Commit 0a7ad645 authored by Ingo Molnar's avatar Ingo Molnar

tracepoints: format documentation

Impact: documentation update

Properly format Documentation/tracepoints.txt - it was full of
overlong lines and other typographical problems.
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 0dcf8fe5
......@@ -3,28 +3,30 @@
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.
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).
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.
which prototypes are described in a tracepoint declaration placed in a
header file.
They can be used for tracing and performance accounting.
......@@ -63,36 +65,41 @@ 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.
- TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function
called by this tracepoint.
- TPARGS(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
- TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the
function called by this tracepoint.
- TPARGS(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.
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.
EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
used to export the defined tracepoints.
* Probe / tracepoint example
......
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