From: "Steven Rostedt (Red Hat)" <[email protected]> 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 <[email protected]> --- samples/trace_events/trace-events-sample.c | 3 ++ samples/trace_events/trace-events-sample.h | 58 ++++++++++++++++++++++++++++++ 2 files changed, 61 insertions(+) diff --git a/samples/trace_events/trace-events-sample.c b/samples/trace_events/trace-events-sample.c index 16c15c08ed38..c396a49b5d78 100644 --- a/samples/trace_events/trace-events-sample.c +++ b/samples/trace_events/trace-events-sample.c @@ -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) diff --git a/samples/trace_events/trace-events-sample.h b/samples/trace_events/trace-events-sample.h index dd65f7b8c0d9..c3232340914d 100644 --- a/samples/trace_events/trace-events-sample.h +++ b/samples/trace_events/trace-events-sample.h @@ -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. *****/ -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
