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perf/core: Fix time on IOC_ENABLE

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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: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2017-08-03 15:42:09 +02:00 committed by Ingo Molnar
parent bfe334924c
commit 9b231d9f47
1 changed files with 36 additions and 5 deletions
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41
kernel/events/core.c
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@ -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);
}
}