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author | 2015-08-04 12:17:53 -0700 | |
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committer | 2015-08-04 15:44:42 -0700 | |
commit | 9ca8dbcc65cfc63d6f5ef3312a33184e1d726e00 (patch) | |
tree | 1c9cafbcd35f783a87880a10f85d1a060db1a563 /kernel/Documentation/trace/tracepoints.txt | |
parent | 98260f3884f4a202f9ca5eabed40b1354c489b29 (diff) |
Add the rt linux 4.1.3-rt3 as base
Import the rt linux 4.1.3-rt3 as OPNFV kvm base.
It's from git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-4.1.y-rt and
the base is:
commit 0917f823c59692d751951bf5ea699a2d1e2f26a2
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Sat Jul 25 12:13:34 2015 +0200
Prepare v4.1.3-rt3
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
We lose all the git history this way and it's not good. We
should apply another opnfv project repo in future.
Change-Id: I87543d81c9df70d99c5001fbdf646b202c19f423
Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>
Diffstat (limited to 'kernel/Documentation/trace/tracepoints.txt')
-rw-r--r-- | kernel/Documentation/trace/tracepoints.txt | 145 |
1 files changed, 145 insertions, 0 deletions
diff --git a/kernel/Documentation/trace/tracepoints.txt b/kernel/Documentation/trace/tracepoints.txt new file mode 100644 index 000000000..a3efac621 --- /dev/null +++ b/kernel/Documentation/trace/tracepoints.txt @@ -0,0 +1,145 @@ + 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/events/subsys.h : + +#undef TRACE_SYSTEM +#define TRACE_SYSTEM subsys + +#if !defined(_TRACE_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) +#define _TRACE_SUBSYS_H + +#include <linux/tracepoint.h> + +DECLARE_TRACE(subsys_eventname, + TP_PROTO(int firstarg, struct task_struct *p), + TP_ARGS(firstarg, p)); + +#endif /* _TRACE_SUBSYS_H */ + +/* This part must be outside protection */ +#include <trace/define_trace.h> + +In subsys/file.c (where the tracing statement must be added) : + +#include <trace/events/subsys.h> + +#define CREATE_TRACE_POINTS +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. + +- if you use the header in multiple source files, #define CREATE_TRACE_POINTS + should appear only in one source file. + +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. + +If you need to do a bit of work for a tracepoint parameter, and +that work is only used for the tracepoint, that work can be encapsulated +within an if statement with the following: + + if (trace_foo_bar_enabled()) { + int i; + int tot = 0; + + for (i = 0; i < count; i++) + tot += calculate_nuggets(); + + trace_foo_bar(tot); + } + +All trace_<tracepoint>() calls have a matching trace_<tracepoint>_enabled() +function defined that returns true if the tracepoint is enabled and +false otherwise. The trace_<tracepoint>() should always be within the +block of the if (trace_<tracepoint>_enabled()) to prevent races between +the tracepoint being enabled and the check being seen. + +The advantage of using the trace_<tracepoint>_enabled() is that it uses +the static_key of the tracepoint to allow the if statement to be implemented +with jump labels and avoid conditional branches. + +Note: The convenience macro TRACE_EVENT provides an alternative way to + define tracepoints. Check http://lwn.net/Articles/379903, + http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362 + for a series of articles with more details. |