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

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Date:	Wed, 26 Aug 2015 13:17:20 +0000
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	"peterz@...radead.org" <peterz@...radead.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
	"arc-linux-dev@...opsys.com" <arc-linux-dev@...opsys.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"acme@...nel.org" <acme@...nel.org>
Subject: Re: [PATCH v3 3/6] ARCv2: perf: Support sampling events using
 overflow interrupts

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--
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