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Li Zefan
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refs/heads/master: be4bdbfbae6b303c21ebe446648f617908a794b5 | ||
refs/heads/master: a4ec5e0c2681b8cf99ddabf118705847f7460f19 |
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piggyback | ||
pnmtologo | ||
ppc_defs.h* | ||
promcon_tbl.c | ||
pss_boot.h | ||
qconf | ||
raid6altivec*.c | ||
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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 | ||
examples of how to insert markers in the kernel and connect probe functions to | ||
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 | ||
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). | ||
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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. | ||
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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); | ||
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. | ||
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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. | ||
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marker_synchronize_unregister() must be called between probe unregistration and | ||
the first occurrence of | ||
- the end of module exit function, | ||
to make sure there is no caller left using the probe; | ||
- the free of any resource used by the probes, | ||
to make sure the probes wont be accessing invalid data. | ||
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. | ||
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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. | ||
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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: | ||
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"Format mismatch for probe probe_name (format), marker (format)" | ||
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Another way to use markers is to simply define the marker without generating any | ||
function call to actually call into the marker. This is useful in combination | ||
with tracepoint probes in a scheme like this : | ||
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void probe_tracepoint_name(unsigned int arg1, struct task_struct *tsk); | ||
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DEFINE_MARKER_TP(marker_eventname, tracepoint_name, probe_tracepoint_name, | ||
"arg1 %u pid %d"); | ||
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notrace void probe_tracepoint_name(unsigned int arg1, struct task_struct *tsk) | ||
{ | ||
struct marker *marker = &GET_MARKER(kernel_irq_entry); | ||
/* write data to trace buffers ... */ | ||
} | ||
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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 : | ||
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 |
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