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Commit a737823d authored by Will Deacon's avatar Will Deacon Committed by Russell King
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ARM: 6835/1: perf: ensure overflows aren't missed due to IRQ latency 



If a counter overflows during a perf stat profiling run it may overtake
the last known value of the counter:

    0        prev     new                0xffffffff
    |----------|-------|----------------------|

In this case, the number of events that have occurred is
(0xffffffff - prev) + new. Unfortunately, the event update code will
not realise an overflow has occurred and will instead report the event
delta as (new - prev) which may be considerably smaller than the real
count.

This patch adds an extra argument to armpmu_event_update which indicates
whether or not an overflow has occurred. If an overflow has occurred
then we use the maximum period of the counter to calculate the elapsed
events.

Acked-by: default avatarJamie Iles <jamie@jamieiles.com>
Reported-by: default avatarAshwin Chaugule <ashwinc@codeaurora.org>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 574b69cb

arch/arm/kernel/perf_event.c

+11 −8
Original line number Diff line number Diff line
@@ -205,11 +205,9 @@ armpmu_event_set_period(struct perf_event *event, 
static u64

armpmu_event_update(struct perf_event *event,

		    struct hw_perf_event *hwc,

		    int idx)

		    int idx, int overflow)

{

	int shift = 64 - 32;

	s64 prev_raw_count, new_raw_count;

	u64 delta;

	u64 delta, prev_raw_count, new_raw_count;



again:

	prev_raw_count = local64_read(&hwc->prev_count);
@@ -219,8 +217,13 @@ armpmu_event_update(struct perf_event *event, 
			     new_raw_count) != prev_raw_count)

		goto again;



	delta = (new_raw_count << shift) - (prev_raw_count << shift);

	delta >>= shift;

	new_raw_count &= armpmu->max_period;

	prev_raw_count &= armpmu->max_period;



	if (overflow)

		delta = armpmu->max_period - prev_raw_count + new_raw_count;

	else

		delta = new_raw_count - prev_raw_count;



	local64_add(delta, &event->count);

	local64_sub(delta, &hwc->period_left);
@@ -237,7 +240,7 @@ armpmu_read(struct perf_event *event) 
	if (hwc->idx < 0)

		return;



	armpmu_event_update(event, hwc, hwc->idx);

	armpmu_event_update(event, hwc, hwc->idx, 0);

}



static void
@@ -255,7 +258,7 @@ armpmu_stop(struct perf_event *event, int flags) 
	if (!(hwc->state & PERF_HES_STOPPED)) {

		armpmu->disable(hwc, hwc->idx);

		barrier(); /* why? */

		armpmu_event_update(event, hwc, hwc->idx);

		armpmu_event_update(event, hwc, hwc->idx, 0);

		hwc->state |= PERF_HES_STOPPED | PERF_HES_UPTODATE;

	}

}

arch/arm/kernel/perf_event_v6.c

+1 −1
Original line number Diff line number Diff line
@@ -474,7 +474,7 @@ armv6pmu_handle_irq(int irq_num, 
			continue;



		hwc = &event->hw;

		armpmu_event_update(event, hwc, idx);

		armpmu_event_update(event, hwc, idx, 1);

		data.period = event->hw.last_period;

		if (!armpmu_event_set_period(event, hwc, idx))

			continue;

arch/arm/kernel/perf_event_v7.c

+1 −1
Original line number Diff line number Diff line
@@ -782,7 +782,7 @@ static irqreturn_t armv7pmu_handle_irq(int irq_num, void *dev) 
			continue;



		hwc = &event->hw;

		armpmu_event_update(event, hwc, idx);

		armpmu_event_update(event, hwc, idx, 1);

		data.period = event->hw.last_period;

		if (!armpmu_event_set_period(event, hwc, idx))

			continue;

arch/arm/kernel/perf_event_xscale.c

+2 −2
Original line number Diff line number Diff line
@@ -246,7 +246,7 @@ xscale1pmu_handle_irq(int irq_num, void *dev) 
			continue;



		hwc = &event->hw;

		armpmu_event_update(event, hwc, idx);

		armpmu_event_update(event, hwc, idx, 1);

		data.period = event->hw.last_period;

		if (!armpmu_event_set_period(event, hwc, idx))

			continue;
@@ -578,7 +578,7 @@ xscale2pmu_handle_irq(int irq_num, void *dev) 
			continue;



		hwc = &event->hw;

		armpmu_event_update(event, hwc, idx);

		armpmu_event_update(event, hwc, idx, 1);

		data.period = event->hw.last_period;

		if (!armpmu_event_set_period(event, hwc, idx))

			continue;