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: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (Red Hat) 2015-02-09 16:05:55 -05:00 committed by Steven Rostedt
parent 4e20e3a60b
commit c4c7eb2938
2 changed files with 61 additions and 0 deletions

View File

@ -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)

View File

@ -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. *****/