mirror of
https://github.com/FEX-Emu/linux.git
synced 2024-12-26 19:36:41 +00:00
24b8d831d5
Documentation of tracepoint usage. Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Acked-by: 'Peter Zijlstra' <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
102 lines
3.7 KiB
Plaintext
102 lines
3.7 KiB
Plaintext
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>
|
|
|
|
DEFINE_TRACE(subsys_eventname,
|
|
TPPTOTO(int firstarg, struct task_struct *p),
|
|
TPARGS(firstarg, p));
|
|
|
|
In subsys/file.c (where the tracing statement must be added) :
|
|
|
|
#include <trace/subsys.h>
|
|
|
|
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.
|
|
- 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 sure there is no
|
|
caller left using the probe when it returns. Probe removal is preempt-safe
|
|
because preemption is disabled around the probe call. 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.
|
|
|
|
|
|
* Probe / tracepoint example
|
|
|
|
See the example provided in samples/tracepoints/src
|
|
|
|
Compile them with your kernel.
|
|
|
|
Run, as root :
|
|
modprobe tracepoint-example (insmod order is not important)
|
|
modprobe tracepoint-probe-example
|
|
cat /proc/tracepoint-example (returns an expected error)
|
|
rmmod tracepoint-example tracepoint-probe-example
|
|
dmesg
|