linux-kernel - Re: [REGRESSION] x86, perf: counter freezing breaks rr

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Date:   Thu, 29 Nov 2018 09:50:09 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     Kyle Huey <me@...ehuey.com>, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, robert@...llahan.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [REGRESSION] x86, perf: counter freezing breaks rr



On 11/27/2018 8:25 PM, Stephane Eranian wrote:
> On Tue, Nov 27, 2018 at 3:36 PM Andi Kleen <ak@...ux.intel.com> wrote:
>>
>>> It does seem that FREEZE_PERFMON_ON_PMI (misnamed as it is) is of
>>> rather limited use (or even negative, in our case) to a counter that's
>>> already restricted to ring 3.
>>
>> It's much faster. The PMI cost goes down dramatically.
>>
>> I still the the right fix is to add an perf event opt-out and let it be
>> used by rr.
>>
>>     V3 is without counter freezing.
>>      V4 is with counter freezing.
>>      The value is the average cost of the PMI handler.
>>      (lower is better)
>>
>>      perf options    `           V3(ns) V4(ns)  delta
>>      -c 100000                   1088   894     -18%
>>      -g -c 100000                1862   1646    -12%
>>      --call-graph lbr -c 100000  3649   3367    -8%
>>      --c.g. dwarf -c 100000      2248   1982    -12%
>>
> Is that measured on the same machine, i.e., do you force V3 on Skylake?

Yes, it's measured on same Kabylake machine with counter_freezing option 
disabled/enabled.


> All it does, I think, is save one wrmsr(GLOBAL_CTLR) on entry to the
> PMU interrupt handler or am I missing something?
> Or does it save two? The wrmsr(GLOBAL_CTRL) at the end to reactivate.

__intel_pmu_disable_all() and __intel_pmu_enable_all() are not called in 
V4 handler. So save at least two wrmsrl.

Thanks,
Kan