linux-kernel - Re: [PATCH v3] perf/core: Avoid context switch overheads

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Date:   Wed, 9 Aug 2017 18:37:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     石祤 <linxiulei@...il.com>
Cc:     yang_oliver@...mail.com, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, eranian@...il.com,
        torvalds@...ux-foundation.org, jolsa@...hat.com,
        "leilei.lin" <leilei.lin@...baba-inc.com>
Subject: Re: [PATCH v3] perf/core: Avoid context switch overheads

On Wed, Aug 09, 2017 at 08:29:21AM +0800, 石祤 wrote:
> From: "leilei.lin" <leilei.lin@...baba-inc.com>
> 
> A performance issue caused by less strickly check in task
> sched when these tasks were once attached by per-task perf_event.
> 
> A task will alloc task->perf_event_ctxp[ctxn] when it was called
> by perf_event_open, and task->perf_event_ctxp[ctxn] would not
> ever be freed to NULL.
> 
> __perf_event_task_sched_in()
>         if (task->perf_event_ctxp[ctxn]) //  here is always true
>                 perf_event_context_sched_in() // operate pmu
> 
> 50% at most performance overhead was observed under some extreme
> test case. Therefore, add a more strick check as to ctx->nr_events,
> when ctx->nr_events == 0, it's no need to continue.
> 
> Signed-off-by: leilei.lin <leilei.lin@...baba-inc.com>

Thanks!