perf: Fix double start/stop in x86_pmu_start()

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

        e050e3f0a71b ("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:

	84f2b9b ("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: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Stephane Eranian <eranian@google.com>
Acked-by: Peter 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: Ingo Molnar <mingo@elte.hu>

1 files changed, 14 insertions, 5 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ba36013cfb21..1b5c081d8b9f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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;
@@ -2322,9 +2322,13 @@ 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) {
-                event->pmu->stop(event, PERF_EF_UPDATE);
+                if (disable)
+                        event->pmu->stop(event, PERF_EF_UPDATE);
+
                 local64_set(&hwc->period_left, 0);
-                event->pmu->start(event, PERF_EF_RELOAD);
+
+                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);
         }
 
         /*