Using the Linux Kernel Tracepoints

                            Mathieu Desnoyers


This document introduces Linux Kernel Tracepoints and their use. It
provides examples of how to insert tracepoints in the kernel and
connect probe functions to them and provides some examples of probe
functions.


* Purpose of tracepoints

A tracepoint placed in code provides a hook to call a function (probe)
that you can provide at runtime. A tracepoint can be "on" (a probe is
connected to it) or "off" (no probe is attached). When a tracepoint is
"off" it has no effect, except for adding a tiny time penalty
(checking a condition for a branch) and space penalty (adding a few
bytes for the function call at the end of the instrumented function
and adds a data structure in a separate section).  When a tracepoint
is "on", the function you provide is called each time the tracepoint
is executed, in the execution context of the caller. When the function
provided ends its execution, it returns to the caller (continuing from
the tracepoint site).

You can put tracepoints at important locations in the code. They are
lightweight hooks that can pass an arbitrary number of parameters,
which prototypes are described in a tracepoint declaration placed in a
header file.

They can be used for tracing and performance accounting.


* Usage

Two elements are required for tracepoints :

- A tracepoint definition, placed in a header file.
- The tracepoint statement, in C code.

In order to use tracepoints, you should include linux/tracepoint.h.

In include/trace/subsys.h :

#include <linux/tracepoint.h>

DECLARE_TRACE(subsys_eventname,
        TP_PROTO(int firstarg, struct task_struct *p),
        TP_ARGS(firstarg, p));

In subsys/file.c (where the tracing statement must be added) :

#include <trace/subsys.h>

DEFINE_TRACE(subsys_eventname);

void somefct(void)
{
        ...
        trace_subsys_eventname(arg, task);
        ...
}

Where :
- subsys_eventname is an identifier unique to your event
    - subsys is the name of your subsystem.
    - eventname is the name of the event to trace.

- TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the
  function called by this tracepoint.

- TP_ARGS(firstarg, p) are the parameters names, same as found in the
  prototype.

Connecting a function (probe) to a tracepoint is done by providing a
probe (function to call) for the specific tracepoint through
register_trace_subsys_eventname().  Removing a probe is done through
unregister_trace_subsys_eventname(); it will remove the probe.

tracepoint_synchronize_unregister() must be called before the end of
the module exit function to make sure there is no caller left using
the probe. This, and the fact that preemption is disabled around the
probe call, make sure that probe removal and module unload are safe.
See the "Probe example" section below for a sample probe module.

The tracepoint mechanism supports inserting multiple instances of the
same tracepoint, but a single definition must be made of a given
tracepoint name over all the kernel to make sure no type conflict will
occur. Name mangling of the tracepoints is done using the prototypes
to make sure typing is correct. Verification of probe type correctness
is done at the registration site by the compiler. Tracepoints can be
put in inline functions, inlined static functions, and unrolled loops
as well as regular functions.

The naming scheme "subsys_event" is suggested here as a convention
intended to limit collisions. Tracepoint names are global to the
kernel: they are considered as being the same whether they are in the
core kernel image or in modules.

If the tracepoint has to be used in kernel modules, an
EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
used to export the defined tracepoints.

* Probe / tracepoint example

See the example provided in samples/tracepoints

Compile them with your kernel.  They are built during 'make' (not
'make modules') when CONFIG_SAMPLE_TRACEPOINTS=m.

Run, as root :
modprobe tracepoint-sample (insmod order is not important)
modprobe tracepoint-probe-sample
cat /proc/tracepoint-sample (returns an expected error)
rmmod tracepoint-sample tracepoint-probe-sample
dmesg