linux-kernel - Re: [PATCH 06/23] sched,psci: Convert to sched_set

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Message-ID: <jhjtv1bloom.mognet@arm.com>
Date:   Wed, 22 Apr 2020 12:55:53 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        rostedt@...dmis.org, qais.yousef@....com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, daniel.lezcano@...aro.org,
        sudeep.holla@....com
Subject: Re: [PATCH 06/23] sched,psci: Convert to sched_set_fifo*()


On 22/04/20 12:27, Peter Zijlstra wrote:
> Because SCHED_FIFO is a broken scheduler model (see previous patches)
> take away the priority field, the kernel can't possibly make an
> informed decision.
>
> Effectively changes prio from 99 to 50.
>
> XXX this thing is horrific, it basically open-codes a stop-machine and
> idle.
>

I went and tried to clean up the thing. I didn't find a clean way to use
stop_machine() + play_idle() (stoppers can't sleep), but I did manage to
get something usable with the existing FIFO threads, play_idle_precise()
and the cpuidle state usage stats, so the whole homebrewed idle thing can
go.

I also just tested it with 50 prio and big surprise it didn't change
anything (it ends up being sibling of idle injection). So FWIW:

Acked-by: Valentin Schneider <valentin.schneider@....com>

> Cc: daniel.lezcano@...aro.org
> Cc: sudeep.holla@....com
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Ingo Molnar <mingo@...nel.org>