Hi Peter,
On Wed, 2015-08-26 at 16:35 +0200, Peter Zijlstra wrote:
> 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.
Well indeed there's a room for improvement always.
But from our current experience existing implementation works pretty fine.
Moreover having now PCT IRQs we mostly use sampling events that allow doing
real profiling.
-Alexey--
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