linux-kernel - Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

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Message-ID: <20201109174059.GN2611@hirez.programming.kicks-ass.net>
Date:   Mon, 9 Nov 2020 18:40:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        namhyung@...nel.org, eranian@...gle.com, irogers@...gle.com,
        gmx@...gle.com, acme@...nel.org, jolsa@...hat.com,
        ak@...ux.intel.com
Subject: Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU
 events

On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:

> - The patch 1 tries to fix broken per-CPU events. The CPU context cannot be
> retrieved from the task->perf_event_ctxp. So it has to be tracked in the
> sched_cb_list. Yes, the code is very similar to the original codes, but it
> is actually the new code for per-CPU events. The optimization for per-task
> events is still kept.
>   For the case, which has both a CPU context and a task context, yes, the
> __perf_pmu_sched_task() in this patch is not invoked. Because the
> sched_task() only need to be invoked once in a context switch. The
> sched_task() will be eventually invoked in the task context.
> 
> 
> - The following patch 3 tries to optimize the sched_task() further.
>   - We handle the per-CPU event and per-task event in different places. In a
> pure per-task context, we don't need to go through the sched_cb_list. We can
> get the information from task->perf_event_ctxp. We introduce a flag to
> distinguish them.
>   - For most of the cases, we don't need to invoke the sched_task() in both
> sched in and sched out. We also introduce a flag for PMUs to avoid
> unnecessary calls.
> 

There's a further optimiation possible; we can have pmu::swap_task_ctx()
imply pmu::sched_task() (both in and out).


It's all a bit of a mess though.. let me consider.