Re: [PATCH v3 3/6] ARCv2: perf: Support sampling events using overflow interrupts

Alexey Brodkin
2015-08-26T13:17:31Z

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:
> > @@ -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.
>
> > 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