linux-kernel - Re: [PATCH v2] perf/core: Avoid context switch overheads

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Message-ID: <CALPjY3=dGwBmZj_Ek2J7AuMY_ytLk-e0gBu7PSKvXBOrs1SfCA@mail.gmail.com>
Date:   Wed, 9 Aug 2017 08:28:01 +0800
From:   林守磊 <linxiulei@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     yang_oliver@...mail.com, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, Stephane Eranian <eranian@...il.com>,
        torvalds@...ux-foundation.org, jolsa@...hat.com,
        "leilei.lin" <leilei.lin@...baba-inc.com>
Subject: Re: [PATCH v2] perf/core: Avoid context switch overheads

2017-08-08 18:37 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> On Tue, Aug 08, 2017 at 06:00:45PM +0800, 石祤 wrote:
>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 426c2ff..3d86695 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -3180,6 +3180,13 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
>>               return;
>>
>>       perf_ctx_lock(cpuctx, ctx);
>> +     /*
>> +      * We must check ctx->nr_events while holding ctx->lock, such
>> +      * that we serialize against perf_install_in_context().
>> +      */
>> +     if (!cpuctx->task_ctx && !ctx->nr_events)
>> +             goto unlock;
>
> Do we really need the cpuctx->task_ctx test? I think that task_ctx is
> 'tight' these days. We never have it set unless there are events
> scheduled for that context.
>
> I even think the cpuctx->task_ctx == ctx test right above here is
> superfluous these days. That could only happen when the
> perf_install_in_context() IPI came before perf_event_task_sched_in(),
> but we removed the arch option to do context switches with IRQs enabled.
>

It looks that cpuctx->task_ctx exists somewhere else, so I thought it was
conservative making this patch.

For a centain, during my process of debugging I didn't figure out any value
of cpuctx->task_ctx. I shall make a v3.

Thanks

>> +
>>       perf_pmu_disable(ctx->pmu);
>>       /*
>>        * We want to keep the following priority order:
>> @@ -3193,6 +3200,8 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
>>               cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
>>       perf_event_sched_in(cpuctx, ctx, task);
>>       perf_pmu_enable(ctx->pmu);
>> +
>> +unlock:
>>       perf_ctx_unlock(cpuctx, ctx);
>>  }
>>
>> --
>> 2.8.4.31.g9ed660f
>>