linux-kernel - Re: [PATCH] perf: correct ctx_event_type in ctx

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Date:   Tue, 6 Mar 2018 09:54:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, jolsa@...hat.com, kernel-team@...com,
        ephiepark@...com
Subject: Re: [PATCH] perf: correct ctx_event_type in ctx_resched()

On Mon, Mar 05, 2018 at 09:55:04PM -0800, Song Liu wrote:
> In ctx_resched(), EVENT_FLEXIBLE should be sched_out when EVENT_PINNED is
> added. However, ctx_resched() calculates ctx_event_type before checking
> this condition. As a result, pinned events will NOT get higher priority
> than flexible events.
> 
> The following shows this issue on an Intel CPU (where ref-cycles can
> only use one hardware counter).
> 
>   1. First start:
>        perf stat -C 0 -e ref-cycles  -I 1000
>   2. Then, in the second console, run:
>        perf stat -C 0 -e ref-cycles:D -I 1000
> 
> The second perf uses pinned events, which is expected to have higher
> priority. However, because it failed in ctx_resched(). It is never
> run.
> 
> This patch fixes this by calculating ctx_event_type after re-evaluating
> event_type.
> 
> Fixes: 487f05e18aa4 ("perf/core: Optimize event rescheduling on active contexts")
> Signed-off-by: Song Liu <songliubraving@...com>
> Reported-by: Ephraim Park <ephiepark@...com>

Thanks!