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               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). The immediate values are used to minimize the impact on data cache, encoding the condition in the instruction stream. 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
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)"
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 :
void probe_tracepoint_name(unsigned int arg1, struct task_struct *tsk);
DEFINE_MARKER_TP(marker_eventname, tracepoint_name, probe_tracepoint_name,
"arg1 %u pid %d");
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 ... */
}
* Optimization for a given architecture
To force use of a non-optimized version of the markers, _trace_mark() should be used. It takes the same parameters as the normal markers, but it does not use the immediate values based on code patching.
* 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


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