ARM: 7556/1: perf: fix updated event period in response to PERF_EVENT_IOC_PERIOD

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.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reported-by: Andreas Sandberg <andreas.sandberg@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
index 93971b1a4f0bb0d38eebd573f8618e994273b079..53c0304b734a4e6ad7b1806b147baa3d2d43986f 100644 (file)
--- a/arch/arm/kernel/perf_event.c
+++ b/arch/arm/kernel/perf_event.c
@@ -96,6 +96,10 @@ 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);