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

Peter Zijlstra
2015-08-26T14:35:27Z

On Wed, Aug 26, 2015 at 01:17:20PM +0000, Alexey Brodkin wrote:
> 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.

Tricky, I was seeing is_isa_arcv2() calls elsewhere, so I figured you'd
make it conditional on that.

But sure, if you think you can live with 1 this'll work.