102 строки
3.6 KiB
Plaintext
102 строки
3.6 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>
|
|
|
|
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.
|
|
|
|
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.
|