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

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Date:   Tue, 29 Nov 2016 20:32:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     Stephane Eranian <eranian@...gle.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Shishkin <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 07:07:25PM +0000, Liang, Kan wrote:
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index a74a2dbc0180..cb8522290e6a 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -4034,7 +4034,7 @@ __init int intel_pmu_init(void)
> > 
> >  	/* 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.max_period = x86_pmu.cntval_mask >> 1;
> >  		x86_pmu.perfctr = MSR_IA32_PMC0;
> >  		pr_cont("full-width counters, ");
> >  	}
> 
> It doesn't work. 
> perf stat -x, -C1 -e cycles -- sudo taskset 0x2 ./loop 100000000000
> 18446743727217821696,,cycles,313837854019,100.00
> 
> delta 0xffffff8000001803 new 0x1804 prev 0xffffff8000000001
> 
> I guess we need at least x86_pmu.cntval_mask >> 2 to prevent
> the sign flag set.

Possible delta should be u64, as we know the counter cannot decrement.