diff options
Diffstat (limited to 'Documentation/trace/tracepoints.txt')
| -rw-r--r-- | Documentation/trace/tracepoints.txt | 48 |
1 files changed, 46 insertions, 2 deletions
diff --git a/Documentation/trace/tracepoints.txt b/Documentation/trace/tracepoints.txt index da49437d5ae..a3efac621c5 100644 --- a/Documentation/trace/tracepoints.txt +++ b/Documentation/trace/tracepoints.txt @@ -40,7 +40,13 @@ Two elements are required for tracepoints : In order to use tracepoints, you should include linux/tracepoint.h. -In include/trace/subsys.h : +In include/trace/events/subsys.h : + +#undef TRACE_SYSTEM +#define TRACE_SYSTEM subsys + +#if !defined(_TRACE_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) +#define _TRACE_SUBSYS_H #include <linux/tracepoint.h> @@ -48,10 +54,16 @@ DECLARE_TRACE(subsys_eventname, TP_PROTO(int firstarg, struct task_struct *p), TP_ARGS(firstarg, p)); +#endif /* _TRACE_SUBSYS_H */ + +/* This part must be outside protection */ +#include <trace/define_trace.h> + In subsys/file.c (where the tracing statement must be added) : -#include <trace/subsys.h> +#include <trace/events/subsys.h> +#define CREATE_TRACE_POINTS DEFINE_TRACE(subsys_eventname); void somefct(void) @@ -72,6 +84,9 @@ Where : - TP_ARGS(firstarg, p) are the parameters names, same as found in the prototype. +- if you use the header in multiple source files, #define CREATE_TRACE_POINTS + should appear only in one source file. + 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 @@ -99,3 +114,32 @@ core kernel image or in modules. If the tracepoint has to be used in kernel modules, an EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be used to export the defined tracepoints. + +If you need to do a bit of work for a tracepoint parameter, and +that work is only used for the tracepoint, that work can be encapsulated +within an if statement with the following: + + if (trace_foo_bar_enabled()) { + int i; + int tot = 0; + + for (i = 0; i < count; i++) + tot += calculate_nuggets(); + + trace_foo_bar(tot); + } + +All trace_<tracepoint>() calls have a matching trace_<tracepoint>_enabled() +function defined that returns true if the tracepoint is enabled and +false otherwise. The trace_<tracepoint>() should always be within the +block of the if (trace_<tracepoint>_enabled()) to prevent races between +the tracepoint being enabled and the check being seen. + +The advantage of using the trace_<tracepoint>_enabled() is that it uses +the static_key of the tracepoint to allow the if statement to be implemented +with jump labels and avoid conditional branches. + +Note: The convenience macro TRACE_EVENT provides an alternative way to + define tracepoints. Check http://lwn.net/Articles/379903, + http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362 + for a series of articles with more details. |