linux-kernel - Re: [PATCH] perf/x86/intel: Avoid PEBS

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Date:   Fri, 15 Nov 2019 09:46:25 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, eranian@...gle.com
Subject: Re: [PATCH] perf/x86/intel: Avoid PEBS_ENABLE MSR access in PMI



On 11/15/2019 9:07 AM, Peter Zijlstra wrote:
> On Fri, Nov 15, 2019 at 05:39:17AM -0800, kan.liang@...ux.intel.com wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> The perf PMI handler, intel_pmu_handle_irq(), currently does
>> unnecessary MSR accesses when PEBS is enabled.
>>
>> When entering the handler, global ctrl is explicitly disabled. All
>> counters do not count anymore. It doesn't matter if the PEBS is
>> enabled or disabled. Furthermore, cpuc->pebs_enabled is not changed
>> in PMI. The PEBS status doesn't change. The PEBS_ENABLE MSR doesn't need
>> to be changed either.
> 
> PMI can throttle, and iirc x86_pmu_stop() ends up in
> intel_pmu_pebs_disable()
>

Right, the declaration is inaccurate. I will fix it in v2.
But the patch still works for the case of PMI throttle.
The intel_pmu_pebs_disable() will update cpuc->pebs_enabled and 
unconditionally modify MSR_IA32_PEBS_ENABLE.
When exiting the handler, current perf re-write the PEBS MSR according 
to the updated cpuc->pebs_enabled, which is still unnecessary.

Thanks,
Kan