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Commit c4c7eb29 authored by Steven Rostedt (Red Hat)'s avatar Steven Rostedt (Red Hat) Committed by Steven Rostedt
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tracing: Add TRACE_EVENT_CONDITION sample 



The sample code lacks an example of TRACE_EVENT_CONDITION, and it
has been expressed to me that this feature for TRACE_EVENT is not
well known and not used when it could be.

Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent 4e20e3a6

samples/trace_events/trace-events-sample.c

+3 −0
Original line number Diff line number Diff line
@@ -31,8 +31,11 @@ static void simple_thread_func(int cnt) 
		array[i] = i + 1;

	array[i] = 0;



	/* Silly tracepoints */

	trace_foo_bar("hello", cnt, array, random_strings[len],

		      tsk_cpus_allowed(current));



	trace_foo_bar_with_cond("Some times print", cnt);

}



static int simple_thread(void *arg)

samples/trace_events/trace-events-sample.h

+58 −0
Original line number Diff line number Diff line
@@ -212,6 +212,64 @@ TRACE_EVENT(foo_bar, 
				sizeof(int)),

		  __get_str(str), __get_bitmask(cpus))

);



/*

 * There may be a case where a tracepoint should only be called if

 * some condition is set. Otherwise the tracepoint should not be called.

 * But to do something like:

 *

 *  if (cond)

 *     trace_foo();

 *

 * Would cause a little overhead when tracing is not enabled, and that

 * overhead, even if small, is not something we want. As tracepoints

 * use static branch (aka jump_labels), where no branch is taken to

 * skip the tracepoint when not enabled, and a jmp is placed to jump

 * to the tracepoint code when it is enabled, having a if statement

 * nullifies that optimization. It would be nice to place that

 * condition within the static branch. This is where TRACE_EVENT_CONDITION

 * comes in.

 *

 * TRACE_EVENT_CONDITION() is just like TRACE_EVENT, except it adds another

 * parameter just after args. Where TRACE_EVENT has:

 *

 * TRACE_EVENT(name, proto, args, struct, assign, printk)

 *

 * the CONDITION version has:

 *

 * TRACE_EVENT_CONDITION(name, proto, args, cond, struct, assign, printk)

 *

 * Everything is the same as TRACE_EVENT except for the new cond. Think

 * of the cond variable as:

 *

 *   if (cond)

 *      trace_foo_bar_with_cond();

 *

 * Except that the logic for the if branch is placed after the static branch.

 * That is, the if statement that processes the condition will not be

 * executed unless that traecpoint is enabled. Otherwise it still remains

 * a nop.

 */

TRACE_EVENT_CONDITION(foo_bar_with_cond,



	TP_PROTO(const char *foo, int bar),



	TP_ARGS(foo, bar),



	TP_CONDITION(!(bar % 10)),



	TP_STRUCT__entry(

		__string(	foo,    foo		)

		__field(	int,	bar			)

	),



	TP_fast_assign(

		__assign_str(foo, foo);

		__entry->bar	= bar;

	),



	TP_printk("foo %s %d", __get_str(foo), __entry->bar)

);

#endif



/***** NOTICE! The #if protection ends here. *****/