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Commit f39d47ff authored by Stephane Eranian's avatar Stephane Eranian Committed by Ingo Molnar
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perf: Fix double start/stop in x86_pmu_start() 



The following patch fixes a bug introduced by the following
commit:

        e050e3f0 ("perf: Fix broken interrupt rate throttling")

The patch caused the following warning to pop up depending on
the sampling frequency adjustments:

  ------------[ cut here ]------------
  WARNING: at arch/x86/kernel/cpu/perf_event.c:995 x86_pmu_start+0x79/0xd4()

It was caused by the following call sequence:

perf_adjust_freq_unthr_context.part() {
     stop()
     if (delta > 0) {
          perf_adjust_period() {
              if (period > 8*...) {
                  stop()
                  ...
                  start()
              }
          }
      }
      start()
}

Which caused a double start and a double stop, thus triggering
the assert in x86_pmu_start().

The patch fixes the problem by avoiding the double calls. We
pass a new argument to perf_adjust_period() to indicate whether
or not the event is already stopped. We can't just remove the
start/stop from that function because it's called from
__perf_event_overflow where the event needs to be reloaded via a
stop/start back-toback call.

The patch reintroduces the assertion in x86_pmu_start() which
was removed by commit:

	84f2b9b2 ("perf: Remove deprecated WARN_ON_ONCE()")

In this second version, we've added calls to disable/enable PMU
during unthrottling or frequency adjustment based on bug report
of spurious NMI interrupts from Eric Dumazet.

Reported-and-tested-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: default avatarStephane Eranian <eranian@google.com>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: markus@trippelsdorf.de
Cc: paulus@samba.org
Link: http://lkml.kernel.org/r/20120207133956.GA4932@quad


[ Minor edits to the changelog and to the code ]
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 136e0b8e

arch/x86/kernel/cpu/perf_event.c

+3 −0
Original line number Diff line number Diff line
@@ -986,6 +986,9 @@ static void x86_pmu_start(struct perf_event *event, int flags) 
	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);

	int idx = event->hw.idx;



	if (WARN_ON_ONCE(!(event->hw.state & PERF_HES_STOPPED)))

		return;



	if (WARN_ON_ONCE(idx == -1))

		return;

kernel/events/core.c

+14 −5
Original line number Diff line number Diff line
@@ -2303,7 +2303,7 @@ do { \ 
static DEFINE_PER_CPU(int, perf_throttled_count);

static DEFINE_PER_CPU(u64, perf_throttled_seq);



static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count)

static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count, bool disable)

{

	struct hw_perf_event *hwc = &event->hw;

	s64 period, sample_period;
@@ -2322,8 +2322,12 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count) 
	hwc->sample_period = sample_period;



	if (local64_read(&hwc->period_left) > 8*sample_period) {

		if (disable)

			event->pmu->stop(event, PERF_EF_UPDATE);



		local64_set(&hwc->period_left, 0);



		if (disable)

			event->pmu->start(event, PERF_EF_RELOAD);

	}

}
@@ -2350,6 +2354,7 @@ static void perf_adjust_freq_unthr_context(struct perf_event_context *ctx, 
		return;



	raw_spin_lock(&ctx->lock);

	perf_pmu_disable(ctx->pmu);



	list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {

		if (event->state != PERF_EVENT_STATE_ACTIVE)
@@ -2381,13 +2386,17 @@ static void perf_adjust_freq_unthr_context(struct perf_event_context *ctx, 
		/*

		 * restart the event

		 * reload only if value has changed

		 * we have stopped the event so tell that

		 * to perf_adjust_period() to avoid stopping it

		 * twice.

		 */

		if (delta > 0)

			perf_adjust_period(event, period, delta);

			perf_adjust_period(event, period, delta, false);



		event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);

	}



	perf_pmu_enable(ctx->pmu);

	raw_spin_unlock(&ctx->lock);

}
@@ -4562,7 +4571,7 @@ static int __perf_event_overflow(struct perf_event *event, 
		hwc->freq_time_stamp = now;



		if (delta > 0 && delta < 2*TICK_NSEC)

			perf_adjust_period(event, delta, hwc->last_period);

			perf_adjust_period(event, delta, hwc->last_period, true);

	}



	/*