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1 Using the Linux Kernel Markers
2
3 Mathieu Desnoyers
4
5
6This document introduces Linux Kernel Markers and their use. It provides
7examples of how to insert markers in the kernel and connect probe functions to
8them and provides some examples of probe functions.
9
10
11* Purpose of markers
12
13A marker placed in code provides a hook to call a function (probe) that you can
14provide at runtime. A marker can be "on" (a probe is connected to it) or "off"
15(no probe is attached). When a marker is "off" it has no effect, except for
16adding a tiny time penalty (checking a condition for a branch) and space
17penalty (adding a few bytes for the function call at the end of the
18instrumented function and adds a data structure in a separate section). When a
19marker is "on", the function you provide is called each time the marker is
20executed, in the execution context of the caller. When the function provided
21ends its execution, it returns to the caller (continuing from the marker site).
22
23You can put markers at important locations in the code. Markers are
24lightweight hooks that can pass an arbitrary number of parameters,
25described in a printk-like format string, to the attached probe function.
26
27They can be used for tracing and performance accounting.
28
29
30* Usage
31
32In order to use the macro trace_mark, you should include linux/marker.h.
33
34#include <linux/marker.h>
35
36And,
37
38trace_mark(subsystem_event, "%d %s", someint, somestring);
39Where :
40- subsystem_event is an identifier unique to your event
41 - subsystem is the name of your subsystem.
42 - event is the name of the event to mark.
43- "%d %s" is the formatted string for the serializer.
44- someint is an integer.
45- somestring is a char pointer.
46
47Connecting a function (probe) to a marker is done by providing a probe (function
48to call) for the specific marker through marker_probe_register() and can be
49activated by calling marker_arm(). Marker deactivation can be done by calling
50marker_disarm() as many times as marker_arm() has been called. Removing a probe
51is done through marker_probe_unregister(); it will disarm the probe and make
52sure there is no caller left using the probe when it returns. Probe removal is
53preempt-safe because preemption is disabled around the probe call. See the
54"Probe example" section below for a sample probe module.
55
56The marker mechanism supports inserting multiple instances of the same marker.
57Markers can be put in inline functions, inlined static functions, and
58unrolled loops as well as regular functions.
59
60The naming scheme "subsystem_event" is suggested here as a convention intended
61to limit collisions. Marker names are global to the kernel: they are considered
62as being the same whether they are in the core kernel image or in modules.
63Conflicting format strings for markers with the same name will cause the markers
64to be detected to have a different format string not to be armed and will output
65a printk warning which identifies the inconsistency:
66
67"Format mismatch for probe probe_name (format), marker (format)"
68
69
70* Probe / marker example
71
72See the example provided in samples/markers/src
73
74Compile them with your kernel.
75
76Run, as root :
77modprobe marker-example (insmod order is not important)
78modprobe probe-example
79cat /proc/marker-example (returns an expected error)
80rmmod marker-example probe-example
81dmesg