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, "myint %d mystring %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.
- "myint %d mystring %s" is the formatted string for the serializer. "myint" and
  "mystring" are repectively the field names associated with the first and
  second parameter.
- 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.
marker_synchronize_unregister() must be called before the end of the module exit
function to make sure there is no caller left using the probe. This, and the
fact that preemption is disabled around the probe call, make sure that probe
removal and module unload are safe. 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