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author | Ingo Molnar <mingo@elte.hu> | 2009-06-07 12:22:15 +0200 |
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committer | Ingo Molnar <mingo@elte.hu> | 2009-06-07 12:22:15 +0200 |
commit | 5f4457a4f62cc9d78e04c0eb12ff0540899aad89 (patch) | |
tree | 0b973d527ea6b2ae31e08da0746b4965a3c5a6d8 /Documentation/trace/tracepoints.txt | |
parent | 9b94b3a19b13e094c10f65f24bc358f6ffe4eacd (diff) | |
parent | b87297fb405ef13cac375f202d114323b076a56d (diff) |
Merge branch 'linus' into x86/cpu
Diffstat (limited to 'Documentation/trace/tracepoints.txt')
-rw-r--r-- | Documentation/trace/tracepoints.txt | 116 |
1 files changed, 116 insertions, 0 deletions
diff --git a/Documentation/trace/tracepoints.txt b/Documentation/trace/tracepoints.txt new file mode 100644 index 00000000000..c0e1ceed75a --- /dev/null +++ b/Documentation/trace/tracepoints.txt @@ -0,0 +1,116 @@ + 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 |