diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/markers.txt | 10 | ||||
| -rw-r--r-- | Documentation/tracepoints.txt | 101 | ||||
| -rw-r--r-- | Documentation/tracers/mmiotrace.txt | 5 |
3 files changed, 109 insertions, 7 deletions
diff --git a/Documentation/markers.txt b/Documentation/markers.txt index d9f50a19fa0..089f6138fcd 100644 --- a/Documentation/markers.txt +++ b/Documentation/markers.txt @@ -50,10 +50,12 @@ 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 and make -sure there is no caller left using the probe when it returns. Probe removal is -preempt-safe because preemption is disabled around the probe call. See the -"Probe example" section below for a sample probe module. +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 diff --git a/Documentation/tracepoints.txt b/Documentation/tracepoints.txt new file mode 100644 index 00000000000..5d354e16749 --- /dev/null +++ b/Documentation/tracepoints.txt @@ -0,0 +1,101 @@ + Using the Linux Kernel Tracepoints + + Mathieu Desnoyers + + +This document introduces Linux Kernel Tracepoints and their use. It provides +examples of how to insert tracepoints in the kernel and connect probe functions +to them and provides some examples of probe functions. + + +* Purpose of tracepoints + +A tracepoint placed in code provides a hook to call a function (probe) that you +can provide at runtime. A tracepoint can be "on" (a probe is connected to it) or +"off" (no probe is attached). When a tracepoint 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 +tracepoint is "on", the function you provide is called each time the tracepoint +is executed, in the execution context of the caller. When the function provided +ends its execution, it returns to the caller (continuing from the tracepoint +site). + +You can put tracepoints at important locations in the code. They are +lightweight hooks that can pass an arbitrary number of parameters, +which prototypes are described in a tracepoint declaration placed in a header +file. + +They can be used for tracing and performance accounting. + + +* Usage + +Two elements are required for tracepoints : + +- A tracepoint definition, placed in a header file. +- The tracepoint statement, in C code. + +In order to use tracepoints, you should include linux/tracepoint.h. + +In include/trace/subsys.h : + +#include <linux/tracepoint.h> + +DEFINE_TRACE(subsys_eventname, + TPPTOTO(int firstarg, struct task_struct *p), + TPARGS(firstarg, p)); + +In subsys/file.c (where the tracing statement must be added) : + +#include <trace/subsys.h> + +void somefct(void) +{ + ... + trace_subsys_eventname(arg, task); + ... +} + +Where : +- subsys_eventname is an identifier unique to your event + - subsys is the name of your subsystem. + - eventname is the name of the event to trace. +- TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function + called by this tracepoint. +- TPARGS(firstarg, p) are the parameters names, same as found in the prototype. + +Connecting a function (probe) to a tracepoint is done by providing a probe +(function to call) for the specific tracepoint through +register_trace_subsys_eventname(). Removing a probe is done through +unregister_trace_subsys_eventname(); it will remove the probe sure there is no +caller left using the probe when it returns. Probe removal is preempt-safe +because preemption is disabled around the probe call. See the "Probe example" +section below for a sample probe module. + +The tracepoint mechanism supports inserting multiple instances of the same +tracepoint, but a single definition must be made of a given tracepoint name over +all the kernel to make sure no type conflict will occur. Name mangling of the +tracepoints is done using the prototypes to make sure typing is correct. +Verification of probe type correctness is done at the registration site by the +compiler. Tracepoints can be put in inline functions, inlined static functions, +and unrolled loops as well as regular functions. + +The naming scheme "subsys_event" is suggested here as a convention intended +to limit collisions. Tracepoint names are global to the kernel: they are +considered as being the same whether they are in the core kernel image or in +modules. + + +* Probe / tracepoint example + +See the example provided in samples/tracepoints/src + +Compile them with your kernel. + +Run, as root : +modprobe tracepoint-example (insmod order is not important) +modprobe tracepoint-probe-example +cat /proc/tracepoint-example (returns an expected error) +rmmod tracepoint-example tracepoint-probe-example +dmesg diff --git a/Documentation/tracers/mmiotrace.txt b/Documentation/tracers/mmiotrace.txt index a4afb560a45..5bbbe209622 100644 --- a/Documentation/tracers/mmiotrace.txt +++ b/Documentation/tracers/mmiotrace.txt @@ -36,7 +36,7 @@ $ mount -t debugfs debugfs /debug $ echo mmiotrace > /debug/tracing/current_tracer $ cat /debug/tracing/trace_pipe > mydump.txt & Start X or whatever. -$ echo "X is up" > /debug/tracing/marker +$ echo "X is up" > /debug/tracing/trace_marker $ echo none > /debug/tracing/current_tracer Check for lost events. @@ -59,9 +59,8 @@ The 'cat' process should stay running (sleeping) in the background. Load the driver you want to trace and use it. Mmiotrace will only catch MMIO accesses to areas that are ioremapped while mmiotrace is active. -[Unimplemented feature:] During tracing you can place comments (markers) into the trace by -$ echo "X is up" > /debug/tracing/marker +$ echo "X is up" > /debug/tracing/trace_marker This makes it easier to see which part of the (huge) trace corresponds to which action. It is recommended to place descriptive markers about what you do. |