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Subject[PATCH] perf: fix assertion failure in x86_pmu_start()

The following patch fixes an issue 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:

[89214.962603] ------------[ cut here ]------------
[89214.967441] WARNING: at arch/x86/kernel/cpu/perf_event.c:995 x86_pmu_start+0x79/0xd4()
[89214.975825] Hardware name: X8DTN
[89214.979268] Modules linked in:
[89214.982560] Pid: 0, comm: swapper/6 Not tainted 3.3.0-rc2-tip+ #1
[89214.988865] Call Trace:
[89214.991533] <IRQ> [<ffffffff81065cc7>] warn_slowpath_common+0x7e/0x97
[89214.998379] [<ffffffff81065cf5>] warn_slowpath_null+0x15/0x17
[89215.004428] [<ffffffff8103f626>] x86_pmu_start+0x79/0xd4
[89215.010042] [<ffffffff810e30d1>] perf_adjust_freq_unthr_context.part.63+0xef/0x123
[89215.018123] [<ffffffff810e318c>] perf_event_task_tick+0x87/0x1c1
[89215.024463] [<ffffffff810a2370>] ? tick_nohz_handler+0xda/0xda
[89215.030595] [<ffffffff8108b819>] scheduler_tick+0xd1/0xf3
[89215.036296] [<ffffffff810720b0>] update_process_times+0x5e/0x6f
[89215.042512] [<ffffffff810a23e0>] tick_sched_timer+0x70/0x99
[89215.048387] [<ffffffff810823f9>] __run_hrtimer+0x8c/0x148
[89215.054087] [<ffffffff81082add>] hrtimer_interrupt+0xc1/0x18c
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()

Signed-off-by: Stephane Eranian <eranian@google.com>
---
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 3c44b71..f8bddb5 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -988,6 +988,9 @@ 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;

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7c3b9de..ee151f4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2303,7 +2303,9 @@ 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 dostop)
{
struct hw_perf_event *hwc = &event->hw;
s64 period, sample_period;
@@ -2322,9 +2324,13 @@ 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 (dostop)
+ event->pmu->stop(event, PERF_EF_UPDATE);
+
local64_set(&hwc->period_left, 0);
- event->pmu->start(event, PERF_EF_RELOAD);
+
+ if (dostop)
+ event->pmu->start(event, PERF_EF_RELOAD);
}
}

@@ -2381,9 +2387,12 @@ 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);
}
@@ -4567,7 +4576,8 @@ 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);
}

/*

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