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

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Date:   Tue, 29 Nov 2016 20:33:28 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
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.

Yes, the patch as below fixes the issue on my SLM.

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 6c3b0ef..abd97e8 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -69,7 +69,7 @@ u64 x86_perf_event_update(struct perf_event *event)
 	int shift = 64 - x86_pmu.cntval_bits;
 	u64 prev_raw_count, new_raw_count;
 	int idx = hwc->idx;
-	s64 delta;
+	u64 delta;
 
 	if (idx == INTEL_PMC_IDX_FIXED_BTS)
 		return 0;
diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index a74a2db..cb85222 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, ");
 	}