names are + * NOT strings: they are in fact parts of variables that the + * macros in hello-tp.h create. + */ + tracepoint(hello_world, my_first_tracepoint, 23, "hi there!"); + + for (x = 0; x < argc; ++x) { + tracepoint(hello_world, my_first_tracepoint, x, argv[x]); + } + + puts("Quitting now!"); + tracepoint(hello_world, my_first_tracepoint, x * x, "x^2"); + + return 0; +} +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -c hello.c +---- +-- + +. Link the application with the tracepoint provider package, + `liblttng-ust`, and `libdl`: ++ +-- +[role="term"] +---- +gcc -o hello hello.o hello-tp.o -llttng-ust -ldl +---- +-- + +Here's the whole build process: + +[role="img-100"] +.User space tracing tutorial's build steps. +image::ust-flow.png[] + +To trace the user application: + +. Run the application with a few arguments: ++ +-- +[role="term"] +---- +./hello world and beyond +---- +-- ++ +You see: ++ +-- +---- +Hello, World! +Press Enter to continue... +---- +-- + +. Start an LTTng <>: ++ +-- +[role="term"] +---- +lttng-sessiond --daemonize +---- +-- ++ +Note that a session daemon might already be running, for example as +a service that the distribution's service manager started. + +. List the available user space tracepoints: ++ +-- +[role="term"] +---- +lttng list --userspace +---- +-- ++ +You see the `hello_world:my_first_tracepoint` tracepoint listed +under the `./hello` process. + +. Create a <>: ++ +-- +[role="term"] +---- +lttng create my-user-space-session +---- +-- + +. Create an <> which matches the + `hello_world:my_first_tracepoint` event name: ++ +-- +[role="term"] +---- +lttng enable-event --userspace hello_world:my_first_tracepoint +---- +-- + +. Start tracing: ++ +-- +[role="term"] +---- +lttng start +---- +-- + +. Go back to the running `hello` application and press Enter. The + program executes all `tracepoint()` instrumentation points and exits. +. Stop tracing and destroy the tracing session: ++ +-- +[role="term"] +---- +sudo lttng stop +sudo lttng destroy +---- +-- ++ +The man:lttng-destroy(1) command does not destroy the trace data; it +only destroys the state of the tracing session. + +By default, LTTng saves the traces in ++$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+, +where +__name__+ is the tracing session name. Note that the +env:LTTNG_HOME environment variable defaults to `$HOME` if not set. + +See <> to view the recorded events. + + +[[viewing-and-analyzing-your-traces]] +=== View and analyze the recorded events + +Once you have completed the <> and <> tutorials, you can inspect the recorded events. + +Many tools are available to read LTTng traces: + +* **cmd:babeltrace** is a command-line utility which converts trace + formats; it supports the format that LTTng produces, CTF, as well as a + basic text output which can be ++grep++ed. The cmd:babeltrace command + is part of the http://diamon.org/babeltrace[Babeltrace] project. +* Babeltrace also includes + **https://www.python.org/[Python] bindings** so + that you can easily open and read an LTTng trace with your own script, + benefiting from the power of Python. +* http://tracecompass.org/[**Trace Compass**] + is a graphical user interface for viewing and analyzing any type of + logs or traces, including LTTng's. +* https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a + project which includes many high-level analyses of LTTng kernel + traces, like scheduling statistics, interrupt frequency distribution, + top CPU usage, and more. + +NOTE: This section assumes that the traces recorded during the previous +tutorials were saved to their default location, in the +dir:{$LTTNG_HOME/lttng-traces} directory. Note that the env:LTTNG_HOME +environment variable defaults to `$HOME` if not set. + + +[[viewing-and-analyzing-your-traces-bt]] +==== Use the cmd:babeltrace command-line tool + +The simplest way to list all the recorded events of a trace is to pass +its path to cmd:babeltrace with no options: + +[role="term"] +---- +babeltrace ~/lttng-traces/my-user-space-session* +---- + +cmd:babeltrace finds all traces recursively within the given path and +prints all their events, merging them in chronological order. + +You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for +further filtering: + +[role="term"] +---- +babeltrace ~/lttng-traces/my-kernel-session* | grep sys_ +---- + +You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to +count the recorded events: + +[role="term"] +---- +babeltrace ~/lttng-traces/my-kernel-session* | grep sys_read | wc --lines +---- + + +[[viewing-and-analyzing-your-traces-bt-python]] +==== Use the Babeltrace Python bindings + +The <> +is useful to isolate events by simple matching using man:grep(1) and +similar utilities. However, more elaborate filters, such as keeping only +event records with a field value falling within a specific range, are +not trivial to write using a shell. Moreover, reductions and even the +most basic computations involving multiple event records are virtually +impossible to implement. + +Fortunately, Babeltrace ships with Python 3 bindings which makes it easy +to read the event records of an LTTng trace sequentially and compute the +desired information. + +The following script accepts an LTTng Linux kernel trace path as its +first argument and prints the short names of the top 5 running processes +on CPU 0 during the whole trace: + +[source,python] +.path:{top5proc.py} +---- +from collections import Counter +import babeltrace +import sys + + +def top5proc(): + if len(sys.argv) != 2: + msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0]) + print(msg, file=sys.stderr) + return False + + # A trace collection contains one or more traces + col = babeltrace.TraceCollection() + + # Add the trace provided by the user (LTTng traces always have + # the 'ctf' format) + if col.add_trace(sys.argv[1], 'ctf') is None: + raise RuntimeError('Cannot add trace') + + # This counter dict contains execution times: + # + # task command name -> total execution time (ns) + exec_times = Counter() + + # This contains the last `sched_switch` timestamp + last_ts = None + + # Iterate on events + for event in col.events: + # Keep only `sched_switch` events + if event.name != 'sched_switch': + continue + + # Keep only events which happened on CPU 0 + if event['cpu_id'] != 0: + continue + + # Event timestamp + cur_ts = event.timestamp + + if last_ts is None: + # We start here + last_ts = cur_ts + + # Previous task command (short) name + prev_comm = event['prev_comm'] + + # Initialize entry in our dict if not yet done + if prev_comm not in exec_times: + exec_times[prev_comm] = 0 + + # Compute previous command execution time + diff = cur_ts - last_ts + + # Update execution time of this command + exec_times[prev_comm] += diff + + # Update last timestamp + last_ts = cur_ts + + # Display top 5 + for name, ns in exec_times.most_common(5): + s = ns / 1000000000 + print('{:20}{} s'.format(name, s)) + + return True + + +if __name__ == '__main__': + sys.exit(0 if top5proc() else 1) +---- + +Run this script: + +[role="term"] +---- +python3 top5proc.py ~/lttng-traces/my-kernel-session*/kernel +---- + +Output example: + +---- +swapper/0 48.607245889 s +chromium 7.192738188 s +pavucontrol 0.709894415 s +Compositor 0.660867933 s +Xorg.bin 0.616753786 s +---- + +Note that `swapper/0` is the "idle" process of CPU 0 on Linux; since we +weren't using the CPU that much when tracing, its first position in the +list makes sense. + + +[[core-concepts]] +== [[understanding-lttng]]Core concepts + +From a user's perspective, the LTTng system is built on a few concepts, +or objects, on which the <> +operates by sending commands to the <>. +Understanding how those objects relate to eachother is key in mastering +the toolkit. + +The core concepts are: + +* <> +* <> +* <> +* <<"event","Instrumentation point, event rule, event, and event record">> + + +[[tracing-session]] +=== Tracing session + +A _tracing session_ is a stateful dialogue between you and +a <>. You can +<> with the `lttng create` command. + +Anything that you do when you control LTTng tracers happens within a +tracing session. In particular, a tracing session: + +* Has its own name. +* Has its own set of trace files. +* Has its own state of activity (started or stopped). +* Has its own <> (local, network streaming, + snapshot, or live). +* Has its own <> which have their own + <>. + +[role="img-100"] +.A _tracing session_ contains <> that are members of <> and contain <>. +image::concepts.png[] + +Those attributes and objects are completely isolated between different +tracing sessions. + +A tracing session is analogous to a cash machine session: +the operations you do on the banking system through the cash machine do +not alter the data of other users of the same system. In the case of +the cash machine, a session lasts as long as your bank card is inside. +In the case of LTTng, a tracing session lasts from the `lttng create` +command to the `lttng destroy` command. + +[role="img-100"] +.Each Unix user has its own set of tracing sessions. +image::many-sessions.png[] + + +[[tracing-session-mode]] +==== Tracing session mode + +LTTng can send the generated trace data to different locations. The +_tracing session mode_ dictates where to send it. The following modes +are available in LTTng{nbsp}{revision}: + +Local mode:: + LTTng writes the traces to the file system of the machine being traced + (target system). + +Network streaming mode:: + LTTng sends the traces over the network to a + <> running on a remote system. + +Snapshot mode:: + LTTng does not write the traces by default. Instead, you can request + LTTng to <>, that is, a copy of the + current tracing buffers, and to write it to the target's file system + or to send it over the network to a <> + running on a remote system. + +Live mode:: + This mode is similar to the network streaming mode, but a live + trace viewer can connect to the distant relay daemon to + <> by + the tracers. + + +[[domain]] +=== Tracing domain + +A _tracing domain_ is a namespace for event sources. A tracing domain +has its own properties and features. + +There are currently five available tracing domains: + +* Linux kernel +* User space +* `java.util.logging` (JUL) +* log4j +* Python + +You must specify a tracing domain when using some commands to avoid +ambiguity. For example, since all the domains support named tracepoints +as event sources (instrumentation points that you manually insert in the +source code), you need to specify a tracing domain when +<> because all the +tracing domains could have tracepoints with the same names. + +Some features are reserved to specific tracing domains. Dynamic function +entry and return instrumentation points, for example, are currently only +supported in the Linux kernel tracing domain, but support for other +tracing domains could be added in the future. + +You can create <> in the Linux kernel and user space +tracing domains. The other tracing domains have a single default +channel. + + +[[channel]] +=== Channel and ring buffer + +A _channel_ is an object which is responsible for a set of ring buffers. +Each ring buffer is divided into multiple sub-buffers. When an LTTng +tracer emits an event, it can record it to one or more +sub-buffers. The attributes of a channel determine what to do when +there's no space left for a new event record because all sub-buffers +are full, where to send a full sub-buffer, and other behaviours. + +A channel is always associated to a <>. The +`java.util.logging` (JUL), log4j, and Python tracing domains each have +a default channel which you cannot configure. + +A channel also owns <>. When an LTTng tracer emits +an event, it records it to the sub-buffers of all +the enabled channels with a satisfied event rule, as long as those +channels are part of active <>. + + +[[channel-buffering-schemes]] +==== Per-user vs. per-process buffering schemes + +A channel has at least one ring buffer _per CPU_. LTTng always +records an event to the ring buffer associated to the CPU on which it +occurred. + +Two _buffering schemes_ are available when you +<> in the +user space <>: + +Per-user buffering:: + Allocate one set of ring buffers--one per CPU--shared by all the + instrumented processes of each Unix user. ++ +-- +[role="img-100"] +.Per-user buffering scheme. +image::per-user-buffering.png[] +-- + +Per-process buffering:: + Allocate one set of ring buffers--one per CPU--for each + instrumented process. ++ +-- +[role="img-100"] +.Per-process buffering scheme. +image::per-process-buffering.png[] +-- ++ +The per-process buffering scheme tends to consume more memory than the +per-user option because systems generally have more instrumented +processes than Unix users running instrumented processes. However, the +per-process buffering scheme ensures that one process having a high +event throughput won't fill all the shared sub-buffers of the same +user, only its own. + +The Linux kernel tracing domain has only one available buffering scheme +which is to allocate a single set of ring buffers for the whole system. +This scheme is similar to the per-user option, but with a single, global +user "running" the kernel. + + +[[channel-overwrite-mode-vs-discard-mode]] +==== Overwrite vs. discard event loss modes + +When an event occurs, LTTng records it to a specific sub-buffer (yellow +arc in the following animation) of a specific channel's ring buffer. +When there's no space left in a sub-buffer, the tracer marks it as +consumable (red) and another, empty sub-buffer starts receiving the +following event records. A <> +eventually consumes the marked sub-buffer (returns to white). + +[NOTE] +[role="docsvg-channel-subbuf-anim"] +==== +{note-no-anim} +==== + +In an ideal world, sub-buffers are consumed faster than they are filled, +as is the case in the previous animation. In the real world, +however, all sub-buffers can be full at some point, leaving no space to +record the following events. + +By design, LTTng is a _non-blocking_ tracer: when no empty sub-buffer is +available, it is acceptable to lose event records when the alternative +would be to cause substantial delays in the instrumented application's +execution. LTTng privileges performance over integrity; it aims at +perturbing the traced system as little as possible in order to make +tracing of subtle race conditions and rare interrupt cascades possible. + +When it comes to losing event records because no empty sub-buffer is +available, the channel's _event loss mode_ determines what to do. The +available event loss modes are: + +Discard mode:: + Drop the newest event records until a the tracer + releases a sub-buffer. + +Overwrite mode:: + Clear the sub-buffer containing the oldest event records and start + writing the newest event records there. ++ +This mode is sometimes called _flight recorder mode_ because it's +similar to a +https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]: +always keep a fixed amount of the latest data. + +Which mechanism you should choose depends on your context: prioritize +the newest or the oldest event records in the ring buffer? + +Beware that, in overwrite mode, the tracer abandons a whole sub-buffer +as soon as a there's no space left for a new event record, whereas in +discard mode, the tracer only discards the event record that doesn't +fit. + +In discard mode, LTTng increments a count of lost event records when +an event record is lost and saves this count to the trace. In +overwrite mode, LTTng keeps no information when it overwrites a +sub-buffer before consuming it. + +There are a few ways to decrease your probability of losing event +records. +<> shows +how you can fine-une the sub-buffer count and size of a channel to +virtually stop losing event records, though at the cost of greater +memory usage. + + +[[channel-subbuf-size-vs-subbuf-count]] +==== Sub-buffer count and size + +When you <>, you can +set its number of sub-buffers and their size. + +Note that there is noticeable CPU overhead introduced when +switching sub-buffers (marking a full one as consumable and switching +to an empty one for the following events to be recorded). Knowing this, +the following list presents a few practical situations along with how +to configure the sub-buffer count and size for them: + +* **High event throughput**: In general, prefer bigger sub-buffers to + lower the risk of losing event records. ++ +Having bigger sub-buffers also ensures a lower sub-buffer switching +frequency. ++ +The number of sub-buffers is only meaningful if you create the channel +in overwrite mode: in this case, if a sub-buffer overwrite happens, the +other sub-buffers are left unaltered. + +* **Low event throughput**: In general, prefer smaller sub-buffers + since the risk of losing event records is low. ++ +Because events occur less frequently, the sub-buffer switching frequency +should remain low and thus the tracer's overhead should not be a +problem. + +* **Low memory system**: If your target system has a low memory + limit, prefer fewer first, then smaller sub-buffers. ++ +Even if the system is limited in memory, you want to keep the +sub-buffers as big as possible to avoid a high sub-buffer switching +frequency. + +Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format, +which means event data is very compact. For example, the average +LTTng kernel event record weights about 32{nbsp}bytes. Thus, a +sub-buffer size of 1{nbsp}MiB is considered big. + +The previous situations highlight the major trade-off between a few big +sub-buffers and more, smaller sub-buffers: sub-buffer switching +frequency vs. how much data is lost in overwrite mode. Assuming a +constant event throughput and using the overwrite mode, the two +following configurations have the same ring buffer total size: + +[NOTE] +[role="docsvg-channel-subbuf-size-vs-count-anim"] +==== +{note-no-anim} +==== + +* **2 sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer + switching frequency, but if a sub-buffer overwrite happens, half of + the event records so far (4{nbsp}MiB) are definitely lost. +* **8 sub-buffers of 1{nbsp}MiB each**: Expect 4{nbsp}times the tracer's + overhead as the previous configuration, but if a sub-buffer + overwrite happens, only the eighth of event records so far are + definitely lost. + +In discard mode, the sub-buffers count parameter is pointless: use two +sub-buffers and set their size according to the requirements of your +situation. + + +[[channel-switch-timer]] +==== Switch timer period + +The _switch timer period_ is an important configurable attribute of +a channel to ensure periodic sub-buffer flushing. + +When the _switch timer_ expires, a sub-buffer switch happens. You can +set the switch timer period attribute when you +<> to ensure that event +data is consumed and committed to trace files or to a distant relay +daemon periodically in case of a low event throughput. + +[NOTE] +[role="docsvg-channel-switch-timer"] +==== +{note-no-anim} +==== + +This attribute is also convenient when you use big sub-buffers to cope +with a sporadic high event throughput, even if the throughput is +normally low. + + +[[channel-read-timer]] +==== Read timer period + +By default, the LTTng tracers use a notification mechanism to signal a +full sub-buffer so that a consumer daemon can consume it. When such +notifications must be avoided, for example in real-time applications, +you can use the channel's _read timer_ instead. When the read timer +fires, the <> checks for full, +consumable sub-buffers. + + +[[tracefile-rotation]] +==== Trace file count and size + +By default, trace files can grow as large as needed. You can set the +maximum size of each trace file that a channel writes when you +<>. When the size of +a trace file reaches the channel's fixed maximum size, LTTng creates +another file to contain the next event records. LTTng appends a file +count to each trace file name in this case. + +If you set the trace file size attribute when you create a channel, the +maximum number of trace files that LTTng creates is _unlimited_ by +default. To limit them, you can also set a maximum number of trace +files. When the number of trace files reaches the channel's fixed +maximum count, the oldest trace file is overwritten. This mechanism is +called _trace file rotation_. + + +[[event]] +=== Instrumentation point, event rule, event, and event record + +An _event rule_ is a set of conditions which must be **all** satisfied +for LTTng to record an occuring event. + +You set the conditions when you <>. + +You always attach an event rule to <> when you create +it. + +When an event passes the conditions of an event rule, LTTng records it +in one of the attached channel's sub-buffers. + +The available conditions, as of LTTng{nbsp}{revision}, are: + +* The event rule _is enabled_. +* The instrumentation point's type _is{nbsp}T_. +* The instrumentation point's name (sometimes called _event name_) + _matches{nbsp}N_, but _is not{nbsp}E_. +* The instrumentation point's log level _is as severe as{nbsp}L_, or + _is exactly{nbsp}L_. +* The fields of the event's payload _satisfy_ a filter + expression{nbsp}__F__. + +As you can see, all the conditions but the dynamic filter are related to +the event rule's status or to the instrumentation point, not to the +occurring events. This is why, without a filter, checking if an event +passes an event rule is not a dynamic task: when you create or modify an +event rule, all the tracers of its tracing domain enable or disable the +instrumentation points themselves once. This is possible because the +attributes of an instrumentation point (type, name, and log level) are +defined statically. In other words, without a dynamic filter, the tracer +_does not evaluate_ the arguments of an instrumentation point unless it +matches an enabled event rule. + +Note that, for LTTng to record an event, the <> to +which a matching event rule is attached must also be enabled, and the +tracing session owning this channel must be active. + +[role="img-100"] +.Logical path from an instrumentation point to an event record. +image::event-rule.png[] + +.Event, event record, or event rule? +**** +With so many similar terms, it's easy to get confused. + +An **event** is the consequence of the execution of an _instrumentation +point_, like a tracepoint that you manually place in some source code, +or a Linux kernel KProbe. An event is said to _occur_ at a specific +time. Different actions can be taken upon the occurance of an event, +like record the event's payload to a buffer. + +An **event record** is the representation of an event in a sub-buffer. A +tracer is responsible for capturing the payload of an event, current +context variables, the event's ID, and the event's timestamp. LTTng +can append this sub-buffer to a trace file. + +An **event rule** is a set of conditions which must all be satisfied for +LTTng to record an occuring event. Events still occur without +satisfying event rules, but LTTng does not record them. +**** + + +[[plumbing]] +== Components of noch:{LTTng} + +The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong +to call LTTng a simple _tool_ since it is composed of multiple +interacting components. This section describes those components, +explains their respective roles, and shows how they connect together to +form the LTTng ecosystem. + +The following diagram shows how the most important components of LTTng +interact with user applications, the Linux kernel, and you: + +[role="img-100"] +.Control and trace data paths between LTTng components. +image::plumbing.png[] + +The LTTng project incorporates: + +* **LTTng-tools**: Libraries and command-line interface to + control tracing sessions. +** <> (man:lttng-sessiond(8)). +** <> (man:lttng-consumerd(8)). +** <> (man:lttng-relayd(8)). +** <> (`liblttng-ctl`). +** <> (man:lttng(1)). +* **LTTng-UST**: Libraries and Java/Python packages to trace user + applications. +** <> (`liblttng-ust`) and its + headers to instrument and trace any native user application. +** <>: +*** `liblttng-ust-libc-wrapper` +*** `liblttng-ust-pthread-wrapper` +*** `liblttng-ust-cyg-profile` +*** `liblttng-ust-cyg-profile-fast` +*** `liblttng-ust-dl` +** User space tracepoint provider source files generator command-line + tool (man:lttng-gen-tp(1)). +** <> to instrument and trace + Java applications using `java.util.logging` or + Apache log4j 1.2 logging. +** <> to instrument + Python applications using the standard `logging` package. +* **LTTng-modules**: <> to trace + the kernel. +** LTTng kernel tracer module. +** Tracing ring buffer kernel modules. +** Probe kernel modules. +** LTTng logger kernel module. + + +[[lttng-cli]] +=== Tracing control command-line interface + +[role="img-100"] +.The tracing control command-line interface. +image::plumbing-lttng-cli.png[] + +The _man:lttng(1) command-line tool_ is the standard user interface to +control LTTng <>. The cmd:lttng tool +is part of LTTng-tools. + +The cmd:lttng tool is linked with +<> to communicate with +one or more <> behind the scenes. + +The cmd:lttng tool has a Git-like interface: + +[role="term"] +---- +lttng +---- + +The <> section explores the +available features of LTTng using the cmd:lttng tool. + + +[[liblttng-ctl-lttng]] +=== Tracing control library + +[role="img-100"] +.The tracing control library. +image::plumbing-liblttng-ctl.png[] + +The _LTTng control library_, `liblttng-ctl`, is used to communicate +with a <> using a C API that hides the +underlying protocol's details. `liblttng-ctl` is part of LTTng-tools. + +The <> +is linked with `liblttng-ctl`. + +You can use `liblttng-ctl` in C or $$C++$$ source code by including its +"master" header: + +[source,c] +---- +#include +---- + +Some objects are referenced by name (C string), such as tracing +sessions, but most of them require to create a handle first using +`lttng_create_handle()`. + +The best available developer documentation for `liblttng-ctl` is, as of +LTTng{nbsp}{revision}, its installed header files. Every function and +structure is thoroughly documented. + + +[[lttng-ust]] +=== User space tracing library + +[role="img-100"] +.The user space tracing library. +image::plumbing-liblttng-ust.png[] + +The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)), +is the LTTng user space tracer. It receives commands from a +<>, for example to +enable and disable specific instrumentation points, and writes event +records to ring buffers shared with a +<>. +`liblttng-ust` is part of LTTng-UST. + +Public C header files are installed beside `liblttng-ust` to +instrument any <>. + +<>, which are regular Java and Python +packages, use their own library providing tracepoints which is +linked with `liblttng-ust`. + +An application or library does not have to initialize `liblttng-ust` +manually: its constructor does the necessary tasks to properly register +to a session daemon. The initialization phase also enables the +instrumentation points matching the <> that you +already created. + + +[[lttng-ust-agents]] +=== User space tracing agents + +[role="img-100"] +.The user space tracing agents. +image::plumbing-lttng-ust-agents.png[] + +The _LTTng-UST Java and Python agents_ are regular Java and Python +packages which add LTTng tracing capabilities to the +native logging frameworks. The LTTng-UST agents are part of LTTng-UST. + +In the case of Java, the +https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging` +core logging facilities] and +https://logging.apache.org/log4j/1.2/[Apache log4j 1.2] are supported. +Note that Apache Log4{nbsp}2 is not supported. + +In the case of Python, the standard +https://docs.python.org/3/library/logging.html[`logging`] package +is supported. Both Python 2 and Python 3 modules can import the +LTTng-UST Python agent package. + +The applications using the LTTng-UST agents are in the +`java.util.logging` (JUL), +log4j, and Python <>. + +Both agents use the same mechanism to trace the log statements. When an +agent is initialized, it creates a log handler that attaches to the root +logger. The agent also registers to a <>. +When the application executes a log statement, it is passed to the +agent's log handler by the root logger. The agent's log handler calls a +native function in a tracepoint provider package shared library linked +with <>, passing the formatted log message and +other fields, like its logger name and its log level. This native +function contains a user space instrumentation point, hence tracing the +log statement. + +The log level condition of an +<> is considered when tracing +a Java or a Python application, and it's compatible with the standard +JUL, log4j, and Python log levels. + + +[[lttng-modules]] +=== LTTng kernel modules + +[role="img-100"] +.The LTTng kernel modules. +image::plumbing-lttng-modules.png[] + +The _LTTng kernel modules_ are a set of Linux kernel modules +which implement the kernel tracer of the LTTng project. The LTTng +kernel modules are part of LTTng-modules. + +The LTTng kernel modules include: + +* A set of _probe_ modules. ++ +Each module attaches to a specific subsystem +of the Linux kernel using its tracepoint instrument points. There are +also modules to attach to the entry and return points of the Linux +system call functions. + +* _Ring buffer_ modules. ++ +A ring buffer implementation is provided as kernel modules. The LTTng +kernel tracer writes to the ring buffer; a +<> reads from the ring buffer. + +* The _LTTng kernel tracer_ module. +* The _LTTng logger_ module. ++ +The LTTng logger module implements the special path:{/proc/lttng-logger} +file so that any executable can generate LTTng events by opening and +writing to this file. ++ +See <>. + +Generally, you do not have to load the LTTng kernel modules manually +(using man:modprobe(8), for example): a root <> loads the necessary modules when starting. If you have extra +probe modules, you can specify to load them to the session daemon on +the command line. + +The LTTng kernel modules are installed in ++/usr/lib/modules/__release__/extra+ by default, where +__release__+ is +the kernel release (see `uname --kernel-release`). + + +[[lttng-sessiond]] +=== Session daemon + +[role="img-100"] +.The session daemon. +image::plumbing-sessiond.png[] + +The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for +managing tracing sessions and for controlling the various components of +LTTng. The session daemon is part of LTTng-tools. + +The session daemon sends control requests to and receives control +responses from: + +* The <>. ++ +Any instance of the user space tracing library first registers to +a session daemon. Then, the session daemon can send requests to +this instance, such as: ++ +-- +** Get the list of tracepoints. +** Share an <> so that the user space tracing library + can enable or disable tracepoints. Amongst the possible conditions + of an event rule is a filter expression which `liblttng-ust` evalutes + when an event occurs. +** Share <> attributes and ring buffer locations. +-- ++ +The session daemon and the user space tracing library use a Unix +domain socket for their communication. + +* The <>. ++ +Any instance of a user space tracing agent first registers to +a session daemon. Then, the session daemon can send requests to +this instance, such as: ++ +-- +** Get the list of loggers. +** Enable or disable a specific logger. +-- ++ +The session daemon and the user space tracing agent use a TCP connection +for their communication. + +* The <>. +* The <>. ++ +The session daemon sends requests to the consumer daemon to instruct +it where to send the trace data streams, amongst other information. + +* The <>. + +The session daemon receives commands from the +<>. + +The root session daemon loads the appropriate +<> on startup. It also spawns +a <> as soon as you create +an <>. + +The session daemon does not send and receive trace data: this is the +role of the <> and +<>. It does, however, generate the +http://diamon.org/ctf/[CTF] metadata stream. + +Each Unix user can have its own session daemon instance. The +tracing sessions managed by different session daemons are completely +independent. + +The root user's session daemon is the only one which is +allowed to control the LTTng kernel tracer, and its spawned consumer +daemon is the only one which is allowed to consume trace data from the +LTTng kernel tracer. Note, however, that any Unix user which is a member +of the <> is allowed +to create <> in the +Linux kernel <>, and thus to trace the Linux +kernel. + +The <> automatically starts a +session daemon when using its `create` command if none is currently +running. You can also start the session daemon manually. + + +[[lttng-consumerd]] +=== Consumer daemon + +[role="img-100"] +.The consumer daemon. +image::plumbing-consumerd.png[] + +The _consumer daemon_, man:lttng-consumerd(8), is a daemon which shares +ring buffers with user applications or with the LTTng kernel modules to +collect trace data and send it to some location (on disk or to a +<> over the network). The consumer daemon +is part of LTTng-tools. + +You do not start a consumer daemon manually: a consumer daemon is always +spawned by a <> as soon as you create an +<>, that is, before you start tracing. When you kill +its owner session daemon, the consumer daemon also exits because it is +the session daemon's child process. Command-line options of +man:lttng-sessiond(8) target the consumer daemon process. + +There are up to two running consumer daemons per Unix user, whereas only +one session daemon can run per user. This is because each process can be +either 32-bit or 64-bit: if the target system runs a mixture of 32-bit +and 64-bit processes, it is more efficient to have separate +corresponding 32-bit and 64-bit consumer daemons. The root user is an +exception: it can have up to _three_ running consumer daemons: 32-bit +and 64-bit instances for its user applications, and one more +reserved for collecting kernel trace data. + + +[[lttng-relayd]] +=== Relay daemon + +[role="img-100"] +.The relay daemon. +image::plumbing-relayd.png[] + +The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge +between remote session and consumer daemons, local trace files, and a +remote live trace viewer. The relay daemon is part of LTTng-tools. + +The main purpose of the relay daemon is to implement a receiver of +<>. +This is useful when the target system does not have much file system +space to record trace files locally. + +The relay daemon is also a server to which a +<> can +connect. The live trace viewer sends requests to the relay daemon to +receive trace data as the target system emits events. The +communication protocol is named _LTTng live_; it is used over TCP +connections. + +Note that you can start the relay daemon on the target system directly. +This is the setup of choice when the use case is to view events as +the target system emits them without the need of a remote system. + + +[[instrumenting]] +== [[using-lttng]]Instrumentation + +There are many examples of tracing and monitoring in our everyday life: + +* You have access to real-time and historical weather reports and + forecasts thanks to weather stations installed around the country. +* You know your heart is safe thanks to an electrocardiogram. +* You make sure not to drive your car too fast and to have enough fuel + to reach your destination thanks to gauges visible on your dashboard. + +All the previous examples have something in common: they rely on +**instruments**. Without the electrodes attached to the surface of your +body's skin, cardiac monitoring is futile. + +LTTng, as a tracer, is no different from those real life examples. If +you're about to trace a software system or, in other words, record its +history of execution, you better have **instrumentation points** in the +subject you're tracing, that is, the actual software. + +Various ways were developed to instrument a piece of software for LTTng +tracing. The most straightforward one is to manually place +instrumentation points, called _tracepoints_, in the software's source +code. It is also possible to add instrumentation points dynamically in +the Linux kernel <>. + +If you're only interested in tracing the Linux kernel, your +instrumentation needs are probably already covered by LTTng's built-in +<>. You may also wish to trace a +user application which is already instrumented for LTTng tracing. +In such cases, you can skip this whole section and read the topics of +the <> section. + +Many methods are available to instrument a piece of software for LTTng +tracing. They are: + +* <>. +* <>. +* <>. +* <>. +* <>. +* <>. + + +[[c-application]] +=== [[cxx-application]]User space instrumentation for C and $$C++$$ applications + +The procedure to instrument a C or $$C++$$ user application with +the <>, `liblttng-ust`, is: + +. <>. +. <>. +. <>. + +If you need quick, man:printf(3)-like instrumentation, you can skip +those steps and use <> or <> +instead. + +IMPORTANT: You need to <> LTTng-UST to +instrument a user application with `liblttng-ust`. + + +[[tracepoint-provider]] +==== Create the source files of a tracepoint provider package + +A _tracepoint provider_ is a set of compiled functions which provide +**tracepoints** to an application, the type of instrumentation point +supported by LTTng-UST. Those functions can emit events with +user-defined fields and serialize those events as event records to one +or more LTTng-UST <> sub-buffers. The `tracepoint()` +macro, which you <>, calls those functions. + +A _tracepoint provider package_ is an object file (`.o`) or a shared +library (`.so`) which contains one or more tracepoint providers. +Its source files are: + +* One or more <> (`.h`). +* A <> (`.c`). + +A tracepoint provider package is dynamically linked with `liblttng-ust`, +the LTTng user space tracer, at run time. + +[role="img-100"] +.User application linked with `liblttng-ust` and containing a tracepoint provider. +image::ust-app.png[] + +NOTE: If you need quick, man:printf(3)-like instrumentation, you can +skip creating and using a tracepoint provider and use +<> or <> instead. + + +[[tpp-header]] +===== Create a tracepoint provider header file template + +A _tracepoint provider header file_ contains the tracepoint +definitions of a tracepoint provider. + +To create a tracepoint provider header file: + +. Start from this template: ++ +-- +[source,c] +.Tracepoint provider header file template (`.h` file extension). +---- +#undef TRACEPOINT_PROVIDER +#define TRACEPOINT_PROVIDER provider_name + +#undef TRACEPOINT_INCLUDE +#define TRACEPOINT_INCLUDE "./tp.h" + +#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ) +#define _TP_H + +#include + +/* + * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(), + * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here. + */ + +#endif /* _TP_H */ + +#include +---- +-- + +. Replace: ++ +* `provider_name` with the name of your tracepoint provider. +* `"tp.h"` with the name of your tracepoint provider header file. + +. Below the `#include ` line, put your + <>. + +Your tracepoint provider name must be unique amongst all the possible +tracepoint provider names used on the same target system. We +suggest to include the name of your project or company in the name, +for example, `org_lttng_my_project_tpp`. + +TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create +this boilerplate for you. When using cmd:lttng-gen-tp, all you need to +write are the <>. + + +[[defining-tracepoints]] +===== Create a tracepoint definition + +A _tracepoint definition_ defines, for a given tracepoint: + +* Its **input arguments**. They are the macro parameters that the + `tracepoint()` macro accepts for this particular tracepoint + in the user application's source code. +* Its **output event fields**. They are the sources of event fields + that form the payload of any event that the execution of the + `tracepoint()` macro emits for this particular tracepoint. + +You can create a tracepoint definition by using the +`TRACEPOINT_EVENT()` macro below the `#include ` +line in the +<>. + +The syntax of the `TRACEPOINT_EVENT()` macro is: + +[source,c] +.`TRACEPOINT_EVENT()` macro syntax. +---- +TRACEPOINT_EVENT( + /* Tracepoint provider name */ + provider_name, + + /* Tracepoint name */ + tracepoint_name, + + /* Input arguments */ + TP_ARGS( + arguments + ), + + /* Output event fields */ + TP_FIELDS( + fields + ) +) +---- + +Replace: + +* `provider_name` with your tracepoint provider name. +* `tracepoint_name` with your tracepoint name. +* `arguments` with the <>. +* `fields` with the <> + definitions. + +This tracepoint emits events named `provider_name:tracepoint_name`. + +[IMPORTANT] +.Event name's length limitation +==== +The concatenation of the tracepoint provider name and the +tracepoint name must not exceed **254 characters**. If it does, the +instrumented application compiles and runs, but LTTng throws multiple +warnings and you could experience serious issues. +==== + +[[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is: + +[source,c] +.`TP_ARGS()` macro syntax. +---- +TP_ARGS( + type, arg_name +) +---- + +Replace: + +* `type` with the C type of the argument. +* `arg_name` with the argument name. + +You can repeat `type` and `arg_name` up to 10 times to have +more than one argument. + +.`TP_ARGS()` usage with three arguments. +==== +[source,c] +---- +TP_ARGS( + int, count, + float, ratio, + const char*, query +) +---- +==== + +The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a +tracepoint definition with no input arguments. + +[[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of +`ctf_*()` macros. Each `ctf_*()` macro defines one event field. See +man:lttng-ust(3) for a complete description of the available `ctf_*()` +macros. A `ctf_*()` macro specifies the type, size, and byte order of +one event field. + +Each `ctf_*()` macro takes an _argument expression_ parameter. This is a +C expression that the tracer evalutes at the `tracepoint()` macro site +in the application's source code. This expression provides a field's +source of data. The argument expression can include input argument names +listed in the `TP_ARGS()` macro. + +Each `ctf_*()` macro also takes a _field name_ parameter. Field names +must be unique within a given tracepoint definition. + +Here's a complete tracepoint definition example: + +.Tracepoint definition. +==== +The following tracepoint definition defines a tracepoint which takes +three input arguments and has four output event fields. + +[source,c] +---- +#include "my-custom-structure.h" + +TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + TP_ARGS( + const struct my_custom_structure*, my_custom_structure, + float, ratio, + const char*, query + ), + TP_FIELDS( + ctf_string(query_field, query) + ctf_float(double, ratio_field, ratio) + ctf_integer(int, recv_size, my_custom_structure->recv_size) + ctf_integer(int, send_size, my_custom_structure->send_size) + ) +) +---- + +You can refer to this tracepoint definition with the `tracepoint()` +macro in your application's source code like this: + +[source,c] +---- +tracepoint(my_provider, my_tracepoint, + my_structure, some_ratio, the_query); +---- +==== + +NOTE: The LTTng tracer only evaluates tracepoint arguments at run time +if they satisfy an enabled <>. + + +[[using-tracepoint-classes]] +===== Use a tracepoint class + +A _tracepoint class_ is a class of tracepoints which share the same +output event field definitions. A _tracepoint instance_ is one +instance of such a defined tracepoint class, with its own tracepoint +name. + +The <> is actually a +shorthand which defines both a tracepoint class and a tracepoint +instance at the same time. + +When you build a tracepoint provider package, the C or $$C++$$ compiler +creates one serialization function for each **tracepoint class**. A +serialization function is responsible for serializing the event fields +of a tracepoint to a sub-buffer when tracing. + +For various performance reasons, when your situation requires multiple +tracepoint definitions with different names, but with the same event +fields, we recommend that you manually create a tracepoint class +and instantiate as many tracepoint instances as needed. One positive +effect of such a design, amongst other advantages, is that all +tracepoint instances of the same tracepoint class reuse the same +serialization function, thus reducing +https://en.wikipedia.org/wiki/Cache_pollution[cache pollution]. + +.Use a tracepoint class and tracepoint instances. +==== +Consider the following three tracepoint definitions: + +[source,c] +---- +TRACEPOINT_EVENT( + my_app, + get_account, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +TRACEPOINT_EVENT( + my_app, + get_settings, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +TRACEPOINT_EVENT( + my_app, + get_transaction, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) +---- + +In this case, we create three tracepoint classes, with one implicit +tracepoint instance for each of them: `get_account`, `get_settings`, and +`get_transaction`. However, they all share the same event field names +and types. Hence three identical, yet independent serialization +functions are created when you build the tracepoint provider package. + +A better design choice is to define a single tracepoint class and three +tracepoint instances: + +[source,c] +---- +/* The tracepoint class */ +TRACEPOINT_EVENT_CLASS( + /* Tracepoint provider name */ + my_app, + + /* Tracepoint class name */ + my_class, + + /* Input arguments */ + TP_ARGS( + int, userid, + size_t, len + ), + + /* Output event fields */ + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +/* The tracepoint instances */ +TRACEPOINT_EVENT_INSTANCE( + /* Tracepoint provider name */ + my_app, + + /* Tracepoint class name */ + my_class, + + /* Tracepoint name */ + get_account, + + /* Input arguments */ + TP_ARGS( + int, userid, + size_t, len + ) +) +TRACEPOINT_EVENT_INSTANCE( + my_app, + my_class, + get_settings, + TP_ARGS( + int, userid, + size_t, len + ) +) +TRACEPOINT_EVENT_INSTANCE( + my_app, + my_class, + get_transaction, + TP_ARGS( + int, userid, + size_t, len + ) +) +---- +==== + + +[[assigning-log-levels]] +===== Assign a log level to a tracepoint definition + +You can assign an optional _log level_ to a +<>. + +Assigning different levels of severity to tracepoint definitions can +be useful: when you <>, +you can target tracepoints having a log level as severe as a specific +value. + +The concept of LTTng-UST log levels is similar to the levels found +in typical logging frameworks: + +* In a logging framework, the log level is given by the function + or method name you use at the log statement site: `debug()`, + `info()`, `warn()`, `error()`, and so on. +* In LTTng-UST, you statically assign the log level to a tracepoint + definition; any `tracepoint()` macro invocation which refers to + this definition has this log level. + +You can assign a log level to a tracepoint definition with the +`TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the +<> or +<> macro for a given +tracepoint. + +The syntax of the `TRACEPOINT_LOGLEVEL()` macro is: + +[source,c] +.`TRACEPOINT_LOGLEVEL()` macro syntax. +---- +TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level) +---- + +Replace: + +* `provider_name` with the tracepoint provider name. +* `tracepoint_name` with the tracepoint name. +* `log_level` with the log level to assign to the tracepoint + definition named `tracepoint_name` in the `provider_name` + tracepoint provider. ++ +See man:lttng-ust(3) for a list of available log level names. + +.Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition. +==== +[source,c] +---- +/* Tracepoint definition */ +TRACEPOINT_EVENT( + my_app, + get_transaction, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +/* Log level assignment */ +TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT) +---- +==== + + +[[tpp-source]] +===== Create a tracepoint provider package source file + +A _tracepoint provider package source file_ is a C source file which +includes a <> to expand its +macros into event serialization and other functions. + +You can always use the following tracepoint provider package source +file template: + +[source,c] +.Tracepoint provider package source file template. +---- +#define TRACEPOINT_CREATE_PROBES + +#include "tp.h" +---- + +Replace `tp.h` with the name of your <> name. You may also include more than one tracepoint +provider header file here to create a tracepoint provider package +holding more than one tracepoint providers. + + +[[probing-the-application-source-code]] +==== Add tracepoints to an application's source code + +Once you <>, you +can use the `tracepoint()` macro in your application's +source code to insert the tracepoints that this header +<>. + +The `tracepoint()` macro takes at least two parameters: the tracepoint +provider name and the tracepoint name. The corresponding tracepoint +definition defines the other parameters. + +.`tracepoint()` usage. +==== +The following <> defines a +tracepoint which takes two input arguments and has two output event +fields. + +[source,c] +.Tracepoint provider header file. +---- +#include "my-custom-structure.h" + +TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + TP_ARGS( + int, argc, + const char*, cmd_name + ), + TP_FIELDS( + ctf_string(cmd_name, cmd_name) + ctf_integer(int, number_of_args, argc) + ) +) +---- + +You can refer to this tracepoint definition with the `tracepoint()` +macro in your application's source code like this: + +[source,c] +.Application's source file. +---- +#include "tp.h" + +int main(int argc, char* argv[]) +{ + tracepoint(my_provider, my_tracepoint, argc, argv[0]); + + return 0; +} +---- + +Note how the application's source code includes +the tracepoint provider header file containing the tracepoint +definitions to use, path:{tp.h}. +==== + +.`tracepoint()` usage with a complex tracepoint definition. +==== +Consider this complex tracepoint definition, where multiple event +fields refer to the same input arguments in their argument expression +parameter: + +[source,c] +.Tracepoint provider header file. +---- +/* For `struct stat` */ +#include +#include +#include + +TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + TP_ARGS( + int, my_int_arg, + char*, my_str_arg, + struct stat*, st + ), + TP_FIELDS( + ctf_integer(int, my_constant_field, 23 + 17) + ctf_integer(int, my_int_arg_field, my_int_arg) + ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg) + ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] + + my_str_arg[2] + my_str_arg[3]) + ctf_string(my_str_arg_field, my_str_arg) + ctf_integer_hex(off_t, size_field, st->st_size) + ctf_float(double, size_dbl_field, (double) st->st_size) + ctf_sequence_text(char, half_my_str_arg_field, my_str_arg, + size_t, strlen(my_str_arg) / 2) + ) +) +---- + +You can refer to this tracepoint definition with the `tracepoint()` +macro in your application's source code like this: + +[source,c] +.Application's source file. +---- +#define TRACEPOINT_DEFINE +#include "tp.h" + +int main(void) +{ + struct stat s; + + stat("/etc/fstab", &s); + tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s); + + return 0; +} +---- + +If you look at the event record that LTTng writes when tracing this +program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes, +it should look like this: + +.Event record fields +|==== +|Field's name |Field's value +|`my_constant_field` |40 +|`my_int_arg_field` |23 +|`my_int_arg_field2` |529 +|`sum4_field` |389 +|`my_str_arg_field` |`Hello, World!` +|`size_field` |0x12d +|`size_dbl_field` |301.0 +|`half_my_str_arg_field` |`Hello,` +|==== +==== + +Sometimes, the arguments you pass to `tracepoint()` are expensive to +compute--they use the call stack, for example. To avoid this +computation when the tracepoint is disabled, you can use the +`tracepoint_enabled()` and `do_tracepoint()` macros. + +The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros +is: + +[source,c] +.`tracepoint_enabled()` and `do_tracepoint()` macros syntax. +---- +tracepoint_enabled(provider_name, tracepoint_name) +do_tracepoint(provider_name, tracepoint_name, ...) +---- + +Replace: + +* `provider_name` with the tracepoint provider name. +* `tracepoint_name` with the tracepoint name. + +`tracepoint_enabled()` returns a non-zero value if the tracepoint named +`tracepoint_name` from the provider named `provider_name` is enabled +**at run time**. + +`do_tracepoint()` is like `tracepoint()`, except that it doesn't check +if the tracepoint is enabled. Using `tracepoint()` with +`tracepoint_enabled()` is dangerous since `tracepoint()` also contains +the `tracepoint_enabled()` check, thus a race condition is +possible in this situation: + +[source,c] +.Possible race condition when using `tracepoint_enabled()` with `tracepoint()`. +---- +if (tracepoint_enabled(my_provider, my_tracepoint)) { + stuff = prepare_stuff(); +} + +tracepoint(my_provider, my_tracepoint, stuff); +---- + +If the tracepoint is enabled after the condition, then `stuff` is not +prepared: the emitted event will either contain wrong data, or the whole +application could crash (segmentation fault, for example). + +NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an +`STAP_PROBEV()` call. If you need it, you must emit +this call yourself. + + +[[building-tracepoint-providers-and-user-application]] +==== Build and link a tracepoint provider package and an application + +Once you have one or more <> and a <>, +you can create the tracepoint provider package by compiling its source +file. From here, multiple build and run scenarios are possible. The +following table shows common application and library configurations +along with the required command lines to achieve them. + +In the following diagrams, we use the following file names: + +`app`:: + Executable application. + +`app.o`:: + Application's object file. + +`tpp.o`:: + Tracepoint provider package object file. + +`tpp.a`:: + Tracepoint provider package archive file. + +`libtpp.so`:: + Tracepoint provider package shared object file. + +`emon.o`:: + User library object file. + +`libemon.so`:: + User library shared object file. + +We use the following symbols in the diagrams of table below: + +[role="img-100"] +.Symbols used in the build scenario diagrams. +image::ust-sit-symbols.png[] + +We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment +variable in the following instructions. + +[role="growable ust-scenarios",cols="asciidoc,asciidoc"] +.Common tracepoint provider package scenarios. +|==== +|Scenario |Instructions + +| +The instrumented application is statically linked with +the tracepoint provider package object. + +image::ust-sit+app-linked-with-tp-o+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o tpp.o -llttng-ust -ldl +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The instrumented application is statically linked with the +tracepoint provider package archive file. + +image::ust-sit+app-linked-with-tp-a+app-instrumented.png[] + +| +To create the tracepoint provider package archive file: + +. Compile the <>: ++ +-- +[role="term"] +---- +gcc -I. -c tpp.c +---- +-- + +. Create the tracepoint provider package archive file: ++ +-- +[role="term"] +---- +ar rcs tpp.a tpp.o +---- +-- + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o tpp.a -llttng-ust -ldl +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The instrumented application is linked with the tracepoint provider +package shared object. + +image::ust-sit+app-linked-with-tp-so+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -ldl -L. -ltpp +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The tracepoint provider package shared object is preloaded before the +instrumented application starts. + +image::ust-sit+tp-so-preloaded+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -ldl +---- +-- + +To run the instrumented application with tracing support: + +* Preload the tracepoint provider package shared object and + start the application: ++ +-- +[role="term"] +---- +LD_PRELOAD=./libtpp.so ./app +---- +-- + +To run the instrumented application without tracing support: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The instrumented application dynamically loads the tracepoint provider +package shared object. + +See the <>. + +image::ust-sit+app-dlopens-tp-so+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -ldl +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application is linked with the instrumented user library. + +The instrumented user library is statically linked with the tracepoint +provider package object file. + +image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o-fpic.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application is linked with the instrumented user library. + +The instrumented user library is linked with the tracepoint provider +package shared object. + +image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o -ldl -L. -ltpp +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The tracepoint provider package shared object is preloaded before the +application starts. + +The application is linked with the instrumented user library. + +image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -L. -lemon +---- +-- + +To run the application with tracing support: + +* Preload the tracepoint provider package shared object and + start the application: ++ +-- +[role="term"] +---- +LD_PRELOAD=./libtpp.so ./app +---- +-- + +To run the application without tracing support: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application is linked with the instrumented user library. + +The instrumented user library dynamically loads the tracepoint provider +package shared object. + +See the <>. + +image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application dynamically loads the instrumented user library. + +The instrumented user library is linked with the tracepoint provider +package shared object. + +See the <>. + +image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o -ldl -L. -ltpp +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -ldl -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application dynamically loads the instrumented user library. + +The instrumented user library dynamically loads the tracepoint provider +package shared object. + +See the <>. + +image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -ldl -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The tracepoint provider package shared object is preloaded before the +application starts. + +The application dynamically loads the instrumented user library. + +image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o -L. -lemon +---- +-- + +To run the application with tracing support: + +* Preload the tracepoint provider package shared object and + start the application: ++ +-- +[role="term"] +---- +LD_PRELOAD=./libtpp.so ./app +---- +-- + +To run the application without tracing support: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application is statically linked with the tracepoint provider +package object file. + +The application is linked with the instrumented user library. + +image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- + +| +The application is statically linked with the tracepoint provider +package object file. + +The application dynamically loads the instrumented user library. + +image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o.txt[] + +To build the application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +gcc -Wl,--export-dynamic -o app app.o tpp.o \ + -llttng-ust -ldl +---- +-- ++ +The `--export-dynamic` option passed to the linker is necessary for the +dynamically loaded library to ``see'' the tracepoint symbols defined in +the application. + +To build the instrumented user library: + +. Compile the user library source file: ++ +-- +[role="term"] +---- +gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +gcc -shared -o libemon.so emon.o +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +./app +---- +-- +|==== + +[[dlclose-warning]] +[IMPORTANT] +.Do not use man:dlclose(3) on a tracepoint provider package +==== +Never use man:dlclose(3) on any shared object which: + +* Is linked with, statically or dynamically, a tracepoint provider + package. +* Calls man:dlopen(3) itself to dynamically open a tracepoint provider + package shared object. + +This is currently considered **unsafe** due to a lack of reference +counting from LTTng-UST to the shared object. + +A known workaround (available since glibc 2.2) is to use the +`RTLD_NODELETE` flag when calling man:dlopen(3) initially. This has the +effect of not unloading the loaded shared object, even if man:dlclose(3) +is called. + +You can also preload the tracepoint provider package shared object with +the env:LD_PRELOAD environment variable to overcome this limitation. +==== + + +[[using-lttng-ust-with-daemons]] +===== Use noch:{LTTng-UST} with daemons + +If your instrumented application calls man:fork(2), man:clone(2), +or BSD's man:rfork(2), without a following man:exec(3)-family +system call, you must preload the path:{liblttng-ust-fork.so} shared +object when you start the application. + +[role="term"] +---- +LD_PRELOAD=liblttng-ust-fork.so ./my-app +---- + +If your tracepoint provider package is +a shared library which you also preload, you must put both +shared objects in env:LD_PRELOAD: + +[role="term"] +---- +LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app +---- + + +[role="since-2.9"] +[[liblttng-ust-fd]] +===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them + +If your instrumented application closes one or more file descriptors +which it did not open itself, you must preload the +path:{liblttng-ust-fd.so} shared object when you start the application: + +[role="term"] +---- +LD_PRELOAD=liblttng-ust-fd.so ./my-app +---- + +Typical use cases include closing all the file descriptors after +man:fork(2) or man:rfork(2) and buggy applications doing +``double closes''. + + +[[lttng-ust-pkg-config]] +===== Use noch:{pkg-config} + +On some distributions, LTTng-UST ships with a +https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config] +metadata file. If this is your case, then you can use cmd:pkg-config to +build an application on the command line: + +[role="term"] +---- +gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust) +---- + + +[[instrumenting-32-bit-app-on-64-bit-system]] +===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system + +In order to trace a 32-bit application running on a 64-bit system, +LTTng must use a dedicated 32-bit +<>. + +The following steps show how to build and install a 32-bit consumer +daemon, which is _not_ part of the default 64-bit LTTng build, how to +build and install the 32-bit LTTng-UST libraries, and how to build and +link an instrumented 32-bit application in that context. + +To build a 32-bit instrumented application for a 64-bit target system, +assuming you have a fresh target system with no installed Userspace RCU +or LTTng packages: + +. Download, build, and install a 32-bit version of Userspace RCU: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 && +tar -xf userspace-rcu-latest-0.9.tar.bz2 && +cd userspace-rcu-0.9.* && +./configure --libdir=/usr/local/lib32 CFLAGS=-m32 && +make && +sudo make install && +sudo ldconfig +---- +-- + +. Using your distribution's package manager, or from source, install + the following 32-bit versions of the following dependencies of + LTTng-tools and LTTng-UST: ++ +-- +* https://sourceforge.net/projects/libuuid/[libuuid] +* http://directory.fsf.org/wiki/Popt[popt] +* http://www.xmlsoft.org/[libxml2] +-- + +. Download, build, and install a 32-bit version of the latest + LTTng-UST{nbsp}{revision}: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.9.tar.bz2 && +tar -xf lttng-ust-latest-2.9.tar.bz2 && +cd lttng-ust-2.9.* && +./configure --libdir=/usr/local/lib32 \ + CFLAGS=-m32 CXXFLAGS=-m32 \ + LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' && +make && +sudo make install && +sudo ldconfig +---- +-- ++ +[NOTE] +==== +Depending on your distribution, +32-bit libraries could be installed at a different location than +`/usr/lib32`. For example, Debian is known to install +some 32-bit libraries in `/usr/lib/i386-linux-gnu`. + +In this case, make sure to set `LDFLAGS` to all the +relevant 32-bit library paths, for example: + +[role="term"] +---- +LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32' +---- +==== + +. Download the latest LTTng-tools{nbsp}{revision}, build, and install + the 32-bit consumer daemon: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.9.tar.bz2 && +tar -xf lttng-tools-latest-2.9.tar.bz2 && +cd lttng-tools-2.9.* && +./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \ + LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' && +make && +cd src/bin/lttng-consumerd && +sudo make install && +sudo ldconfig +---- +-- + +. From your distribution or from source, + <> the 64-bit versions of + LTTng-UST and Userspace RCU. +. Download, build, and install the 64-bit version of the + latest LTTng-tools{nbsp}{revision}: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.9.tar.bz2 && +tar -xf lttng-tools-latest-2.9.tar.bz2 && +cd lttng-tools-2.9.* && +./configure --with-consumerd32-libdir=/usr/local/lib32 \ + --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd && +make && +sudo make install && +sudo ldconfig +---- +-- + +. Pass the following options to man:gcc(1), man:g++(1), or man:clang(1) + when linking your 32-bit application: ++ +---- +-m32 -L/usr/lib32 -L/usr/local/lib32 \ +-Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 +---- ++ +For example, let's rebuild the quick start example in +<> as an +instrumented 32-bit application: ++ +-- +[role="term"] +---- +gcc -m32 -c -I. hello-tp.c +gcc -m32 -c hello.c +gcc -m32 -o hello hello.o hello-tp.o \ + -L/usr/lib32 -L/usr/local/lib32 \ + -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \ + -llttng-ust -ldl +---- +-- + +No special action is required to execute the 32-bit application and +to trace it: use the command-line man:lttng(1) tool as usual. + + +[role="since-2.5"] +[[tracef]] +==== Use `tracef()` + +man:tracef(3) is a small LTTng-UST API designed for quick, +man:printf(3)-like instrumentation without the burden of +<> and +<> +a tracepoint provider package. + +To use `tracef()` in your application: + +. In the C or C++ source files where you need to use `tracef()`, + include ``: ++ +-- +[source,c] +---- +#include +---- +-- + +. In the application's source code, use `tracef()` like you would use + man:printf(3): ++ +-- +[source,c] +---- + /* ... */ + + tracef("my message: %d (%s)", my_integer, my_string); + + /* ... */ +---- +-- + +. Link your application with `liblttng-ust`: ++ +-- +[role="term"] +---- +gcc -o app app.c -llttng-ust +---- +-- + +To trace the events that `tracef()` calls emit: + +* <> which matches the + `lttng_ust_tracef:*` event name: ++ +-- +[role="term"] +---- +lttng enable-event --userspace 'lttng_ust_tracef:*' +---- +-- + +[IMPORTANT] +.Limitations of `tracef()` +==== +The `tracef()` utility function was developed to make user space tracing +super simple, albeit with notable disadvantages compared to +<>: + +* All the emitted events have the same tracepoint provider and + tracepoint names, respectively `lttng_ust_tracef` and `event`. +* There is no static type checking. +* The only event record field you actually get, named `msg`, is a string + potentially containing the values you passed to `tracef()` + using your own format string. This also means that you cannot filter + events with a custom expression at run time because there are no + isolated fields. +* Since `tracef()` uses the C standard library's man:vasprintf(3) + function behind the scenes to format the strings at run time, its + expected performance is lower than with user-defined tracepoints, + which do not require a conversion to a string. + +Taking this into consideration, `tracef()` is useful for some quick +prototyping and debugging, but you should not consider it for any +permanent and serious applicative instrumentation. +==== + + +[role="since-2.7"] +[[tracelog]] +==== Use `tracelog()` + +The man:tracelog(3) API is very similar to <>, with +the difference that it accepts an additional log level parameter. + +The goal of `tracelog()` is to ease the migration from logging to +tracing. + +To use `tracelog()` in your application: + +. In the C or C++ source files where you need to use `tracelog()`, + include ``: ++ +-- +[source,c] +---- +#include +---- +-- + +. In the application's source code, use `tracelog()` like you would use + man:printf(3), except for the first parameter which is the log + level: ++ +-- +[source,c] +---- + /* ... */ + + tracelog(TRACE_WARNING, "my message: %d (%s)", + my_integer, my_string); + + /* ... */ +---- +-- ++ +See man:lttng-ust(3) for a list of available log level names. + +. Link your application with `liblttng-ust`: ++ +-- +[role="term"] +---- +gcc -o app app.c -llttng-ust +---- +-- + +To trace the events that `tracelog()` calls emit with a log level +_as severe as_ a specific log level: + +* <> which matches the + `lttng_ust_tracelog:*` event name and a minimum level + of severity: ++ +-- +[role="term"] +---- +lttng enable-event --userspace 'lttng_ust_tracelog:*' + --loglevel=TRACE_WARNING +---- +-- + +To trace the events that `tracelog()` calls emit with a +_specific log level_: + +* Create an event rule which matches the `lttng_ust_tracelog:*` + event name and a specific log level: ++ +-- +[role="term"] +---- +lttng enable-event --userspace 'lttng_ust_tracelog:*' + --loglevel-only=TRACE_INFO +---- +-- + + +[[prebuilt-ust-helpers]] +=== Prebuilt user space tracing helpers + +The LTTng-UST package provides a few helpers in the form or preloadable +shared objects which automatically instrument system functions and +calls. + +The helper shared objects are normally found in dir:{/usr/lib}. If you +built LTTng-UST <>, they are probably +located in dir:{/usr/local/lib}. + +The installed user space tracing helpers in LTTng-UST{nbsp}{revision} +are: + +path:{liblttng-ust-libc-wrapper.so}:: +path:{liblttng-ust-pthread-wrapper.so}:: + <>. + +path:{liblttng-ust-cyg-profile.so}:: +path:{liblttng-ust-cyg-profile-fast.so}:: + <>. + +path:{liblttng-ust-dl.so}:: + <>. + +To use a user space tracing helper with any user application: + +* Preload the helper shared object when you start the application: ++ +-- +[role="term"] +---- +LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app +---- +-- ++ +You can preload more than one helper: ++ +-- +[role="term"] +---- +LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app +---- +-- + + +[role="since-2.3"] +[[liblttng-ust-libc-pthread-wrapper]] +==== Instrument C standard library memory and POSIX threads functions + +The path:{liblttng-ust-libc-wrapper.so} and +path:{liblttng-ust-pthread-wrapper.so} helpers +add instrumentation to some C standard library and POSIX +threads functions. + +[role="growable"] +.Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}. +|==== +|TP provider name |TP name |Instrumented function + +.6+|`lttng_ust_libc` |`malloc` |man:malloc(3) + |`calloc` |man:calloc(3) + |`realloc` |man:realloc(3) + |`free` |man:free(3) + |`memalign` |man:memalign(3) + |`posix_memalign` |man:posix_memalign(3) +|==== + +[role="growable"] +.Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}. +|==== +|TP provider name |TP name |Instrumented function + +.4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time) + |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time) + |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p) + |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p) +|==== + +When you preload the shared object, it replaces the functions listed +in the previous tables by wrappers which contain tracepoints and call +the replaced functions. + + +[[liblttng-ust-cyg-profile]] +==== Instrument function entry and exit + +The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation +to the entry and exit points of functions. + +man:gcc(1) and man:clang(1) have an option named +https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html[`-finstrument-functions`] +which generates instrumentation calls for entry and exit to functions. +The LTTng-UST function tracing helpers, +path:{liblttng-ust-cyg-profile.so} and +path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature +to add tracepoints to the two generated functions (which contain +`cyg_profile` in their names, hence the helper's name). + +To use the LTTng-UST function tracing helper, the source files to +instrument must be built using the `-finstrument-functions` compiler +flag. + +There are two versions of the LTTng-UST function tracing helper: + +* **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant + that you should only use when it can be _guaranteed_ that the + complete event stream is recorded without any lost event record. + Any kind of duplicate information is left out. ++ +Assuming no event record is lost, having only the function addresses on +entry is enough to create a call graph, since an event record always +contains the ID of the CPU that generated it. ++ +You can use a tool like man:addr2line(1) to convert function addresses +back to source file names and line numbers. + +* **path:{liblttng-ust-cyg-profile.so}** is a more robust variant +which also works in use cases where event records might get discarded or +not recorded from application startup. +In these cases, the trace analyzer needs more information to be +able to reconstruct the program flow. + +See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation +points of this helper. + +All the tracepoints that this helper provides have the +log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)). + +TIP: It's sometimes a good idea to limit the number of source files that +you compile with the `-finstrument-functions` option to prevent LTTng +from writing an excessive amount of trace data at run time. When using +man:gcc(1), you can use the +`-finstrument-functions-exclude-function-list` option to avoid +instrument entries and exits of specific function names. + + +[role="since-2.4"] +[[liblttng-ust-dl]] +==== Instrument the dynamic linker + +The path:{liblttng-ust-dl.so} helper adds instrumentation to the +man:dlopen(3) and man:dlclose(3) function calls. + +See man:lttng-ust-dl(3) to learn more about the instrumentation points +of this helper. + + +[role="since-2.4"] +[[java-application]] +=== User space Java agent + +You can instrument any Java application which uses one of the following +logging frameworks: + +* The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**] + (JUL) core logging facilities. +* http://logging.apache.org/log4j/1.2/[**Apache log4j 1.2**], since + LTTng 2.6. Note that Apache Log4j{nbsp}2 is not supported. + +[role="img-100"] +.LTTng-UST Java agent imported by a Java application. +image::java-app.png[] + +Note that the methods described below are new in LTTng{nbsp}{revision}. +Previous LTTng versions use another technique. + +NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development +and https://ci.lttng.org/[continuous integration], thus this version is +directly supported. However, the LTTng-UST Java agent is also tested +with OpenJDK{nbsp}7. + + +[role="since-2.8"] +[[jul]] +==== Use the LTTng-UST Java agent for `java.util.logging` + +To use the LTTng-UST Java agent in a Java application which uses +`java.util.logging` (JUL): + +. In the Java application's source code, import the LTTng-UST + log handler package for `java.util.logging`: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.jul.LttngLogHandler; +---- +-- + +. Create an LTTng-UST JUL log handler: ++ +-- +[source,java] +---- +Handler lttngUstLogHandler = new LttngLogHandler(); +---- +-- + +. Add this handler to the JUL loggers which should emit LTTng events: ++ +-- +[source,java] +---- +Logger myLogger = Logger.getLogger("some-logger"); + +myLogger.addHandler(lttngUstLogHandler); +---- +-- + +. Use `java.util.logging` log statements and configuration as usual. + The loggers with an attached LTTng-UST log handler can emit + LTTng events. + +. Before exiting the application, remove the LTTng-UST log handler from + the loggers attached to it and call its `close()` method: ++ +-- +[source,java] +---- +myLogger.removeHandler(lttngUstLogHandler); +lttngUstLogHandler.close(); +---- +-- ++ +This is not strictly necessary, but it is recommended for a clean +disposal of the handler's resources. + +. Include the LTTng-UST Java agent's common and JUL-specific JAR files, + path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar}, + in the + https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class + path] when you build the Java application. ++ +The JAR files are typically located in dir:{/usr/share/java}. ++ +IMPORTANT: The LTTng-UST Java agent must be +<> for the logging framework your +application uses. + +.Use the LTTng-UST Java agent for `java.util.logging`. +==== +[source,java] +.path:{Test.java} +---- +import java.io.IOException; +import java.util.logging.Handler; +import java.util.logging.Logger; +import org.lttng.ust.agent.jul.LttngLogHandler; + +public class Test +{ + private static final int answer = 42; + + public static void main(String[] argv) throws Exception + { + // Create a logger + Logger logger = Logger.getLogger("jello"); + + // Create an LTTng-UST log handler + Handler lttngUstLogHandler = new LttngLogHandler(); + + // Add the LTTng-UST log handler to our logger + logger.addHandler(lttngUstLogHandler); + + // Log at will! + logger.info("some info"); + logger.warning("some warning"); + Thread.sleep(500); + logger.finer("finer information; the answer is " + answer); + Thread.sleep(123); + logger.severe("error!"); + + // Not mandatory, but cleaner + logger.removeHandler(lttngUstLogHandler); + lttngUstLogHandler.close(); + } +} +---- + +Build this example: + +[role="term"] +---- +javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java +---- + +<>, +<> matching the +`jello` JUL logger, and <>: + +[role="term"] +---- +lttng create +lttng enable-event --jul jello +lttng start +---- + +Run the compiled class: + +[role="term"] +---- +java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +lttng stop +lttng view +---- +==== + +You can use the opt:lttng-enable-event(1):--loglevel or +opt:lttng-enable-event(1):--loglevel-only option of the +man:lttng-enable-event(1) command to target a range of JUL log levels +or a specific JUL log level. + + +[role="since-2.8"] +[[log4j]] +==== Use the LTTng-UST Java agent for Apache log4j + +To use the LTTng-UST Java agent in a Java application which uses +Apache log4j 1.2: + +. In the Java application's source code, import the LTTng-UST + log appender package for Apache log4j: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.log4j.LttngLogAppender; +---- +-- + +. Create an LTTng-UST log4j log appender: ++ +-- +[source,java] +---- +Appender lttngUstLogAppender = new LttngLogAppender(); +---- +-- + +. Add this appender to the log4j loggers which should emit LTTng events: ++ +-- +[source,java] +---- +Logger myLogger = Logger.getLogger("some-logger"); + +myLogger.addAppender(lttngUstLogAppender); +---- +-- + +. Use Apache log4j log statements and configuration as usual. The + loggers with an attached LTTng-UST log appender can emit LTTng events. + +. Before exiting the application, remove the LTTng-UST log appender from + the loggers attached to it and call its `close()` method: ++ +-- +[source,java] +---- +myLogger.removeAppender(lttngUstLogAppender); +lttngUstLogAppender.close(); +---- +-- ++ +This is not strictly necessary, but it is recommended for a clean +disposal of the appender's resources. + +. Include the LTTng-UST Java agent's common and log4j-specific JAR + files, path:{lttng-ust-agent-common.jar} and + path:{lttng-ust-agent-log4j.jar}, in the + https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class + path] when you build the Java application. ++ +The JAR files are typically located in dir:{/usr/share/java}. ++ +IMPORTANT: The LTTng-UST Java agent must be +<> for the logging framework your +application uses. + +.Use the LTTng-UST Java agent for Apache log4j. +==== +[source,java] +.path:{Test.java} +---- +import org.apache.log4j.Appender; +import org.apache.log4j.Logger; +import org.lttng.ust.agent.log4j.LttngLogAppender; + +public class Test +{ + private static final int answer = 42; + + public static void main(String[] argv) throws Exception + { + // Create a logger + Logger logger = Logger.getLogger("jello"); + + // Create an LTTng-UST log appender + Appender lttngUstLogAppender = new LttngLogAppender(); + + // Add the LTTng-UST log appender to our logger + logger.addAppender(lttngUstLogAppender); + + // Log at will! + logger.info("some info"); + logger.warn("some warning"); + Thread.sleep(500); + logger.debug("debug information; the answer is " + answer); + Thread.sleep(123); + logger.fatal("error!"); + + // Not mandatory, but cleaner + logger.removeAppender(lttngUstLogAppender); + lttngUstLogAppender.close(); + } +} + +---- + +Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR +file): + +[role="term"] +---- +javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java +---- + +<>, +<> matching the +`jello` log4j logger, and <>: + +[role="term"] +---- +lttng create +lttng enable-event --log4j jello +lttng start +---- + +Run the compiled class: + +[role="term"] +---- +java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +lttng stop +lttng view +---- +==== + +You can use the opt:lttng-enable-event(1):--loglevel or +opt:lttng-enable-event(1):--loglevel-only option of the +man:lttng-enable-event(1) command to target a range of Apache log4j log levels +or a specific log4j log level. + + +[role="since-2.8"] +[[java-application-context]] +==== Provide application-specific context fields in a Java application + +A Java application-specific context field is a piece of state provided +by the application which <>, using the +man:lttng-add-context(1) command, to each <> +produced by the log statements of this application. + +For example, a given object might have a current request ID variable. +You can create a context information retriever for this object and +assign a name to this current request ID. You can then, using the +man:lttng-add-context(1) command, add this context field by name to +the JUL or log4j <>. + +To provide application-specific context fields in a Java application: + +. In the Java application's source code, import the LTTng-UST + Java agent context classes and interfaces: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.context.ContextInfoManager; +import org.lttng.ust.agent.context.IContextInfoRetriever; +---- +-- + +. Create a context information retriever class, that is, a class which + implements the `IContextInfoRetriever` interface: ++ +-- +[source,java] +---- +class MyContextInfoRetriever implements IContextInfoRetriever +{ + @Override + public Object retrieveContextInfo(String key) + { + if (key.equals("intCtx")) { + return (short) 17; + } else if (key.equals("strContext")) { + return "context value!"; + } else { + return null; + } + } +} +---- +-- ++ +This `retrieveContextInfo()` method is the only member of the +`IContextInfoRetriever` interface. Its role is to return the current +value of a state by name to create a context field. The names of the +context fields and which state variables they return depends on your +specific scenario. ++ +All primitive types and objects are supported as context fields. +When `retrieveContextInfo()` returns an object, the context field +serializer calls its `toString()` method to add a string field to +event records. The method can also return `null`, which means that +no context field is available for the required name. + +. Register an instance of your context information retriever class to + the context information manager singleton: ++ +-- +[source,java] +---- +IContextInfoRetriever cir = new MyContextInfoRetriever(); +ContextInfoManager cim = ContextInfoManager.getInstance(); +cim.registerContextInfoRetriever("retrieverName", cir); +---- +-- + +. Before exiting the application, remove your context information + retriever from the context information manager singleton: ++ +-- +[source,java] +---- +ContextInfoManager cim = ContextInfoManager.getInstance(); +cim.unregisterContextInfoRetriever("retrieverName"); +---- +-- ++ +This is not strictly necessary, but it is recommended for a clean +disposal of some manager's resources. + +. Build your Java application with LTTng-UST Java agent support as + usual, following the procedure for either the <> or + <> framework. + + +.Provide application-specific context fields in a Java application. +==== +[source,java] +.path:{Test.java} +---- +import java.util.logging.Handler; +import java.util.logging.Logger; +import org.lttng.ust.agent.jul.LttngLogHandler; +import org.lttng.ust.agent.context.ContextInfoManager; +import org.lttng.ust.agent.context.IContextInfoRetriever; + +public class Test +{ + // Our context information retriever class + private static class MyContextInfoRetriever + implements IContextInfoRetriever + { + @Override + public Object retrieveContextInfo(String key) { + if (key.equals("intCtx")) { + return (short) 17; + } else if (key.equals("strContext")) { + return "context value!"; + } else { + return null; + } + } + } + + private static final int answer = 42; + + public static void main(String args[]) throws Exception + { + // Get the context information manager instance + ContextInfoManager cim = ContextInfoManager.getInstance(); + + // Create and register our context information retriever + IContextInfoRetriever cir = new MyContextInfoRetriever(); + cim.registerContextInfoRetriever("myRetriever", cir); + + // Create a logger + Logger logger = Logger.getLogger("jello"); + + // Create an LTTng-UST log handler + Handler lttngUstLogHandler = new LttngLogHandler(); + + // Add the LTTng-UST log handler to our logger + logger.addHandler(lttngUstLogHandler); + + // Log at will! + logger.info("some info"); + logger.warning("some warning"); + Thread.sleep(500); + logger.finer("finer information; the answer is " + answer); + Thread.sleep(123); + logger.severe("error!"); + + // Not mandatory, but cleaner + logger.removeHandler(lttngUstLogHandler); + lttngUstLogHandler.close(); + cim.unregisterContextInfoRetriever("myRetriever"); + } +} +---- + +Build this example: + +[role="term"] +---- +javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java +---- + +<> +and <> matching the +`jello` JUL logger: + +[role="term"] +---- +lttng create +lttng enable-event --jul jello +---- + +<> to the +JUL channel: + +[role="term"] +---- +lttng add-context --jul --type='$app.myRetriever:intCtx' +lttng add-context --jul --type='$app.myRetriever:strContext' +---- + +<>: + +[role="term"] +---- +lttng start +---- + +Run the compiled class: + +[role="term"] +---- +java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +lttng stop +lttng view +---- +==== + + +[role="since-2.7"] +[[python-application]] +=== User space Python agent + +You can instrument a Python 2 or Python 3 application which uses the +standard https://docs.python.org/3/library/logging.html[`logging`] +package. + +Each log statement emits an LTTng event once the +application module imports the +<> package. + +[role="img-100"] +.A Python application importing the LTTng-UST Python agent. +image::python-app.png[] + +To use the LTTng-UST Python agent: + +. In the Python application's source code, import the LTTng-UST Python + agent: ++ +-- +[source,python] +---- +import lttngust +---- +-- ++ +The LTTng-UST Python agent automatically adds its logging handler to the +root logger at import time. ++ +Any log statement that the application executes before this import does +not emit an LTTng event. ++ +IMPORTANT: The LTTng-UST Python agent must be +<>. + +. Use log statements and logging configuration as usual. + Since the LTTng-UST Python agent adds a handler to the _root_ + logger, you can trace any log statement from any logger. + +.Use the LTTng-UST Python agent. +==== +[source,python] +.path:{test.py} +---- +import lttngust +import logging +import time + + +def example(): + logging.basicConfig() + logger = logging.getLogger('my-logger') + + while True: + logger.debug('debug message') + logger.info('info message') + logger.warn('warn message') + logger.error('error message') + logger.critical('critical message') + time.sleep(1) + + +if __name__ == '__main__': + example() +---- + +NOTE: `logging.basicConfig()`, which adds to the root logger a basic +logging handler which prints to the standard error stream, is not +strictly required for LTTng-UST tracing to work, but in versions of +Python preceding 3.2, you could see a warning message which indicates +that no handler exists for the logger `my-logger`. + +<>, +<> matching the +`my-logger` Python logger, and <>: + +[role="term"] +---- +lttng create +lttng enable-event --python my-logger +lttng start +---- + +Run the Python script: + +[role="term"] +---- +python test.py +---- + +<> and inspect the recorded +events: + +[role="term"] +---- +lttng stop +lttng view +---- +==== + +You can use the opt:lttng-enable-event(1):--loglevel or +opt:lttng-enable-event(1):--loglevel-only option of the +man:lttng-enable-event(1) command to target a range of Python log levels +or a specific Python log level. + +When an application imports the LTTng-UST Python agent, the agent tries +to register to a <>. Note that you must +<> _before_ you run the Python +application. If a session daemon is found, the agent tries to register +to it during 5{nbsp}seconds, after which the application continues +without LTTng tracing support. You can override this timeout value with +the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable +(milliseconds). + +If the session daemon stops while a Python application with an imported +LTTng-UST Python agent runs, the agent retries to connect and to +register to a session daemon every 3{nbsp}seconds. You can override this +delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment +variable. + + +[role="since-2.5"] +[[proc-lttng-logger-abi]] +=== LTTng logger + +The `lttng-tracer` Linux kernel module, part of +<>, creates the special LTTng logger file +path:{/proc/lttng-logger} when it's loaded. Any application can write +text data to this file to emit an LTTng event. + +[role="img-100"] +.An application writes to the LTTng logger file to emit an LTTng event. +image::lttng-logger.png[] + +The LTTng logger is the quickest method--not the most efficient, +however--to add instrumentation to an application. It is designed +mostly to instrument shell scripts: + +[role="term"] +---- +echo "Some message, some $variable" > /proc/lttng-logger +---- + +Any event that the LTTng logger emits is named `lttng_logger` and +belongs to the Linux kernel <>. However, unlike +other instrumentation points in the kernel tracing domain, **any Unix +user** can <> which +matches its event name, not only the root user or users in the tracing +group. + +To use the LTTng logger: + +* From any application, write text data to the path:{/proc/lttng-logger} + file. + +The `msg` field of `lttng_logger` event records contains the +recorded message. + +NOTE: The maximum message length of an LTTng logger event is +1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more +than one event to contain the remaining data. + +You should not use the LTTng logger to trace a user application which +can be instrumented in a more efficient way, namely: + +* <>. +* <>. +* <>. + +.Use the LTTng logger. +==== +[source,bash] +.path:{test.bash} +---- +echo 'Hello, World!' > /proc/lttng-logger +sleep 2 +df --human-readable --print-type / > /proc/lttng-logger +---- + +<>, +<> matching the +`lttng_logger` Linux kernel tracepoint, and +<>: + +[role="term"] +---- +lttng create +lttng enable-event --kernel lttng_logger +lttng start +---- + +Run the Bash script: + +[role="term"] +---- +bash test.bash +---- + +<> and inspect the recorded +events: + +[role="term"] +---- +lttng stop +lttng view +---- +==== + + +[[instrumenting-linux-kernel]] +=== LTTng kernel tracepoints + +NOTE: This section shows how to _add_ instrumentation points to the +Linux kernel. The kernel's subsystems are already thoroughly +instrumented at strategic places for LTTng when you +<> the <> +package. + +//// +There are two methods to instrument the Linux kernel: + +. <> over an existing ftrace + tracepoint which uses the `TRACE_EVENT()` API. ++ +Choose this if you want to instrumentation a Linux kernel tree with an +instrumentation point compatible with ftrace, perf, and SystemTap. + +. Use an <> to + instrument an out-of-tree kernel module. ++ +Choose this if you don't need ftrace, perf, or SystemTap support. +//// + + +[[linux-add-lttng-layer]] +==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint + +This section shows how to add an LTTng layer to existing ftrace +instrumentation using the `TRACE_EVENT()` API. + +This section does not document the `TRACE_EVENT()` macro. You can +read the following articles to learn more about this API: + +* http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part 1)] +* http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part 2)] +* http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part 3)] + +The following procedure assumes that your ftrace tracepoints are +correctly defined in their own header and that they are created in +one source file using the `CREATE_TRACE_POINTS` definition. + +To add an LTTng layer over an existing ftrace tracepoint: + +. Make sure the following kernel configuration options are + enabled: ++ +-- +* `CONFIG_MODULES` +* `CONFIG_KALLSYMS` +* `CONFIG_HIGH_RES_TIMERS` +* `CONFIG_TRACEPOINTS` +-- + +. Build the Linux source tree with your custom ftrace tracepoints. +. Boot the resulting Linux image on your target system. ++ +Confirm that the tracepoints exist by looking for their names in the +dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys` +is your subsystem's name. + +. Get a copy of the latest LTTng-modules{nbsp}{revision}: ++ +-- +[role="term"] +---- +cd $(mktemp -d) && +wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.9.tar.bz2 && +tar -xf lttng-modules-latest-2.9.tar.bz2 && +cd lttng-modules-2.9.* +---- +-- + +. In dir:{instrumentation/events/lttng-module}, relative to the root + of the LTTng-modules source tree, create a header file named + +__subsys__.h+ for your custom subsystem +__subsys__+ and write your + LTTng-modules tracepoint definitions using the LTTng-modules + macros in it. ++ +Start with this template: ++ +-- +[source,c] +.path:{instrumentation/events/lttng-module/my_subsys.h} +---- +#undef TRACE_SYSTEM +#define TRACE_SYSTEM my_subsys + +#if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) +#define _LTTNG_MY_SUBSYS_H + +#include "../../../probes/lttng-tracepoint-event.h" +#include + +LTTNG_TRACEPOINT_EVENT( + /* + * Format is identical to TRACE_EVENT()'s version for the three + * following macro parameters: + */ + my_subsys_my_event, + TP_PROTO(int my_int, const char *my_string), + TP_ARGS(my_int, my_string), + + /* LTTng-modules specific macros */ + TP_FIELDS( + ctf_integer(int, my_int_field, my_int) + ctf_string(my_bar_field, my_bar) + ) +) + +#endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */ + +#include "../../../probes/define_trace.h" +---- +-- ++ +The entries in the `TP_FIELDS()` section are the list of fields for the +LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of +ftrace's `TRACE_EVENT()` macro. ++ +See <> for a +complete description of the available `ctf_*()` macros. + +. Create the LTTng-modules probe's kernel module C source file, + +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your + subsystem name: ++ +-- +[source,c] +.path:{probes/lttng-probe-my-subsys.c} +---- +#include +#include "../lttng-tracer.h" + +/* + * Build-time verification of mismatch between mainline + * TRACE_EVENT() arguments and the LTTng-modules adaptation + * layer LTTNG_TRACEPOINT_EVENT() arguments. + */ +#include + +/* Create LTTng tracepoint probes */ +#define LTTNG_PACKAGE_BUILD +#define CREATE_TRACE_POINTS +#define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module + +#include "../instrumentation/events/lttng-module/my_subsys.h" + +MODULE_LICENSE("GPL and additional rights"); +MODULE_AUTHOR("Your name "); +MODULE_DESCRIPTION("LTTng my_subsys probes"); +MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "." + __stringify(LTTNG_MODULES_MINOR_VERSION) "." + __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION) + LTTNG_MODULES_EXTRAVERSION); +---- +-- + +. Edit path:{probes/Makefile} and add your new kernel module object + next to the existing ones: ++ +-- +[source,make] +.path:{probes/Makefile} +---- +# ... + +obj-m += lttng-probe-module.o +obj-m += lttng-probe-power.o + +obj-m += lttng-probe-my-subsys.o + +# ... +---- +-- + +. Build and install the LTTng kernel modules: ++ +-- +[role="term"] +---- +make KERNELDIR=/path/to/linux +sudo make modules_install +---- +-- ++ +Replace `/path/to/linux` with the path to the Linux source tree where +you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro. + +Note that you can also use the +<> +instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and +C code that need to be executed before the event fields are recorded. + +The best way to learn how to use the previous LTTng-modules macros is to +inspect the existing LTTng-modules tracepoint definitions in the +dir:{instrumentation/events/lttng-module} header files. Compare them +with the Linux kernel mainline versions in the +dir:{include/trace/events} directory of the Linux source tree. + + +[role="since-2.7"] +[[lttng-tracepoint-event-code]] +===== Use custom C code to access the data for tracepoint fields + +Although we recommended to always use the +<> macro to describe +the arguments and fields of an LTTng-modules tracepoint when possible, +sometimes you need a more complex process to access the data that the +tracer records as event record fields. In other words, you need local +variables and multiple C{nbsp}statements instead of simple +argument-based expressions that you pass to the +<>. + +You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of +`LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define +a block of C{nbsp}code to be executed before LTTng records the fields. +The structure of this macro is: + +[source,c] +.`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax. +---- +LTTNG_TRACEPOINT_EVENT_CODE( + /* + * Format identical to the LTTNG_TRACEPOINT_EVENT() + * version for the following three macro parameters: + */ + my_subsys_my_event, + TP_PROTO(int my_int, const char *my_string), + TP_ARGS(my_int, my_string), + + /* Declarations of custom local variables */ + TP_locvar( + int a = 0; + unsigned long b = 0; + const char *name = "(undefined)"; + struct my_struct *my_struct; + ), + + /* + * Custom code which uses both tracepoint arguments + * (in TP_ARGS()) and local variables (in TP_locvar()). + * + * Local variables are actually members of a structure pointed + * to by the special variable tp_locvar. + */ + TP_code( + if (my_int) { + tp_locvar->a = my_int + 17; + tp_locvar->my_struct = get_my_struct_at(tp_locvar->a); + tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct); + tp_locvar->name = my_struct_get_name(tp_locvar->my_struct); + put_my_struct(tp_locvar->my_struct); + + if (tp_locvar->b) { + tp_locvar->a = 1; + } + } + ), + + /* + * Format identical to the LTTNG_TRACEPOINT_EVENT() + * version for this, except that tp_locvar members can be + * used in the argument expression parameters of + * the ctf_*() macros. + */ + TP_FIELDS( + ctf_integer(unsigned long, my_struct_b, tp_locvar->b) + ctf_integer(int, my_struct_a, tp_locvar->a) + ctf_string(my_string_field, my_string) + ctf_string(my_struct_name, tp_locvar->name) + ) +) +---- + +IMPORTANT: The C code defined in `TP_code()` must not have any side +effects when executed. In particular, the code must not allocate +memory or get resources without deallocating this memory or putting +those resources afterwards. + + +[[instrumenting-linux-kernel-tracing]] +==== Load and unload a custom probe kernel module + +You must load a <> in the kernel before it can emit LTTng events. + +To load the default probe kernel modules and a custom probe kernel +module: + +* Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra + probe modules to load when starting a root <>: ++ +-- +.Load the `my_subsys`, `usb`, and the default probe modules. +==== +[role="term"] +---- +sudo lttng-sessiond --extra-kmod-probes=my_subsys,usb +---- +==== +-- ++ +You only need to pass the subsystem name, not the whole kernel module +name. + +To load _only_ a given custom probe kernel module: + +* Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe + modules to load when starting a root session daemon: ++ +-- +.Load only the `my_subsys` and `usb` probe modules. +==== +[role="term"] +---- +sudo lttng-sessiond --kmod-probes=my_subsys,usb +---- +==== +-- + +To confirm that a probe module is loaded: + +* Use man:lsmod(8): ++ +-- +[role="term"] +---- +lsmod | grep lttng_probe_usb +---- +-- + +To unload the loaded probe modules: + +* Kill the session daemon with `SIGTERM`: ++ +-- +[role="term"] +---- +sudo pkill lttng-sessiond +---- +-- ++ +You can also use man:modprobe(8)'s `--remove` option if the session +daemon terminates abnormally. + + +[[controlling-tracing]] +== Tracing control + +Once an application or a Linux kernel is +<> for LTTng tracing, +you can _trace_ it. + +This section is divided in topics on how to use the various +<>, in particular the <>, to _control_ the LTTng daemons and tracers. + +NOTE: In the following subsections, we refer to an man:lttng(1) command +using its man page name. For example, instead of _Run the `create` +command to..._, we use _Run the man:lttng-create(1) command to..._. + + +[[start-sessiond]] +=== Start a session daemon + +In some situations, you need to run a <> +(man:lttng-sessiond(8)) _before_ you can use the man:lttng(1) +command-line tool. + +You will see the following error when you run a command while no session +daemon is running: + +---- +Error: No session daemon is available +---- + +The only command that automatically runs a session daemon is +man:lttng-create(1), which you use to +<>. While +this is most of the time the first operation that you do, sometimes it's +not. Some examples are: + +* <>. +* <>. + +[[tracing-group]] Each Unix user must have its own running session +daemon to trace user applications. The session daemon that the root user +starts is the only one allowed to control the LTTng kernel tracer. Users +that are part of the _tracing group_ can control the root session +daemon. The default tracing group name is `tracing`; you can set it to +something else with the opt:lttng-sessiond(8):--group option when you +start the root session daemon. + +To start a user session daemon: + +* Run man:lttng-sessiond(8): ++ +-- +[role="term"] +---- +lttng-sessiond --daemonize +---- +-- + +To start the root session daemon: + +* Run man:lttng-sessiond(8) as the root user: ++ +-- +[role="term"] +---- +sudo lttng-sessiond --daemonize +---- +-- + +In both cases, remove the opt:lttng-sessiond(8):--daemonize option to +start the session daemon in foreground. + +To stop a session daemon, use man:kill(1) on its process ID (standard +`TERM` signal). + +Note that some Linux distributions could manage the LTTng session daemon +as a service. In this case, you should use the service manager to +start, restart, and stop session daemons. + + +[[creating-destroying-tracing-sessions]] +=== Create and destroy a tracing session + +Almost all the LTTng control operations happen in the scope of +a <>, which is the dialogue between the +<> and you. + +To create a tracing session with a generated name: + +* Use the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +lttng create +---- +-- + +The created tracing session's name is `auto` followed by the +creation date. + +To create a tracing session with a specific name: + +* Use the optional argument of the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +lttng create my-session +---- +-- ++ +Replace `my-session` with the specific tracing session name. + +LTTng appends the creation date to the created tracing session's name. + +LTTng writes the traces of a tracing session in ++$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the +name of the tracing session. Note that the env:LTTNG_HOME environment +variable defaults to `$HOME` if not set. + +To output LTTng traces to a non-default location: + +* Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +lttng create --output=/tmp/some-directory my-session +---- +-- + +You may create as many tracing sessions as you wish. + +To list all the existing tracing sessions for your Unix user: + +* Use the man:lttng-list(1) command: ++ +-- +[role="term"] +---- +lttng list +---- +-- + +When you create a tracing session, it is set as the _current tracing +session_. The following man:lttng(1) commands operate on the current +tracing session when you don't specify one: + +[role="list-3-cols"] +* `add-context` +* `destroy` +* `disable-channel` +* `disable-event` +* `enable-channel` +* `enable-event` +* `load` +* `regenerate` +* `save` +* `snapshot` +* `start` +* `stop` +* `track` +* `untrack` +* `view` + +To change the current tracing session: + +* Use the man:lttng-set-session(1) command: ++ +-- +[role="term"] +---- +lttng set-session new-session +---- +-- ++ +Replace `new-session` by the name of the new current tracing session. + +When you are done tracing in a given tracing session, you can destroy +it. This operation frees the resources taken by the tracing session +to destroy; it does not destroy the trace data that LTTng wrote for +this tracing session. + +To destroy the current tracing session: + +* Use the man:lttng-destroy(1) command: ++ +-- +[role="term"] +---- +lttng destroy +---- +-- + + +[[list-instrumentation-points]] +=== List the available instrumentation points + +The <> can query the running instrumented +user applications and the Linux kernel to get a list of available +instrumentation points. For the Linux kernel <>, +they are tracepoints and system calls. For the user space tracing +domain, they are tracepoints. For the other tracing domains, they are +logger names. + +To list the available instrumentation points: + +* Use the man:lttng-list(1) command with the requested tracing domain's + option amongst: ++ +-- +* opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user + must be a root user, or it must be a member of the + <>). +* opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux + kernel system calls (your Unix user must be a root user, or it must be + a member of the tracing group). +* opt:lttng-list(1):--userspace: user space tracepoints. +* opt:lttng-list(1):--jul: `java.util.logging` loggers. +* opt:lttng-list(1):--log4j: Apache log4j loggers. +* opt:lttng-list(1):--python: Python loggers. +-- + +.List the available user space tracepoints. +==== +[role="term"] +---- +lttng list --userspace +---- +==== + +.List the available Linux kernel system call tracepoints. +==== +[role="term"] +---- +lttng list --kernel --syscall +---- +==== + + +[[enabling-disabling-events]] +=== Create and enable an event rule + +Once you <>, you can create <> with the +man:lttng-enable-event(1) command. + +You specify each condition with a command-line option. The available +condition options are shown in the following table. + +[role="growable",cols="asciidoc,asciidoc,default"] +.Condition command-line options for the man:lttng-enable-event(1) command. +|==== +|Option |Description |Applicable tracing domains + +| +One of: + +. `--syscall` +. +--probe=__ADDR__+ +. +--function=__ADDR__+ + +| +Instead of using the default _tracepoint_ instrumentation type, use: + +. A Linux system call. +. A Linux https://lwn.net/Articles/132196/[KProbe] (symbol or address). +. The entry and return points of a Linux function (symbol or address). + +|Linux kernel. + +|First positional argument. + +| +Tracepoint or system call name. In the case of a Linux KProbe or +function, this is a custom name given to the event rule. With the +JUL, log4j, and Python domains, this is a logger name. + +With a tracepoint, logger, or system call name, the last character +can be `*` to match anything that remains. + +|All. + +| +One of: + +. +--loglevel=__LEVEL__+ +. +--loglevel-only=__LEVEL__+ + +| +. Match only tracepoints or log statements with a logging level at + least as severe as +__LEVEL__+. +. Match only tracepoints or log statements with a logging level + equal to +__LEVEL__+. + +See man:lttng-enable-event(1) for the list of available logging level +names. + +|User space, JUL, log4j, and Python. + +|+--exclude=__EXCLUSIONS__+ + +| +When you use a `*` character at the end of the tracepoint or logger +name (first positional argument), exclude the specific names in the +comma-delimited list +__EXCLUSIONS__+. + +| +User space, JUL, log4j, and Python. + +|+--filter=__EXPR__+ + +| +Match only events which satisfy the expression +__EXPR__+. + +See man:lttng-enable-event(1) to learn more about the syntax of a +filter expression. + +|All. + +|==== + +You attach an event rule to a <> on creation. If you do +not specify the channel with the opt:lttng-enable-event(1):--channel +option, and if the event rule to create is the first in its +<> for a given tracing session, then LTTng +creates a _default channel_ for you. This default channel is reused in +subsequent invocations of the man:lttng-enable-event(1) command for the +same tracing domain. + +An event rule is always enabled at creation time. + +The following examples show how you can combine the previous +command-line options to create simple to more complex event rules. + +.Create an event rule targetting a Linux kernel tracepoint (default channel). +==== +[role="term"] +---- +lttng enable-event --kernel sched_switch +---- +==== + +.Create an event rule matching four Linux kernel system calls (default channel). +==== +[role="term"] +---- +lttng enable-event --kernel --syscall open,write,read,close +---- +==== + +.Create event rules matching tracepoints with filter expressions (default channel). +==== +[role="term"] +---- +lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"' +---- + +[role="term"] +---- +lttng enable-event --kernel --all \ + --filter='$ctx.tid == 1988 || $ctx.tid == 1534' +---- + +[role="term"] +---- +lttng enable-event --jul my_logger \ + --filter='$app.retriever:cur_msg_id > 3' +---- + +IMPORTANT: Make sure to always quote the filter string when you +use man:lttng(1) from a shell. +==== + +.Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel). +==== +[role="term"] +---- +lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO +---- + +IMPORTANT: Make sure to always quote the wildcard character when you +use man:lttng(1) from a shell. +==== + +.Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel). +==== +[role="term"] +---- +lttng enable-event --python my-app.'*' \ + --exclude='my-app.module,my-app.hello' +---- +==== + +.Create an event rule matching any Apache log4j logger with a specific log level (default channel). +==== +[role="term"] +---- +lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN +---- +==== + +.Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint. +==== +[role="term"] +---- +lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel +---- +==== + +The event rules of a given channel form a whitelist: as soon as an +emitted event passes one of them, LTTng can record the event. For +example, an event named `my_app:my_tracepoint` emitted from a user space +tracepoint with a `TRACE_ERROR` log level passes both of the following +rules: + +[role="term"] +---- +lttng enable-event --userspace my_app:my_tracepoint +lttng enable-event --userspace my_app:my_tracepoint \ + --loglevel=TRACE_INFO +---- + +The second event rule is redundant: the first one includes +the second one. + + +[[disable-event-rule]] +=== Disable an event rule + +To disable an event rule that you <> +previously, use the man:lttng-disable-event(1) command. This command +disables _all_ the event rules (of a given tracing domain and channel) +which match an instrumentation point. The other conditions are not +supported as of LTTng{nbsp}{revision}. + +The LTTng tracer does not record an emitted event which passes +a _disabled_ event rule. + +.Disable an event rule matching a Python logger (default channel). +==== +[role="term"] +---- +lttng disable-event --python my-logger +---- +==== + +.Disable an event rule matching all `java.util.logging` loggers (default channel). +==== +[role="term"] +---- +lttng disable-event --jul '*' +---- +==== + +.Disable _all_ the event rules of the default channel. +==== +The opt:lttng-disable-event(1):--all-events option is not, like the +opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the +equivalent of the event name `*` (wildcard): it disables _all_ the event +rules of a given channel. + +[role="term"] +---- +lttng disable-event --jul --all-events +---- +==== + +NOTE: You cannot delete an event rule once you create it. + + +[[status]] +=== Get the status of a tracing session + +To get the status of the current tracing session, that is, its +parameters, its channels, event rules, and their attributes: + +* Use the man:lttng-status(1) command: ++ +-- +[role="term"] +---- +lttng status +---- +-- ++ + +To get the status of any tracing session: + +* Use the man:lttng-list(1) command with the tracing session's name: ++ +-- +[role="term"] +---- +lttng list my-session +---- +-- ++ +Replace `my-session` with the desired tracing session's name. + + +[[basic-tracing-session-control]] +=== Start and stop a tracing session + +Once you <> and +<>, +you can start and stop the tracers for this tracing session. + +To start tracing in the current tracing session: + +* Use the man:lttng-start(1) command: ++ +-- +[role="term"] +---- +lttng start +---- +-- + +LTTng is very flexible: you can launch user applications before +or after the you start the tracers. The tracers only record the events +if they pass enabled event rules and if they occur while the tracers are +started. + +To stop tracing in the current tracing session: + +* Use the man:lttng-stop(1) command: ++ +-- +[role="term"] +---- +lttng stop +---- +-- ++ +If there were <> or lost sub-buffers since the last time you ran +man:lttng-start(1), warnings are printed when you run the +man:lttng-stop(1) command. + + +[[enabling-disabling-channels]] +=== Create a channel + +Once you create a tracing session, you can create a <> +with the man:lttng-enable-channel(1) command. + +Note that LTTng automatically creates a default channel when, for a +given <>, no channels exist and you +<> the first event rule. This default +channel is named `channel0` and its attributes are set to reasonable +values. Therefore, you only need to create a channel when you need +non-default attributes. + +You specify each non-default channel attribute with a command-line +option when you use the man:lttng-enable-channel(1) command. The +available command-line options are: + +[role="growable",cols="asciidoc,asciidoc"] +.Command-line options for the man:lttng-enable-channel(1) command. +|==== +|Option |Description + +|`--overwrite` + +| +Use the _overwrite_ +<> instead of +the default _discard_ mode. + +|`--buffers-pid` (user space tracing domain only) + +| +Use the per-process <> +instead of the default per-user buffering scheme. + +|+--subbuf-size=__SIZE__+ + +| +Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU, +either for each Unix user (default), or for each instrumented process. + +See <>. + +|+--num-subbuf=__COUNT__+ + +| +Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either +for each Unix user (default), or for each instrumented process. + +See <>. + +|+--tracefile-size=__SIZE__+ + +| +Set the maximum size of each trace file that this channel writes within +a stream to +__SIZE__+ bytes instead of no maximum. + +See <>. + +|+--tracefile-count=__COUNT__+ + +| +Limit the number of trace files that this channel creates to ++__COUNT__+ channels instead of no limit. + +See <>. + +|+--switch-timer=__PERIODUS__+ + +| +Set the <> +to +__PERIODUS__+{nbsp}µs. + +|+--read-timer=__PERIODUS__+ + +| +Set the <> +to +__PERIODUS__+{nbsp}µs. + +|+--output=__TYPE__+ (Linux kernel tracing domain only) + +| +Set the channel's output type to +__TYPE__+, either `mmap` or `splice`. + +|==== + +You can only create a channel in the Linux kernel and user space +<>: other tracing domains have their own channel +created on the fly when <>. + +[IMPORTANT] +==== +Because of a current LTTng limitation, you must create all channels +_before_ you <> in a given +tracing session, that is, before the first time you run +man:lttng-start(1). + +Since LTTng automatically creates a default channel when you use the +man:lttng-enable-event(1) command with a specific tracing domain, you +cannot, for example, create a Linux kernel event rule, start tracing, +and then create a user space event rule, because no user space channel +exists yet and it's too late to create one. + +For this reason, make sure to configure your channels properly +before starting the tracers for the first time! +==== + +The following examples show how you can combine the previous +command-line options to create simple to more complex channels. + +.Create a Linux kernel channel with default attributes. +==== +[role="term"] +---- +lttng enable-channel --kernel my-channel +---- +==== + +.Create a user space channel with 4 sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process. +==== +[role="term"] +---- +lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \ + --buffers-pid my-channel +---- +==== + +.Create a Linux kernel channel which rotates 8 trace files of 4{nbsp}MiB each for each stream +==== +[role="term"] +---- +lttng enable-channel --kernel --tracefile-count=8 \ + --tracefile-size=4194304 my-channel +---- +==== + +.Create a user space channel in overwrite (or _flight recorder_) mode. +==== +[role="term"] +---- +lttng enable-channel --userspace --overwrite my-channel +---- +==== + +You can <> the same event rule in +two different channels: + +[role="term"] +---- +lttng enable-event --userspace --channel=my-channel app:tp +lttng enable-event --userspace --channel=other-channel app:tp +---- + +If both channels are enabled, when a tracepoint named `app:tp` is +reached, LTTng records two events, one for each channel. + + +[[disable-channel]] +=== Disable a channel + +To disable a specific channel that you <> +previously, use the man:lttng-disable-channel(1) command. + +.Disable a specific Linux kernel channel. +==== +[role="term"] +---- +lttng disable-channel --kernel my-channel +---- +==== + +The state of a channel precedes the individual states of event rules +attached to it: event rules which belong to a disabled channel, even if +they are enabled, are also considered disabled. + + +[[adding-context]] +=== Add context fields to a channel + +Event record fields in trace files provide important information about +events that occured previously, but sometimes some external context may +help you solve a problem faster. Examples of context fields are: + +* The **process ID**, **thread ID**, **process name**, and + **process priority** of the thread in which the event occurs. +* The **hostname** of the system on which the event occurs. +* The current values of many possible **performance counters** using + perf, for example: +** CPU cycles, stalled cycles, idle cycles, and the other cycle types. +** Cache misses. +** Branch instructions, misses, and loads. +** CPU faults. +* Any context defined at the application level (supported for the + JUL and log4j <>). + +To get the full list of available context fields, see +`lttng add-context --list`. Some context fields are reserved for a +specific <> (Linux kernel or user space). + +You add context fields to <>. All the events +that a channel with added context fields records contain those fields. + +To add context fields to one or all the channels of a given tracing +session: + +* Use the man:lttng-add-context(1) command. + +.Add context fields to all the channels of the current tracing session. +==== +The following command line adds the virtual process identifier and +the per-thread CPU cycles count fields to all the user space channels +of the current tracing session. + +[role="term"] +---- +lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles +---- +==== + +.Add performance counter context fields by raw ID +==== +See man:lttng-add-context(1) for the exact format of the context field +type, which is partly compatible with the format used in +man:perf-record(1). + +[role="term"] +---- +lttng add-context --userspace --type=perf:thread:raw:r0110:test +lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted +---- +==== + +.Add a context field to a specific channel. +==== +The following command line adds the thread identifier context field +to the Linux kernel channel named `my-channel` in the current +tracing session. + +[role="term"] +---- +lttng add-context --kernel --channel=my-channel --type=tid +---- +==== + +.Add an application-specific context field to a specific channel. +==== +The following command line adds the `cur_msg_id` context field of the +`retriever` context retriever for all the instrumented +<> recording <> +in the channel named `my-channel`: + +[role="term"] +---- +lttng add-context --kernel --channel=my-channel \ + --type='$app:retriever:cur_msg_id' +---- + +IMPORTANT: Make sure to always quote the `$` character when you +use man:lttng-add-context(1) from a shell. +==== + +NOTE: You cannot remove context fields from a channel once you add it. + + +[role="since-2.7"] +[[pid-tracking]] +=== Track process IDs + +It's often useful to allow only specific process IDs (PIDs) to emit +events. For example, you may wish to record all the system calls made by +a given process (à la http://linux.die.net/man/1/strace[strace]). + +The man:lttng-track(1) and man:lttng-untrack(1) commands serve this +purpose. Both commands operate on a whitelist of process IDs. You _add_ +entries to this whitelist with the man:lttng-track(1) command and remove +entries with the man:lttng-untrack(1) command. Any process which has one +of the PIDs in the whitelist is allowed to emit LTTng events which pass +an enabled <>. + +NOTE: The PID tracker tracks the _numeric process IDs_. Should a +process with a given tracked ID exit and another process be given this +ID, then the latter would also be allowed to emit events. + +.Track and untrack process IDs. +==== +For the sake of the following example, assume the target system has 16 +possible PIDs. + +When you +<>, +the whitelist contains all the possible PIDs: + +[role="img-100"] +.All PIDs are tracked. +image::track-all.png[] + +When the whitelist is full and you use the man:lttng-track(1) command to +specify some PIDs to track, LTTng first clears the whitelist, then it +tracks the specific PIDs. After: + +[role="term"] +---- +lttng track --pid=3,4,7,10,13 +---- + +the whitelist is: + +[role="img-100"] +.PIDs 3, 4, 7, 10, and 13 are tracked. +image::track-3-4-7-10-13.png[] + +You can add more PIDs to the whitelist afterwards: + +[role="term"] +---- +lttng track --pid=1,15,16 +---- + +The result is: + +[role="img-100"] +.PIDs 1, 15, and 16 are added to the whitelist. +image::track-1-3-4-7-10-13-15-16.png[] + +The man:lttng-untrack(1) command removes entries from the PID tracker's +whitelist. Given the previous example, the following command: + +[role="term"] +---- +lttng untrack --pid=3,7,10,13 +---- + +leads to this whitelist: + +[role="img-100"] +.PIDs 3, 7, 10, and 13 are removed from the whitelist. +image::track-1-4-15-16.png[] + +LTTng can track all possible PIDs again using the opt:track(1):--all +option: + +[role="term"] +---- +lttng track --pid --all +---- + +The result is, again: + +[role="img-100"] +.All PIDs are tracked. +image::track-all.png[] +==== + +.Track only specific PIDs +==== +A very typical use case with PID tracking is to start with an empty +whitelist, then <>, and +then add PIDs manually while tracers are active. You can accomplish this +by using the opt:lttng-untrack(1):--all option of the +man:lttng-untrack(1) command to clear the whitelist after you +<>: + +[role="term"] +---- +lttng untrack --pid --all +---- + +gives: + +[role="img-100"] +.No PIDs are tracked. +image::untrack-all.png[] + +If you trace with this whitelist configuration, the tracer records no +events for this <> because no processes are +tracked. You can use the man:lttng-track(1) command as usual to track +specific PIDs, for example: + +[role="term"] +---- +lttng track --pid=6,11 +---- + +Result: + +[role="img-100"] +.PIDs 6 and 11 are tracked. +image::track-6-11.png[] +==== + + +[role="since-2.5"] +[[saving-loading-tracing-session]] +=== Save and load tracing session configurations + +Configuring a <> can be long. Some of +the tasks involved are: + +* <> with + specific attributes. +* <> to specific channels. +* <> with specific log + level and filter conditions. + +If you use LTTng to solve real world problems, chances are you have to +record events using the same tracing session setup over and over, +modifying a few variables each time in your instrumented program +or environment. To avoid constant tracing session reconfiguration, +the man:lttng(1) command-line tool can save and load tracing session +configurations to/from XML files. + +To save a given tracing session configuration: + +* Use the man:lttng-save(1) command: ++ +-- +[role="term"] +---- +lttng save my-session +---- +-- ++ +Replace `my-session` with the name of the tracing session to save. + +LTTng saves tracing session configurations to +dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the +env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use +the opt:lttng-save(1):--output-path option to change this destination +directory. + +LTTng saves all configuration parameters, for example: + +* The tracing session name. +* The trace data output path. +* The channels with their state and all their attributes. +* The context fields you added to channels. +* The event rules with their state, log level and filter conditions. + +To load a tracing session: + +* Use the man:lttng-load(1) command: ++ +-- +[role="term"] +---- +lttng load my-session +---- +-- ++ +Replace `my-session` with the name of the tracing session to load. + +When LTTng loads a configuration, it restores your saved tracing session +as if you just configured it manually. + +See man:lttng(1) for the complete list of command-line options. You +can also save and load all many sessions at a time, and decide in which +directory to output the XML files. + + +[[sending-trace-data-over-the-network]] +=== Send trace data over the network + +LTTng can send the recorded trace data to a remote system over the +network instead of writing it to the local file system. + +To send the trace data over the network: + +. On the _remote_ system (which can also be the target system), + start an LTTng <> (man:lttng-relayd(8)): ++ +-- +[role="term"] +---- +lttng-relayd +---- +-- + +. On the _target_ system, create a tracing session configured to + send trace data over the network: ++ +-- +[role="term"] +---- +lttng create my-session --set-url=net://remote-system +---- +-- ++ +Replace `remote-system` by the host name or IP address of the +remote system. See man:lttng-create(1) for the exact URL format. + +. On the target system, use the man:lttng(1) command-line tool as usual. + When tracing is active, the target's consumer daemon sends sub-buffers + to the relay daemon running on the remote system intead of flushing + them to the local file system. The relay daemon writes the received + packets to the local file system. + +The relay daemon writes trace files to ++$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where ++__hostname__+ is the host name of the target system and +__session__+ +is the tracing session name. Note that the env:LTTNG_HOME environment +variable defaults to `$HOME` if not set. Use the +opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write +trace files to another base directory. + + +[role="since-2.4"] +[[lttng-live]] +=== View events as LTTng emits them (noch:{LTTng} live) + +LTTng live is a network protocol implemented by the <> (man:lttng-relayd(8)) to allow compatible trace viewers to +display events as LTTng emits them on the target system while tracing is +active. + +The relay daemon creates a _tee_: it forwards the trace data to both +the local file system and to connected live viewers: + +[role="img-90"] +.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer. +image::live.png[] + +To use LTTng live: + +. On the _target system_, create a <> + in _live mode_: ++ +-- +[role="term"] +---- +lttng create --live my-session +---- +-- ++ +This spawns a local relay daemon. + +. Start the live viewer and configure it to connect to the relay + daemon. For example, with http://diamon.org/babeltrace[Babeltrace]: ++ +-- +[role="term"] +---- +babeltrace --input-format=lttng-live net://localhost/host/hostname/my-session +---- +-- ++ +Replace: ++ +-- +* `hostname` with the host name of the target system. +* `my-session` with the name of the tracing session to view. +-- + +. Configure the tracing session as usual with the man:lttng(1) + command-line tool, and <>. + +You can list the available live tracing sessions with Babeltrace: + +[role="term"] +---- +babeltrace --input-format=lttng-live net://localhost +---- + +You can start the relay daemon on another system. In this case, you need +to specify the relay daemon's URL when you create the tracing session +with the opt:lttng-create(1):--set-url option. You also need to replace +`localhost` in the procedure above with the host name of the system on +which the relay daemon is running. + +See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of +command-line options. + + +[role="since-2.3"] +[[taking-a-snapshot]] +=== Take a snapshot of the current sub-buffers of a tracing session + +The normal behavior of LTTng is to append full sub-buffers to growing +trace data files. This is ideal to keep a full history of the events +that occurred on the target system, but it can +represent too much data in some situations. For example, you may wish +to trace your application continuously until some critical situation +happens, in which case you only need the latest few recorded +events to perform the desired analysis, not multi-gigabyte trace files. + +With the man:lttng-snapshot(1) command, you can take a snapshot of the +current sub-buffers of a given <>. +LTTng can write the snapshot to the local file system or send it over +the network. + +To take a snapshot: + +. Create a tracing session in _snapshot mode_: ++ +-- +[role="term"] +---- +lttng create --snapshot my-session +---- +-- ++ +The <> of +<> created in this mode is automatically set to +_overwrite_ (flight recorder mode). + +. Configure the tracing session as usual with the man:lttng(1) + command-line tool, and <>. + +. **Optional**: When you need to take a snapshot, + <>. ++ +You can take a snapshot when the tracers are active, but if you stop +them first, you are sure that the data in the sub-buffers does not +change before you actually take the snapshot. + +. Take a snapshot: ++ +-- +[role="term"] +---- +lttng snapshot record --name=my-first-snapshot +---- +-- ++ +LTTng writes the current sub-buffers of all the current tracing +session's channels to trace files on the local file system. Those trace +files have `my-first-snapshot` in their name. + +There is no difference between the format of a normal trace file and the +format of a snapshot: viewers of LTTng traces also support LTTng +snapshots. + +By default, LTTng writes snapshot files to the path shown by +`lttng snapshot list-output`. You can change this path or decide to send +snapshots over the network using either: + +. An output path or URL that you specify when you create the + tracing session. +. An snapshot output path or URL that you add using + `lttng snapshot add-output` +. An output path or URL that you provide directly to the + `lttng snapshot record` command. + +Method 3 overrides method 2, which overrides method 1. When you +specify a URL, a relay daemon must listen on a remote system (see +<>). + + +[role="since-2.6"] +[[mi]] +=== Use the machine interface + +With any command of the man:lttng(1) command-line tool, you can set the +opt:lttng(1):--mi option to `xml` (before the command name) to get an +XML machine interface output, for example: + +[role="term"] +---- +lttng --mi=xml enable-event --kernel --syscall open +---- + +A schema definition (XSD) is +https://github.com/lttng/lttng-tools/blob/stable-2.9/src/common/mi-lttng-3.0.xsd[available] +to ease the integration with external tools as much as possible. + + +[role="since-2.8"] +[[metadata-regenerate]] +=== Regenerate the metadata of an LTTng trace + +An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both +data stream files and a metadata file. This metadata file contains, +amongst other things, information about the offset of the clock sources +used to timestamp <> when tracing. + +If, once a <> is +<>, a major +https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction +happens, the trace's clock offset also needs to be updated. You +can use the `metadata` item of the man:lttng-regenerate(1) command +to do so. + +The main use case of this command is to allow a system to boot with +an incorrect wall time and trace it with LTTng before its wall time +is corrected. Once the system is known to be in a state where its +wall time is correct, it can run `lttng regenerate metadata`. + +To regenerate the metadata of an LTTng trace: + +* Use the `metadata` item of the man:lttng-regenerate(1) command: ++ +-- +[role="term"] +---- +lttng regenerate metadata +---- +-- + +[IMPORTANT] +==== +`lttng regenerate metadata` has the following limitations: + +* Tracing session <> + in non-live mode. +* User space <>, if any, are using + <>. +==== + + +[role="since-2.9"] +[[regenerate-statedump]] +=== Regenerate the state dump of a tracing session + +The LTTng kernel and user space tracers generate state dump +<> when the application starts or when you +<>. An analysis +can use the state dump event records to set an initial state before it +builds the rest of the state from the following event records. +http://tracecompass.org/[Trace Compass] is a notable example of an +application which uses the state dump of an LTTng trace. + +When you <>, it's possible that the +state dump event records are not included in the snapshot because they +were recorded to a sub-buffer that has been consumed or overwritten +already. + +You can use the `lttng regenerate statedump` command to emit the state +dump event records again. + +To regenerate the state dump of the current tracing session, provided +create it in snapshot mode, before you take a snapshot: + +. Use the `statedump` item of the man:lttng-regenerate(1) command: ++ +-- +[role="term"] +---- +lttng regenerate statedump +---- +-- + +. <>: ++ +-- +[role="term"] +---- +lttng stop +---- +-- + +. <>: ++ +-- +[role="term"] +---- +lttng snapshot record --name=my-snapshot +---- +-- + +Depending on the event throughput, you should run steps 1 and 2 +as closely as possible. + +NOTE: To record the state dump events, you need to +<> which enable them. +LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`. +LTTng-modules state dump tracepoints start with `lttng_statedump_`. + + +[role="since-2.7"] +[[persistent-memory-file-systems]] +=== Record trace data on persistent memory file systems + +https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory] +(NVRAM) is random-access memory that retains its information when power +is turned off (non-volatile). Systems with such memory can store data +structures in RAM and retrieve them after a reboot, without flushing +to typical _storage_. + +Linux supports NVRAM file systems thanks to either +http://pramfs.sourceforge.net/[PRAMFS] or +https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem] +(requires Linux 4.1+). + +This section does not describe how to operate such file systems; +we assume that you have a working persistent memory file system. + +When you create a <>, you can specify +the path of the shared memory holding the sub-buffers. If you specify a +location on an NVRAM file system, then you can retrieve the latest +recorded trace data when the system reboots after a crash. + +To record trace data on a persistent memory file system and retrieve the +trace data after a system crash: + +. Create a tracing session with a sub-buffer shared memory path located + on an NVRAM file system: ++ +-- +[role="term"] +---- +lttng create --shm-path=/path/to/shm +---- +-- + +. Configure the tracing session as usual with the man:lttng(1) + command-line tool, and <>. + +. After a system crash, use the man:lttng-crash(1) command-line tool to + view the trace data recorded on the NVRAM file system: ++ +-- +[role="term"] +---- +lttng-crash /path/to/shm +---- +-- + +The binary layout of the ring buffer files is not exactly the same as +the trace files layout. This is why you need to use man:lttng-crash(1) +instead of your preferred trace viewer directly. + +To convert the ring buffer files to LTTng trace files: + +* Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1): ++ +-- +[role="term"] +---- +lttng-crash --extract=/path/to/trace /path/to/shm +---- +-- + + +[[reference]] +== Reference + +[[lttng-modules-ref]] +=== noch:{LTTng-modules} + + +[role="since-2.9"] +[[lttng-tracepoint-enum]] +==== `LTTNG_TRACEPOINT_ENUM()` usage + +Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration: + +[source,c] +---- +LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries)) +---- + +Replace: + +* `name` with the name of the enumeration (C identifier, unique + amongst all the defined enumerations). +* `entries` with a list of enumeration entries. + +The available enumeration entry macros are: + ++ctf_enum_value(__name__, __value__)+:: + Entry named +__name__+ mapped to the integral value +__value__+. + ++ctf_enum_range(__name__, __begin__, __end__)+:: + Entry named +__name__+ mapped to the range of integral values between + +__begin__+ (included) and +__end__+ (included). + ++ctf_enum_auto(__name__)+:: + Entry named +__name__+ mapped to the integral value following the + last mapping's value. ++ +The last value of a `ctf_enum_value()` entry is its +__value__+ +parameter. ++ +The last value of a `ctf_enum_range()` entry is its +__end__+ parameter. ++ +If `ctf_enum_auto()` is the first entry in the list, its integral +value is 0. + +Use the `ctf_enum()` <> +to use a defined enumeration as a tracepoint field. + +.Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`. +==== +[source,c] +---- +LTTNG_TRACEPOINT_ENUM( + my_enum, + TP_ENUM_VALUES( + ctf_enum_auto("AUTO: EXPECT 0") + ctf_enum_value("VALUE: 23", 23) + ctf_enum_value("VALUE: 27", 27) + ctf_enum_auto("AUTO: EXPECT 28") + ctf_enum_range("RANGE: 101 TO 303", 101, 303) + ctf_enum_auto("AUTO: EXPECT 304") + ) +) +---- +==== + + +[role="since-2.7"] +[[lttng-modules-tp-fields]] +==== Tracepoint fields macros (for `TP_FIELDS()`) + +[[tp-fast-assign]][[tp-struct-entry]]The available macros to define +tracepoint fields, which must be listed within `TP_FIELDS()` in +`LTTNG_TRACEPOINT_EVENT()`, are: + +[role="func-desc growable",cols="asciidoc,asciidoc"] +.Available macros to define LTTng-modules tracepoint fields +|==== +|Macro |Description and parameters + +| ++ctf_integer(__t__, __n__, __e__)+ + ++ctf_integer_nowrite(__t__, __n__, __e__)+ + ++ctf_user_integer(__t__, __n__, __e__)+ + ++ctf_user_integer_nowrite(__t__, __n__, __e__)+ +| +Standard integer, displayed in base 10. + ++__t__+:: + Integer C type (`int`, `long`, `size_t`, ...). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_integer_hex(__t__, __n__, __e__)+ + ++ctf_user_integer_hex(__t__, __n__, __e__)+ +| +Standard integer, displayed in base 16. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +|+ctf_integer_oct(__t__, __n__, __e__)+ +| +Standard integer, displayed in base 8. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_integer_network(__t__, __n__, __e__)+ + ++ctf_user_integer_network(__t__, __n__, __e__)+ +| +Integer in network byte order (big-endian), displayed in base 10. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_integer_network_hex(__t__, __n__, __e__)+ + ++ctf_user_integer_network_hex(__t__, __n__, __e__)+ +| +Integer in network byte order, displayed in base 16. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_enum(__N__, __t__, __n__, __e__)+ + ++ctf_enum_nowrite(__N__, __t__, __n__, __e__)+ + ++ctf_user_enum(__N__, __t__, __n__, __e__)+ + ++ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+ +| +Enumeration. + ++__N__+:: + Name of a <>. + ++__t__+:: + Integer C type (`int`, `long`, `size_t`, ...). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_string(__n__, __e__)+ + ++ctf_string_nowrite(__n__, __e__)+ + ++ctf_user_string(__n__, __e__)+ + ++ctf_user_string_nowrite(__n__, __e__)+ +| +Null-terminated string; undefined behavior if +__e__+ is `NULL`. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_array(__t__, __n__, __e__, __s__)+ + ++ctf_array_nowrite(__t__, __n__, __e__, __s__)+ + ++ctf_user_array(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+ +| +Statically-sized array of integers. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__s__+:: + Number of elements. + +| ++ctf_array_bitfield(__t__, __n__, __e__, __s__)+ + ++ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+ +| +Statically-sized array of bits. + +The type of +__e__+ must be an integer type. +__s__+ is the number +of elements of such type in +__e__+, not the number of bits. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__s__+:: + Number of elements. + +| ++ctf_array_text(__t__, __n__, __e__, __s__)+ + ++ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_text(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+ +| +Statically-sized array, printed as text. + +The string does not need to be null-terminated. + ++__t__+:: + Array element C type (always `char`). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__s__+:: + Number of elements. + +| ++ctf_sequence(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of integers. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +| ++ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of integers, displayed in base 16. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +|+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of integers in network byte order (big-endian), +displayed in base 10. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +| ++ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of bits. + +The type of +__e__+ must be an integer type. +__s__+ is the number +of elements of such type in +__e__+, not the number of bits. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +| ++ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array, displayed as text. + +The string does not need to be null-terminated. + +The type of +__E__+ must be unsigned. + +The behaviour is undefined if +__e__+ is `NULL`. + ++__t__+:: + Sequence element C type (always `char`). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. +|==== + +Use the `_user` versions when the argument expression, `e`, is +a user space address. In the cases of `ctf_user_integer*()` and +`ctf_user_float*()`, `&e` must be a user space address, thus `e` must +be addressable. + +The `_nowrite` versions omit themselves from the session trace, but are +otherwise identical. This means the `_nowrite` fields won't be written +in the recorded trace. Their primary purpose is to make some +of the event context available to the +<> without having to +commit the data to sub-buffers. + + +[[glossary]] +== Glossary + +Terms related to LTTng and to tracing in general: + +Babeltrace:: + The http://diamon.org/babeltrace[Babeltrace] project, which includes + the cmd:babeltrace command, some libraries, and Python bindings. + +<>:: + A layout of sub-buffers applied to a given channel. + +<>:: + An entity which is responsible for a set of ring buffers. ++ +<> are always attached to a specific channel. + +clock:: + A reference of time for a tracer. + +<>:: + A process which is responsible for consuming the full sub-buffers + and write them to a file system or send them over the network. + +<>:: The event loss + mode in which the tracer _discards_ new event records when there's no + sub-buffer space left to store them. + +event:: + The consequence of the execution of an instrumentation + point, like a tracepoint that you manually place in some source code, + or a Linux kernel KProbe. ++ +An event is said to _occur_ at a specific time. Different actions can +be taken upon the occurance of an event, like record the event's payload +to a sub-buffer. + +<>:: + The mechanism by which event records of a given channel are lost + (not recorded) when there is no sub-buffer space left to store them. + +[[def-event-name]]event name:: + The name of an event, which is also the name of the event record. + This is also called the _instrumentation point name_. + +event record:: + A record, in a trace, of the payload of an event which occured. + +<>:: + Set of conditions which must be satisfied for one or more occuring + events to be recorded. + +`java.util.logging`:: + Java platform's + https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities]. + +<>:: + The use of LTTng probes to make a piece of software traceable. + +instrumentation point:: + A point in the execution path of a piece of software that, when + reached by this execution, can emit an event. + +instrumentation point name:: + See _<>_. + +log4j:: + A http://logging.apache.org/log4j/1.2/[logging library] for Java + developed by the Apache Software Foundation. + +log level:: + Level of severity of a log statement or user space + instrumentation point. + +LTTng:: + The _Linux Trace Toolkit: next generation_ project. + +<>:: + A command-line tool provided by the LTTng-tools project which you + can use to send and receive control messages to and from a + session daemon. + +LTTng analyses:: + The https://github.com/lttng/lttng-analyses[LTTng analyses] project, + which is a set of analyzing programs that are used to obtain a + higher level view of an LTTng trace. + +cmd:lttng-consumerd:: + The name of the consumer daemon program. + +cmd:lttng-crash:: + A utility provided by the LTTng-tools project which can convert + ring buffer files (usually + <>) + to trace files. + +LTTng Documentation:: + This document. + +<>:: + A communication protocol between the relay daemon and live viewers + which makes it possible to see events "live", as they are received by + the relay daemon. + +<>:: + The https://github.com/lttng/lttng-modules[LTTng-modules] project, + which contains the Linux kernel modules to make the Linux kernel + instrumentation points available for LTTng tracing. + +cmd:lttng-relayd:: + The name of the relay daemon program. + +cmd:lttng-sessiond:: + The name of the session daemon program. + +LTTng-tools:: + The https://github.com/lttng/lttng-tools[LTTng-tools] project, which + contains the various programs and libraries used to + <>. + +<>:: + The https://github.com/lttng/lttng-ust[LTTng-UST] project, which + contains libraries to instrument user applications. + +<>:: + A Java package provided by the LTTng-UST project to allow the + LTTng instrumentation of `java.util.logging` and Apache log4j 1.2 + logging statements. + +<>:: + A Python package provided by the LTTng-UST project to allow the + LTTng instrumentation of Python logging statements. + +<>:: + The event loss mode in which new event records overwrite older + event records when there's no sub-buffer space left to store them. + +<>:: + A buffering scheme in which each instrumented process has its own + sub-buffers for a given user space channel. + +<>:: + A buffering scheme in which all the processes of a Unix user share the + same sub-buffer for a given user space channel. + +<>:: + A process which is responsible for receiving the trace data sent by + a distant consumer daemon. + +ring buffer:: + A set of sub-buffers. + +<>:: + A process which receives control commands from you and orchestrates + the tracers and various LTTng daemons. + +<>:: + A copy of the current data of all the sub-buffers of a given tracing + session, saved as trace files. + +sub-buffer:: + One part of an LTTng ring buffer which contains event records. + +timestamp:: + The time information attached to an event when it is emitted. + +trace (_noun_):: + A set of files which are the concatenations of one or more + flushed sub-buffers. + +trace (_verb_):: + The action of recording the events emitted by an application + or by a system, or to initiate such recording by controlling + a tracer. + +Trace Compass:: + The http://tracecompass.org[Trace Compass] project and application. + +tracepoint:: + An instrumentation point using the tracepoint mechanism of the Linux + kernel or of LTTng-UST. + +tracepoint definition:: + The definition of a single tracepoint. + +tracepoint name:: + The name of a tracepoint. + +tracepoint provider:: + A set of functions providing tracepoints to an instrumented user + application. ++ +Not to be confused with a _tracepoint provider package_: many tracepoint +providers can exist within a tracepoint provider package. + +tracepoint provider package:: + One or more tracepoint providers compiled as an object file or as + a shared library. + +tracer:: + A software which records emitted events. + +<>:: + A namespace for event sources. + +tracing group:: + The Unix group in which a Unix user can be to be allowed to trace the + Linux kernel. + +<>:: + A stateful dialogue between you and a <>. + +user application:: + An application running in user space, as opposed to a Linux kernel + module, for example.