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

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Message-ID: <CABPqkBTubyf48Q9rbXfTLwJcZOV-pG9+=qACOhdMLhATs9htBA@mail.gmail.com>
Date:   Tue, 29 Nov 2016 10:11:23 -0800
From:   Stephane Eranian <eranian@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 9:30 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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, ");
>         }
Ah, yes!
That would make it consistent with the other Intel PMU settings I see
in intel/core.c such as for
intel_core_pmu.max_period = (1<<31) -1 which is 0x7ffffff whereas now
I see max_period of
0x0000ffffffffffff instead of  0x7fffffffffff. So I think Peter's
patch is required no matter what.