[patch 5/5] Linux Kernel Markers - Documentation

From: Mathieu Desnoyers
Date: Fri Sep 28 2007 - 10:55:42 EST

Here is some documentation explaining what is/how to use the Linux
Kernel Markers.

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

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Acked-by: "Frank Ch. Eigler" <fche@xxxxxxxxxx>
CC: Christoph Hellwig <hch@xxxxxxxxxxxxx>

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>
+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

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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