linux-kernel - RE: [PATCH 1/2] perf/x86/intel: enable CPU ref

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Date:   Mon, 22 May 2017 16:55:47 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "vincent.weaver@...ne.edu" <vincent.weaver@...ne.edu>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: RE: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter



> On Fri, May 19, 2017 at 10:06:21AM -0700, kan.liang@...el.com wrote:
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index
> > 580b60f..e8b2326 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -101,6 +101,10 @@ u64 x86_perf_event_update(struct perf_event
> *event)
> >  	delta = (new_raw_count << shift) - (prev_raw_count << shift);
> >  	delta >>= shift;
> >
> > +	/* Correct the count number if applying ref_cycles replacement */
> > +	if (!is_sampling_event(event) &&
> > +	    (hwc->flags & PERF_X86_EVENT_REF_CYCLES_REP))
> > +		delta *= x86_pmu.ref_cycles_factor;
> 
> That condition seems wrong, why only correct for !sampling events?
>

For sampling, it's either fixed freq mode or fixed period mode.
 - In the fixed freq mode, we should do nothing, because the adaptive
   frequency algorithm will handle it.
 - In the fixed period mode, we have already adjusted the period in 
    ref_cycles_rep().
Therefore, we should only handle !sampling events here.

 
> >  	local64_add(delta, &event->count);
> >  	local64_sub(delta, &hwc->period_left);
> >
> 
> 
> > @@ -934,6 +938,21 @@ int x86_schedule_events(struct cpu_hw_events
> *cpuc, int n, int *assign)
> >  		for (i = 0; i < n; i++) {
> >  			e = cpuc->event_list[i];
> >  			e->hw.flags |= PERF_X86_EVENT_COMMITTED;
> > +
> > +			/*
> > +			 * 0x0300 is pseudo-encoding for REF_CPU_CYCLES.
> > +			 * It indicates that fixed counter 2 should be used.
> > +			 *
> > +			 * If fixed counter 2 is occupied and a GP counter
> > +			 * is assigned, an alternative event which can be
> > +			 * counted in GP counter will be used to replace
> > +			 * the pseudo-encoding REF_CPU_CYCLES event.
> > +			 */
> > +			if (((e->hw.config & X86_RAW_EVENT_MASK) ==
> 0x0300) &&
> > +			    (assign[i] < INTEL_PMC_IDX_FIXED) &&
> > +			    x86_pmu.ref_cycles_rep)
> > +				x86_pmu.ref_cycles_rep(e);
> > +
> >  			if (x86_pmu.commit_scheduling)
> >  				x86_pmu.commit_scheduling(cpuc, i,
> assign[i]);
> >  		}
> 
> This looks dodgy, this is the branch were we managed to schedule all events.
> Why would we need to consider anything here?
> 
> I was expecting a retry if there are still unscheduled events and one of the
> events was our 0x0300 event. In that case you have to reset the event and
> retry the whole scheduling thing.

Will do it.

Thanks,
Kan