84 lines
3.4 KiB
Plaintext
84 lines
3.4 KiB
Plaintext
Using the Linux Kernel Markers
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Mathieu Desnoyers
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This document introduces Linux Kernel Markers and their use. It provides
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examples of how to insert markers in the kernel and connect probe functions to
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them and provides some examples of probe functions.
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* Purpose of markers
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A marker placed in code provides a hook to call a function (probe) that you can
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provide at runtime. A marker can be "on" (a probe is connected to it) or "off"
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(no probe is attached). When a marker is "off" it has no effect, except for
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adding a tiny time penalty (checking a condition for a branch) and space
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penalty (adding a few bytes for the function call at the end of the
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instrumented function and adds a data structure in a separate section). When a
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marker is "on", the function you provide is called each time the marker is
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executed, in the execution context of the caller. When the function provided
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ends its execution, it returns to the caller (continuing from the marker site).
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You can put markers at important locations in the code. Markers are
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lightweight hooks that can pass an arbitrary number of parameters,
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described in a printk-like format string, to the attached probe function.
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They can be used for tracing and performance accounting.
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* Usage
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In order to use the macro trace_mark, you should include linux/marker.h.
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#include <linux/marker.h>
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And,
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trace_mark(subsystem_event, "myint %d mystring %s", someint, somestring);
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Where :
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- subsystem_event is an identifier unique to your event
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- subsystem is the name of your subsystem.
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- event is the name of the event to mark.
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- "myint %d mystring %s" is the formatted string for the serializer. "myint" and
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"mystring" are repectively the field names associated with the first and
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second parameter.
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- someint is an integer.
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- somestring is a char pointer.
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Connecting a function (probe) to a marker is done by providing a probe (function
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to call) for the specific marker through marker_probe_register() and can be
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activated by calling marker_arm(). Marker deactivation can be done by calling
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marker_disarm() as many times as marker_arm() has been called. Removing a probe
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is done through marker_probe_unregister(); it will disarm the probe and make
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sure there is no caller left using the probe when it returns. Probe removal is
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preempt-safe because preemption is disabled around the probe call. See the
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"Probe example" section below for a sample probe module.
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The marker mechanism supports inserting multiple instances of the same marker.
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Markers can be put in inline functions, inlined static functions, and
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unrolled loops as well as regular functions.
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The naming scheme "subsystem_event" is suggested here as a convention intended
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to limit collisions. Marker names are global to the kernel: they are considered
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as being the same whether they are in the core kernel image or in modules.
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Conflicting format strings for markers with the same name will cause the markers
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to be detected to have a different format string not to be armed and will output
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a printk warning which identifies the inconsistency:
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"Format mismatch for probe probe_name (format), marker (format)"
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* Probe / marker example
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See the example provided in samples/markers/src
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Compile them with your kernel.
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Run, as root :
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modprobe marker-example (insmod order is not important)
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modprobe probe-example
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cat /proc/marker-example (returns an expected error)
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rmmod marker-example probe-example
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dmesg
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