[PATCH] ARM: perf: ensure overflows aren't missed due to IRQ latency

[Will Deacon]

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 and
the new count value is greater than the previous one, then we add on
the remaining event period (which must have expired for the overflow to
occur) and the previous count value.

Cc: Jamie Iles <jamie at jamieiles.com>
Reported-by: Ashwin Chaugule <ashbertslists at gmail.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
---

Jamie, I've changed quite a bit of the delta calculation code in
armpmu_event_update so that we can detect the problem above.

Please let me know if I've missed/broken anything!

Will

 arch/arm/kernel/perf_event.c        |   19 +++++++++++--------
 arch/arm/kernel/perf_event_v6.c     |    2 +-
 arch/arm/kernel/perf_event_v7.c     |    2 +-
 arch/arm/kernel/perf_event_xscale.c |    4 ++--
 4 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index d150ad1..53f6068 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -204,11 +204,9 @@ 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);
@@ -218,8 +216,13 @@ again:
 			     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;
+
+	delta = (new_raw_count - prev_raw_count);
+
+	if (overflow && new_raw_count > prev_raw_count)
+		delta += local64_read(&hwc->period_left) + prev_raw_count;
 
 	local64_add(delta, &event->count);
 	local64_sub(delta, &hwc->period_left);
@@ -236,7 +239,7 @@ 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
@@ -254,7 +257,7 @@ 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;
 	}
 }
diff --git a/arch/arm/kernel/perf_event_v6.c b/arch/arm/kernel/perf_event_v6.c
index c058bfc..66ce900 100644
--- a/arch/arm/kernel/perf_event_v6.c
+++ b/arch/arm/kernel/perf_event_v6.c
@@ -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;
diff --git a/arch/arm/kernel/perf_event_v7.c b/arch/arm/kernel/perf_event_v7.c
index 2e14025..e144b8c 100644
--- a/arch/arm/kernel/perf_event_v7.c
+++ b/arch/arm/kernel/perf_event_v7.c
@@ -780,7 +780,7 @@ 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;
diff --git a/arch/arm/kernel/perf_event_xscale.c b/arch/arm/kernel/perf_event_xscale.c
index 28cd3b0..39affbe 100644
--- a/arch/arm/kernel/perf_event_xscale.c
+++ b/arch/arm/kernel/perf_event_xscale.c
@@ -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;
-- 
1.7.0.4