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   1 ---
   2 id: mainline-trace-event
   3 ---
   4
   5 The first step is to define tracepoints using the mainline Linux
   6 `TRACE_EVENT()` macro and insert tracepoints where you want them.
   7 Your tracepoint definitions reside in a header file in
   8 `include/trace/events`. If you're adding tracepoints to an existing
   9 subsystem, edit its appropriate header file.
  10
  11 As an example, the following header file (let's call it
  12 `include/trace/events/hello.h`) defines one tracepoint using
  13 `TRACE_EVENT()`:
  14
  15 ~~~ c
  16 /* subsystem name is "hello" */
  17 #undef TRACE_SYSTEM
  18 #define TRACE_SYSTEM hello
  19
  20 #if !defined(_TRACE_HELLO_H) || defined(TRACE_HEADER_MULTI_READ)
  21 #define _TRACE_HELLO_H
  22
  23 #include <linux/tracepoint.h>
  24
  25 TRACE_EVENT(
  26     /* "hello" is the subsystem name, "world" is the event name */
  27     hello_world,
  28
  29     /* tracepoint function prototype */
  30     TP_PROTO(int foo, const char* bar),
  31
  32     /* arguments for this tracepoint */
  33     TP_ARGS(foo, bar),
  34
  35     /* LTTng doesn't need those */
  36     TP_STRUCT__entry(),
  37     TP_fast_assign(),
  38     TP_printk("", 0)
  39 );
  40
  41 #endif
  42
  43 /* this part must be outside protection */
  44 #include <trace/define_trace.h>
  45 ~~~
  46
  47 Notice that we don't use any of the last three arguments: they
  48 are left empty here because LTTng doesn't need them. You would only fill
  49 `TP_STRUCT__entry()`, `TP_fast_assign()` and `TP_printk()` if you were
  50 to also use this tracepoint for ftrace/perf.
  51
  52 Once this is done, you may place calls to `trace_hello_world()`
  53 wherever you want in the Linux source code. As an example, let us place
  54 such a tracepoint in the `usb_probe_device()` static function
  55 (`drivers/usb/core/driver.c`):
  56
  57 ~~~ c
  58 /* called from driver core with dev locked */
  59 static int usb_probe_device(struct device *dev)
  60 {
  61     struct usb_device_driver *udriver = to_usb_device_driver(dev->driver);
  62     struct usb_device *udev = to_usb_device(dev);
  63     int error = 0;
  64
  65     trace_hello_world(udev->devnum, udev->product);
  66
  67     /* ... */
  68 }
  69 ~~~
  70
  71 This tracepoint should fire every time a USB device is plugged in.
  72
  73 At the top of `driver.c`, we need to include our actual tracepoint
  74 definition and, in this case (one place per subsystem), define
  75 `CREATE_TRACE_POINTS`, which will create our tracepoint:
  76
  77 ~~~ c
  78 /* ... */
  79
  80 #include "usb.h"
  81
  82 #define CREATE_TRACE_POINTS
  83 #include <trace/events/hello.h>
  84
  85 /* ... */
  86 ~~~
  87
  88 Build your custom Linux kernel. In order to use LTTng, make sure the
  89 following kernel configuration options are enabled:
  90
  91   * `CONFIG_MODULES` (loadable module support)
  92   * `CONFIG_KALLSYMS` (load all symbols for debugging/kksymoops)
  93   * `CONFIG_HIGH_RES_TIMERS` (high resolution timer support)
  94   * `CONFIG_TRACEPOINTS` (kernel tracepoint instrumentation)
  95
  96 Boot the custom kernel. The directory
  97 `/sys/kernel/debug/tracing/events/hello` should exist if everything
  98 went right, with a `hello_world` subdirectory.