contents/using-lttng/instrumenting/instrumenting-linux-kernel/instrumenting-linux-kernel-itself/lttng-adaptation-layer.md

   1 ---
   2 id: lttng-adaptation-layer
   3 ---
   4
   5 The steps to write the LTTng adaptation layer are, in your
   6 LTTng-modules copy's source code tree:
   7
   8   1. In `instrumentation/events/lttng-module`,
   9      add a header <code><em>subsys</em>.h</code> for your custom
  10      subsystem <code><em>subsys</em></code> and write your
  11      tracepoint definitions using LTTng-modules macros in it.
  12      Those macros look like the mainline kernel equivalents,
  13      but they present subtle, yet important differences.
  14   2. In `probes`, create the C source file of the LTTng probe kernel
  15      module for your subsystem. It should be named
  16      <code>lttng-probe-<em>subsys</em>.c</code>.
  17   3. Edit `probes/Makefile` so that the LTTng-modules project
  18      builds your custom LTTng probe kernel module.
  19   4. Build and install LTTng kernel modules.
  20
  21 Following our `hello_world` event example, here's the content of
  22 `instrumentation/events/lttng-module/hello.h`:
  23
  24 ~~~ c
  25 #undef TRACE_SYSTEM
  26 #define TRACE_SYSTEM hello
  27
  28 #if !defined(_TRACE_HELLO_H) || defined(TRACE_HEADER_MULTI_READ)
  29 #define _TRACE_HELLO_H
  30
  31 #include "../../../probes/lttng-tracepoint-event.h"
  32 #include <linux/tracepoint.h>
  33
  34 LTTNG_TRACEPOINT_EVENT(
  35     /* format identical to mainline version for those */
  36     hello_world,
  37     TP_PROTO(int foo, const char* bar),
  38     TP_ARGS(foo, bar),
  39
  40     /* possible differences */
  41     TP_STRUCT__entry(
  42         __field(int, my_int)
  43         __field(char, char0)
  44         __field(char, char1)
  45         __string(product, bar)
  46     ),
  47
  48     /* notice the use of tp_assign()/tp_strcpy() and no semicolons */
  49     TP_fast_assign(
  50         tp_assign(my_int, foo)
  51         tp_assign(char0, bar[0])
  52         tp_assign(char1, bar[1])
  53         tp_strcpy(product, bar)
  54     ),
  55
  56     /* This one is actually not used by LTTng either, but must be
  57      * present for the moment.
  58      */
  59     TP_printk("", 0)
  60
  61 /* no semicolon after this either */
  62 )
  63
  64 #endif
  65
  66 /* other difference: do NOT include <trace/define_trace.h> */
  67 #include "../../../probes/define_trace.h"
  68 ~~~
  69
  70 Some possible entries for `TP_STRUCT__entry()` and `TP_fast_assign()`,
  71 in the case of LTTng-modules, are shown in the
  72 [LTTng-modules reference](#doc-lttng-modules-ref) section.
  73
  74 The best way to learn how to use the above macros is to inspect
  75 existing LTTng tracepoint definitions in `instrumentation/events/lttng-module`
  76 header files. Compare them with the Linux kernel mainline versions
  77 in `include/trace/events`.
  78
  79 The next step is writing the LTTng probe kernel module C source file.
  80 This one is named <code>lttng-probe-<em>subsys</em>.c</code>
  81 in `probes`. You may always use the following template:
  82
  83 ~~~ c
  84 #include <linux/module.h>
  85 #include "../lttng-tracer.h"
  86
  87 /* Build time verification of mismatch between mainline TRACE_EVENT()
  88  * arguments and LTTng adaptation layer LTTNG_TRACEPOINT_EVENT() arguments.
  89  */
  90 #include <trace/events/hello.h>
  91
  92 /* create LTTng tracepoint probes */
  93 #define LTTNG_PACKAGE_BUILD
  94 #define CREATE_TRACE_POINTS
  95 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
  96
  97 #include "../instrumentation/events/lttng-module/hello.h"
  98
  99 MODULE_LICENSE("GPL and additional rights");
 100 MODULE_AUTHOR("Your name <your-email>");
 101 MODULE_DESCRIPTION("LTTng hello probes");
 102 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
 103     __stringify(LTTNG_MODULES_MINOR_VERSION) "."
 104     __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
 105     LTTNG_MODULES_EXTRAVERSION);
 106 ~~~
 107
 108 Just replace `hello` with your subsystem name. In this example,
 109 `<trace/events/hello.h>`, which is the original mainline tracepoint
 110 definition header, is included for verification purposes: the
 111 LTTng-modules build system is able to emit an error at build time when
 112 the arguments of the mainline `TRACE_EVENT()` definitions do not match
 113 the ones of the LTTng-modules adaptation layer
 114 (`LTTNG_TRACEPOINT_EVENT()`).
 115
 116 Edit `probes/Makefile` and add your new kernel module object
 117 next to existing ones:
 118
 119 ~~~ makefile
 120 # ...
 121
 122 obj-m += lttng-probe-module.o
 123 obj-m += lttng-probe-power.o
 124
 125 obj-m += lttng-probe-hello.o
 126
 127 # ...
 128 ~~~
 129
 130 Time to build! Point to your custom Linux kernel source tree using
 131 the `KERNELDIR` variable:
 132
 133 <pre class="term">
 134 make <strong>KERNELDIR=/path/to/custom/linux</strong>
 135 </pre>
 136
 137 Finally, install modules:
 138
 139 <pre class="term">
 140 sudo make modules_install
 141 </pre>