linux-kernel - Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task

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Date:   Tue, 15 Jun 2021 17:39:52 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Yafang Shao <laoar.shao@...il.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task

On 15/06/21 17:45, Vincent Guittot wrote:
> On Tue, 15 Jun 2021 at 16:55, Valentin Schneider
> <valentin.schneider@....com> wrote:
>> This can be summarized as in the below, no?
>>
>>                         /*
>>                          * Don't cause a higher-than-CFS task to be preempted by
>>                          * the CPU stopper.
>>                          */
>
> IMO, it's worth keeping the explanation that we are there because:
> - a CFS task that was running during the 1st step :  if
> (busiest->nr_running > 1) { ...
> so we didn't pull the task
> - but it has been preempted while lb was deciding if it needs an
> active load balance
>
> so maybe something like:
>                          /*
>                           * Don't kick the active_load_balance_cpu_stop,
>                           * if the CFS task has been preempted by higher
>                           * priority task in the meantime.
>                          */
>

Ack