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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>
101 lines
3.7 KiB
Text
101 lines
3.7 KiB
Text
Using the Linux Kernel Tracepoints
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Mathieu Desnoyers
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This document introduces Linux Kernel Tracepoints and their use. It provides
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examples of how to insert tracepoints in the kernel and connect probe functions
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to them and provides some examples of probe functions.
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* Purpose of tracepoints
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A tracepoint placed in code provides a hook to call a function (probe) that you
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can provide at runtime. A tracepoint can be "on" (a probe is connected to it) or
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"off" (no probe is attached). When a tracepoint is "off" it has no effect,
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except for adding a tiny time penalty (checking a condition for a branch) and
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space penalty (adding a few bytes for the function call at the end of the
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instrumented function and adds a data structure in a separate section). When a
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tracepoint is "on", the function you provide is called each time the tracepoint
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is executed, in the execution context of the caller. When the function provided
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ends its execution, it returns to the caller (continuing from the tracepoint
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site).
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You can put tracepoints at important locations in the code. They are
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lightweight hooks that can pass an arbitrary number of parameters,
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which prototypes are described in a tracepoint declaration placed in a header
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file.
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They can be used for tracing and performance accounting.
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* Usage
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Two elements are required for tracepoints :
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- A tracepoint definition, placed in a header file.
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- The tracepoint statement, in C code.
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In order to use tracepoints, you should include linux/tracepoint.h.
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In include/trace/subsys.h :
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#include <linux/tracepoint.h>
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DEFINE_TRACE(subsys_eventname,
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TPPTOTO(int firstarg, struct task_struct *p),
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TPARGS(firstarg, p));
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In subsys/file.c (where the tracing statement must be added) :
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#include <trace/subsys.h>
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void somefct(void)
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{
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...
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trace_subsys_eventname(arg, task);
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...
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}
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Where :
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- subsys_eventname is an identifier unique to your event
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- subsys is the name of your subsystem.
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- eventname is the name of the event to trace.
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- TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function
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called by this tracepoint.
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- TPARGS(firstarg, p) are the parameters names, same as found in the prototype.
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Connecting a function (probe) to a tracepoint is done by providing a probe
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(function to call) for the specific tracepoint through
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register_trace_subsys_eventname(). Removing a probe is done through
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unregister_trace_subsys_eventname(); it will remove the probe sure there is no
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caller left using the probe when it returns. Probe removal is preempt-safe
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because preemption is disabled around the probe call. See the "Probe example"
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section below for a sample probe module.
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The tracepoint mechanism supports inserting multiple instances of the same
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tracepoint, but a single definition must be made of a given tracepoint name over
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all the kernel to make sure no type conflict will occur. Name mangling of the
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tracepoints is done using the prototypes to make sure typing is correct.
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Verification of probe type correctness is done at the registration site by the
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compiler. Tracepoints can be put in inline functions, inlined static functions,
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and unrolled loops as well as regular functions.
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The naming scheme "subsys_event" is suggested here as a convention intended
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to limit collisions. Tracepoint names are global to the kernel: they are
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considered as being the same whether they are in the core kernel image or in
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modules.
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* Probe / tracepoint example
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See the example provided in samples/tracepoints/src
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Compile them with your kernel.
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Run, as root :
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modprobe tracepoint-example (insmod order is not important)
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modprobe tracepoint-probe-example
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cat /proc/tracepoint-example (returns an expected error)
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rmmod tracepoint-example tracepoint-probe-example
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dmesg
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