[PATCH] ARM: perf: update event period in response to PERF_EVENT_IOC_PERIOD

[Will Deacon]

The PERF_EVENT_IOC_PERIOD ioctl command can be used to change the
sample period of a running perf_event. Consequently, when calculating
the next event period, the new period will only be considered after the
previous one has overflowed.

This patch changes the calculation of the remaining event ticks so that
they are offset if the period has changed.

Reported-by: Andreas Sandberg <andreas.sandberg at arm.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
---
 arch/arm/kernel/perf_event.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index ab243b8..29a6939 100644
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -148,6 +148,10 @@ armpmu_event_set_period(struct perf_event *event,
 	s64 period = hwc->sample_period;
 	int ret = 0;
 
+	/* The period may have been changed by PERF_EVENT_IOC_PERIOD */
+	if (unlikely(period != hwc->last_period))
+		left = period - (hwc->last_period - left);
+
 	if (unlikely(left <= -period)) {
 		left = period;
 		local64_set(&hwc->period_left, left);
-- 
1.7.4.1