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

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Date:   Tue, 29 Nov 2016 18:30:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     "Liang, Kan" <kan.liang@...el.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 09:20:10AM -0800, Stephane Eranian wrote:
> Max period is limited by the number of bits the kernel can write to an MSR.
> Used to be 31, now it is 47 for core PMU as per patch pointed to by Kan.

No, I think it sets it to 48 now, which is the problem. It should be 1
bit less than the total width.

So something like so.

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, ");
 	}