src/patches/suse-2.6.27.25/patches.trace/tracepoints-documentation.patch

   1 From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   2 Subject: Tracepoints Documentation
   3
   4 Documentation of tracepoint usage.
   5
   6 Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   7 Acked-by: 'Peter Zijlstra' <peterz@infradead.org>
   8 CC: Masami Hiramatsu <mhiramat@redhat.com>
   9 CC: "Frank Ch. Eigler" <fche@redhat.com>
  10 CC: 'Ingo Molnar' <mingo@elte.hu>
  11 CC: 'Hideo AOKI' <haoki@redhat.com>
  12 CC: Takashi Nishiie <t-nishiie@np.css.fujitsu.com>
  13 CC: 'Steven Rostedt' <rostedt@goodmis.org>
  14 CC: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
  15 Acked-by: Jan Blunck <jblunck@suse.de>
  16 ---
  17  Documentation/tracepoints.txt |  101 ++++++++++++++++++++++++++++++++++++++++++
  18  1 file changed, 101 insertions(+)
  19
  20 Index: linux-2.6-lttng/Documentation/tracepoints.txt
  21 ===================================================================
  22 --- /dev/null   1970-01-01 00:00:00.000000000 +0000
  23 +++ linux-2.6-lttng/Documentation/tracepoints.txt       2008-07-18 12:15:32.000000000 -0400
  24 @@ -0,0 +1,101 @@
  25 +                    Using the Linux Kernel Tracepoints
  26 +
  27 +                           Mathieu Desnoyers
  28 +
  29 +
  30 +This document introduces Linux Kernel Tracepoints and their use. It provides
  31 +examples of how to insert tracepoints in the kernel and connect probe functions
  32 +to them and provides some examples of probe functions.
  33 +
  34 +
  35 +* Purpose of tracepoints
  36 +
  37 +A tracepoint placed in code provides a hook to call a function (probe) that you
  38 +can provide at runtime. A tracepoint can be "on" (a probe is connected to it) or
  39 +"off" (no probe is attached). When a tracepoint is "off" it has no effect,
  40 +except for adding a tiny time penalty (checking a condition for a branch) and
  41 +space penalty (adding a few bytes for the function call at the end of the
  42 +instrumented function and adds a data structure in a separate section).  When a
  43 +tracepoint is "on", the function you provide is called each time the tracepoint
  44 +is executed, in the execution context of the caller. When the function provided
  45 +ends its execution, it returns to the caller (continuing from the tracepoint
  46 +site).
  47 +
  48 +You can put tracepoints at important locations in the code. They are
  49 +lightweight hooks that can pass an arbitrary number of parameters,
  50 +which prototypes are described in a tracepoint declaration placed in a header
  51 +file.
  52 +
  53 +They can be used for tracing and performance accounting.
  54 +
  55 +
  56 +* Usage
  57 +
  58 +Two elements are required for tracepoints :
  59 +
  60 +- A tracepoint definition, placed in a header file.
  61 +- The tracepoint statement, in C code.
  62 +
  63 +In order to use tracepoints, you should include linux/tracepoint.h.
  64 +
  65 +In include/trace/subsys.h :
  66 +
  67 +#include <linux/tracepoint.h>
  68 +
  69 +DEFINE_TRACE(subsys_eventname,
  70 +       TPPTOTO(int firstarg, struct task_struct *p),
  71 +       TPARGS(firstarg, p));
  72 +
  73 +In subsys/file.c (where the tracing statement must be added) :
  74 +
  75 +#include <trace/subsys.h>
  76 +
  77 +void somefct(void)
  78 +{
  79 +       ...
  80 +       trace_subsys_eventname(arg, task);
  81 +       ...
  82 +}
  83 +
  84 +Where :
  85 +- subsys_eventname is an identifier unique to your event
  86 +    - subsys is the name of your subsystem.
  87 +    - eventname is the name of the event to trace.
  88 +- TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function
  89 +  called by this tracepoint.
  90 +- TPARGS(firstarg, p) are the parameters names, same as found in the prototype.
  91 +
  92 +Connecting a function (probe) to a tracepoint is done by providing a probe
  93 +(function to call) for the specific tracepoint through
  94 +register_trace_subsys_eventname().  Removing a probe is done through
  95 +unregister_trace_subsys_eventname(); it will remove the probe sure there is no
  96 +caller left using the probe when it returns. Probe removal is preempt-safe
  97 +because preemption is disabled around the probe call. See the "Probe example"
  98 +section below for a sample probe module.
  99 +
 100 +The tracepoint mechanism supports inserting multiple instances of the same
 101 +tracepoint, but a single definition must be made of a given tracepoint name over
 102 +all the kernel to make sure no type conflict will occur. Name mangling of the
 103 +tracepoints is done using the prototypes to make sure typing is correct.
 104 +Verification of probe type correctness is done at the registration site by the
 105 +compiler. Tracepoints can be put in inline functions, inlined static functions,
 106 +and unrolled loops as well as regular functions.
 107 +
 108 +The naming scheme "subsys_event" is suggested here as a convention intended
 109 +to limit collisions. Tracepoint names are global to the kernel: they are
 110 +considered as being the same whether they are in the core kernel image or in
 111 +modules.
 112 +
 113 +
 114 +* Probe / tracepoint example
 115 +
 116 +See the example provided in samples/tracepoints/src
 117 +
 118 +Compile them with your kernel.
 119 +
 120 +Run, as root :
 121 +modprobe tracepoint-example (insmod order is not important)
 122 +modprobe tracepoint-probe-example
 123 +cat /proc/tracepoint-example (returns an expected error)
 124 +rmmod tracepoint-example tracepoint-probe-example
 125 +dmesg