Commit 9b231d9f authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

perf/core: Fix time on IOC_ENABLE 



Vince reported that when we do IOC_ENABLE/IOC_DISABLE while the task
is SIGSTOP'ed state the timestamps go wobbly.

It turns out we indeed fail to correctly account time while in 'OFF'
state and doing IOC_ENABLE without getting scheduled in exposes the
problem.

Further thinking about this problem, it occurred to me that we can
suffer a similar fate when we migrate an uncore event between CPUs.
The perf_event_install() on the 'new' CPU will do add_event_to_ctx()
which will reset all the time stamp, resulting in a subsequent
update_event_times() to overwrite the total_time_* fields with smaller
values.

Reported-by: default avatarVince Weaver <vincent.weaver@maine.edu>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent bfe33492

kernel/events/core.c

+36 −5
Original line number Diff line number Diff line
@@ -2217,6 +2217,33 @@ static int group_can_go_on(struct perf_event *event, 
	return can_add_hw;

}



/*

 * Complement to update_event_times(). This computes the tstamp_* values to

 * continue 'enabled' state from @now, and effectively discards the time

 * between the prior tstamp_stopped and now (as we were in the OFF state, or

 * just switched (context) time base).

 *

 * This further assumes '@event->state == INACTIVE' (we just came from OFF) and

 * cannot have been scheduled in yet. And going into INACTIVE state means

 * '@event->tstamp_stopped = @now'.

 *

 * Thus given the rules of update_event_times():

 *

 *   total_time_enabled = tstamp_stopped - tstamp_enabled

 *   total_time_running = tstamp_stopped - tstamp_running

 *

 * We can insert 'tstamp_stopped == now' and reverse them to compute new

 * tstamp_* values.

 */

static void __perf_event_enable_time(struct perf_event *event, u64 now)

{

	WARN_ON_ONCE(event->state != PERF_EVENT_STATE_INACTIVE);



	event->tstamp_stopped = now;

	event->tstamp_enabled = now - event->total_time_enabled;

	event->tstamp_running = now - event->total_time_running;

}



static void add_event_to_ctx(struct perf_event *event,

			       struct perf_event_context *ctx)

{
@@ -2224,9 +2251,12 @@ static void add_event_to_ctx(struct perf_event *event, 


	list_add_event(event, ctx);

	perf_group_attach(event);

	event->tstamp_enabled = tstamp;

	event->tstamp_running = tstamp;

	event->tstamp_stopped = tstamp;

	/*

	 * We can be called with event->state == STATE_OFF when we create with

	 * .disabled = 1. In that case the IOC_ENABLE will call this function.

	 */

	if (event->state == PERF_EVENT_STATE_INACTIVE)

		__perf_event_enable_time(event, tstamp);

}



static void ctx_sched_out(struct perf_event_context *ctx,
@@ -2471,10 +2501,11 @@ static void __perf_event_mark_enabled(struct perf_event *event) 
	u64 tstamp = perf_event_time(event);



	event->state = PERF_EVENT_STATE_INACTIVE;

	event->tstamp_enabled = tstamp - event->total_time_enabled;

	__perf_event_enable_time(event, tstamp);

	list_for_each_entry(sub, &event->sibling_list, group_entry) {

		/* XXX should not be > INACTIVE if event isn't */

		if (sub->state >= PERF_EVENT_STATE_INACTIVE)

			sub->tstamp_enabled = tstamp - sub->total_time_enabled;

			__perf_event_enable_time(sub, tstamp);

	}

}

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