linux-kernel - RE: [PATCH] perf/x86: fix event counter update issue

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Date:   Tue, 29 Nov 2016 17:06:36 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Odzioba, Lukasz" <lukasz.odzioba@...el.com>
Subject: RE: [PATCH] perf/x86: fix event counter update issue



> On Tue, Nov 29, 2016 at 02:46:14PM +0000, Liang, Kan wrote:
> > > And note that we _ALWAYS_ set the IN bits, even for !sampling events.
> > > Also note we set max_period to (1<<31) - 1, so we should never
> > > exceed 31 bits.
> > >
> >
> > The max_period is 0xfffffffff.
> >
> > The limit is breaked by this patch.
> > 069e0c3c4058 ("perf/x86/intel: Support full width counting")
> > https://patchwork.kernel.org/patch/2784191/
> >
> > 	/* Support full width counters using alternative MSR range */
> > 	if (x86_pmu.intel_cap.full_width_write) {
> > 		x86_pmu.max_period = x86_pmu.cntval_mask;
> > 		x86_pmu.perfctr = MSR_IA32_PMC0;
> > 		pr_cont("full-width counters, ");
> > 	}
> >
> 
> Wth do KNL/SLM have full_width_write set if they have short counters?

Yes, the full_width_write is set on KNL/SLM.

> I should the whole point of the full_wdith thing was that in that case the
> counters were actually 64bit wide.

Even for big core, the counter width is 48 bit.
AFAIK, no Intel platform has 64bit wide counter.

Thanks,
Kan