Hi Peter,

On Wed, 2015-08-26 at 15:07 +0200, Peter Zijlstra wrote:

On Mon, Aug 24, 2015 at 05:20:20PM +0300, Alexey Brodkin wrote:

quoted

@@ -139,9 +141,11 @@ static int arc_pmu_event_init(struct perf_event *event)
 	struct hw_perf_event *hwc = &event->hw;
 	int ret;
 
-	hwc->sample_period  = arc_pmu->max_period;
-	hwc->last_period = hwc->sample_period;
-	local64_set(&hwc->period_left, hwc->sample_period);
+	if (!is_sampling_event(event)) {
+		hwc->sample_period  = arc_pmu->max_period;
+		hwc->last_period = hwc->sample_period;
+		local64_set(&hwc->period_left, hwc->sample_period);
+	}

So here we set a max_period sample period for !sampling events such that
we can properly deal with (short) counter overflow and accumulate into a
64bit value.

quoted

 	switch (event->attr.type) {
 	case PERF_TYPE_HARDWARE:

@@ -243,6 +247,11 @@ static void arc_pmu_start(struct perf_event *event, int flags)
 
 	arc_pmu_event_set_period(event);
 
+	/* Enable interrupt for this counter */
+	if (is_sampling_event(event))
+		write_aux_reg(ARC_REG_PCT_INT_CTRL,
+			      read_aux_reg(ARC_REG_PCT_INT_CTRL) | (1 << idx));
+

Yet here you fail to actually enable the interrupt for the non sampling
events, which makes the above not work.

Indeed we intentionally leave interrupts disabled for non-sampling events.
 [1] We have quite large counters so we don't expect to overflow normally
 [2] We may re-use the same code for hardware that lacks support of IRQs in PCT.
     See we check if IRQs are available and if not set PERF_PMU_CAP_NO_INTERRUPT
     that will guarantee we won't get sampling event and for non-sampling events
     we won't use IRQs.

-Alexey