linux/Documentation/trace/tracepoints.txt
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   1                     Using the Linux Kernel Tracepoints
   2
   3                            Mathieu Desnoyers
   4
   5
   6This document introduces Linux Kernel Tracepoints and their use. It
   7provides examples of how to insert tracepoints in the kernel and
   8connect probe functions to them and provides some examples of probe
   9functions.
  10
  11
  12* Purpose of tracepoints
  13
  14A tracepoint placed in code provides a hook to call a function (probe)
  15that you can provide at runtime. A tracepoint can be "on" (a probe is
  16connected to it) or "off" (no probe is attached). When a tracepoint is
  17"off" it has no effect, except for adding a tiny time penalty
  18(checking a condition for a branch) and space penalty (adding a few
  19bytes for the function call at the end of the instrumented function
  20and adds a data structure in a separate section).  When a tracepoint
  21is "on", the function you provide is called each time the tracepoint
  22is executed, in the execution context of the caller. When the function
  23provided ends its execution, it returns to the caller (continuing from
  24the tracepoint site).
  25
  26You can put tracepoints at important locations in the code. They are
  27lightweight hooks that can pass an arbitrary number of parameters,
  28which prototypes are described in a tracepoint declaration placed in a
  29header file.
  30
  31They can be used for tracing and performance accounting.
  32
  33
  34* Usage
  35
  36Two elements are required for tracepoints :
  37
  38- A tracepoint definition, placed in a header file.
  39- The tracepoint statement, in C code.
  40
  41In order to use tracepoints, you should include linux/tracepoint.h.
  42
  43In include/trace/subsys.h :
  44
  45#include <linux/tracepoint.h>
  46
  47DECLARE_TRACE(subsys_eventname,
  48        TP_PROTO(int firstarg, struct task_struct *p),
  49        TP_ARGS(firstarg, p));
  50
  51In subsys/file.c (where the tracing statement must be added) :
  52
  53#include <trace/subsys.h>
  54
  55DEFINE_TRACE(subsys_eventname);
  56
  57void somefct(void)
  58{
  59        ...
  60        trace_subsys_eventname(arg, task);
  61        ...
  62}
  63
  64Where :
  65- subsys_eventname is an identifier unique to your event
  66    - subsys is the name of your subsystem.
  67    - eventname is the name of the event to trace.
  68
  69- TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the
  70  function called by this tracepoint.
  71
  72- TP_ARGS(firstarg, p) are the parameters names, same as found in the
  73  prototype.
  74
  75Connecting a function (probe) to a tracepoint is done by providing a
  76probe (function to call) for the specific tracepoint through
  77register_trace_subsys_eventname().  Removing a probe is done through
  78unregister_trace_subsys_eventname(); it will remove the probe.
  79
  80tracepoint_synchronize_unregister() must be called before the end of
  81the module exit function to make sure there is no caller left using
  82the probe. This, and the fact that preemption is disabled around the
  83probe call, make sure that probe removal and module unload are safe.
  84See the "Probe example" section below for a sample probe module.
  85
  86The tracepoint mechanism supports inserting multiple instances of the
  87same tracepoint, but a single definition must be made of a given
  88tracepoint name over all the kernel to make sure no type conflict will
  89occur. Name mangling of the tracepoints is done using the prototypes
  90to make sure typing is correct. Verification of probe type correctness
  91is done at the registration site by the compiler. Tracepoints can be
  92put in inline functions, inlined static functions, and unrolled loops
  93as well as regular functions.
  94
  95The naming scheme "subsys_event" is suggested here as a convention
  96intended to limit collisions. Tracepoint names are global to the
  97kernel: they are considered as being the same whether they are in the
  98core kernel image or in modules.
  99
 100If the tracepoint has to be used in kernel modules, an
 101EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
 102used to export the defined tracepoints.
 103
 104* Probe / tracepoint example
 105
 106See the example provided in samples/tracepoints
 107
 108Compile them with your kernel.  They are built during 'make' (not
 109'make modules') when CONFIG_SAMPLE_TRACEPOINTS=m.
 110
 111Run, as root :
 112modprobe tracepoint-sample (insmod order is not important)
 113modprobe tracepoint-probe-sample
 114cat /proc/tracepoint-sample (returns an expected error)
 115rmmod tracepoint-sample tracepoint-probe-sample
 116dmesg
 117
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