metag: perf: fix frequency sampling (dynamic period)

Frequency sampling mode dynamically adjusts the sample period so as to
hit a particular frequency of samples. The sample period starts at just
1 and then gets increased if the interrupt rate is too high. This
changed sample period needs handling in metag_pmu_event_set_period to
update period_left (as the ARM equivalent does). The calculated delta
also needs subtracting from period_left in metag_pmu_event_update in
order to hit the conditional blocks in metag_pmu_event_set_period which
update last_period (which is used in the dynamic sampling period
calculation).

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>

arch/metag/kernel/perf/perf_event.c

@@ -214,6 +214,7 @@ void metag_pmu_event_update(struct perf_event *event,
 	delta = (new_raw_count - prev_raw_count) & MAX_PERIOD;
 
 	local64_add(delta, &event->count);
+	local64_sub(delta, &hwc->period_left);
 }
 
 int metag_pmu_event_set_period(struct perf_event *event,
@@ -223,6 +224,10 @@ int metag_pmu_event_set_period(struct perf_event *event,
 	s64 period = hwc->sample_period;
 	int ret = 0;
 
+	/* The period may have been changed */
+	if (unlikely(period != hwc->last_period))
+		left += period - hwc->last_period;
+
 	if (unlikely(left <= -period)) {
 		left = period;
 		local64_set(&hwc->period_left, left);