linux-kernel - Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics

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Date:   Wed, 29 May 2019 09:30:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics

On Tue, May 28, 2019 at 02:23:09PM -0400, Liang, Kan wrote:
> > > RDPMC
> > > =========
> > > The TopDown can be collected per thread/process. To use TopDown
> > > through RDPMC in applications on Icelake, the metrics and slots values
> > > have to be saved/restored during context switching.
> > > 
> > > Add specific set_period() to specially handle the slots and metrics
> > > event. Because,
> > >   - The initial value must be 0.
> > >   - Only need to restore the value in context switch. For other cases,
> > >     the counters have been cleared after read.
> > 
> > So the above claims to explain RDPMC, but doesn't mention that magic
> > value below at all. In fact, I don't see how the above relates to RDPMC
> > at all.
> 
> Current perf only support per-core Topdown RDPMC. On Icelake, it can be
> extended to per-thread Topdown RDPMC.
> It tries to explain the extra work for per-thread topdown RDPMC, e.g.
> save/restore slots and metrics value in context switch.

Right, this has what relation to RDPMC ?

> > > @@ -2141,7 +2157,9 @@ static int x86_pmu_event_idx(struct perf_event *event)
> > >   	if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
> > >   		return 0;
> > > -	if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
> > > +	if (is_metric_idx(idx))
> > > +		idx = 1 << 29;
> > 
> > I can't find this in the SDM RDPMC description. What does it return?
> 
> It will return the value of PERF_METRICS. I will add it in the changelog.

A comment would be even better.