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    Subject[patch 5/5] Linux Kernel Markers - Documentation
    Here is some documentation explaining what is/how to use the Linux
    Kernel Markers.

    Changelog:
    - Move the examples to a separate "samples" patch.

    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    Acked-by: "Frank Ch. Eigler" <fche@redhat.com>
    CC: Christoph Hellwig <hch@infradead.org>
    ---

    Documentation/markers.txt | 81 ++++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 81 insertions(+)

    Index: linux-2.6-lttng/Documentation/markers.txt
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux-2.6-lttng/Documentation/markers.txt 2007-09-24 17:44:56.000000000 -0400
    @@ -0,0 +1,81 @@
    + Using the Linux Kernel Markers
    +
    + Mathieu Desnoyers
    +
    +
    +This document introduces Linux Kernel Markers and their use. It provides
    +examples of how to insert markers in the kernel and connect probe functions to
    +them and provides some examples of probe functions.
    +
    +
    +* Purpose of markers
    +
    +A marker placed in code provides a hook to call a function (probe) that you can
    +provide at runtime. A marker can be "on" (a probe is connected to it) or "off"
    +(no probe is attached). When a marker 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
    +marker is "on", the function you provide is called each time the marker is
    +executed, in the execution context of the caller. When the function provided
    +ends its execution, it returns to the caller (continuing from the marker site).
    +
    +You can put markers at important locations in the code. Markers are
    +lightweight hooks that can pass an arbitrary number of parameters,
    +described in a printk-like format string, to the attached probe function.
    +
    +They can be used for tracing and performance accounting.
    +
    +
    +* Usage
    +
    +In order to use the macro trace_mark, you should include linux/marker.h.
    +
    +#include <linux/marker.h>
    +
    +And,
    +
    +trace_mark(subsystem_event, "%d %s", someint, somestring);
    +Where :
    +- subsystem_event is an identifier unique to your event
    + - subsystem is the name of your subsystem.
    + - event is the name of the event to mark.
    +- "%d %s" is the formatted string for the serializer.
    +- someint is an integer.
    +- somestring is a char pointer.
    +
    +Connecting a function (probe) to a marker is done by providing a probe (function
    +to call) for the specific marker through marker_probe_register() and can be
    +activated by calling marker_arm(). Marker deactivation can be done by calling
    +marker_disarm() as many times as marker_arm() has been called. Removing a probe
    +is done through marker_probe_unregister(); it will disarm the probe and make
    +sure there is no caller left using the probe when it returns. Probe removal is
    +preempt-safe because preemption is disabled around the probe call. See the
    +"Probe example" section below for a sample probe module.
    +
    +The marker mechanism supports inserting multiple instances of the same marker.
    +Markers can be put in inline functions, inlined static functions, and
    +unrolled loops as well as regular functions.
    +
    +The naming scheme "subsystem_event" is suggested here as a convention intended
    +to limit collisions. Marker names are global to the kernel: they are considered
    +as being the same whether they are in the core kernel image or in modules.
    +Conflicting format strings for markers with the same name will cause the markers
    +to be detected to have a different format string not to be armed and will output
    +a printk warning which identifies the inconsistency:
    +
    +"Format mismatch for probe probe_name (format), marker (format)"
    +
    +
    +* Probe / marker example
    +
    +See the example provided in samples/markers/src
    +
    +Compile them with your kernel.
    +
    +Run, as root :
    +modprobe marker-example (insmod order is not important)
    +modprobe probe-example
    +cat /proc/marker-example (returns an expected error)
    +rmmod marker-example probe-example
    +dmesg
    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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