linux-kernel - Re: [PATCH -tip] sched/fair: gracefully handle EEVDF scheduling failures

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Message-ID: <598033f7-f937-4d32-a4ec-b3e0d094c637@antgroup.com>
Date:   Sat, 09 Dec 2023 12:51:27 +0800
From:   "Tiwei Bie" <tiwei.btw@...group.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, wuyun.abel@...edance.com,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        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>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip] sched/fair: gracefully handle EEVDF scheduling
 failures

On 12/8/23 10:32 PM, Peter Zijlstra wrote:
> On Fri, Dec 08, 2023 at 07:20:59PM +0800, Tiwei Bie wrote:
>> The EEVDF scheduling might fail due to unforeseen issues. Previously,
> 
> I might also fly if I jump up. But is there any actual reason to believe
> something like that will happen?

Thanks for the quick reply! Sorry, after re-reading the commit log,
it looks confusing to me as well. I didn't mean something like that
will happen. I just thought it might be worthwhile to have a sanity
check on 'best'. Because, the 'best' is initialized to NULL and is
conditionally updated. The added 'WARN_ONCE' on '!best' is more like
a 'default' case to catch an unreachable case in a 'switch' block.
There was a similar check in the past that was helpful. And there
seems to be no harm in doing it. If this is reasonable, I'd like to
submit a v2 patch.

PS. I just noticed that the subject line should start with a uppercase
letter according to the rules in the tip tree handbook [1]. The subject
line should be something like: "sched/fair: Sanity check best in pick_eevdf()".

[1] https://www.kernel.org/doc/html/next/process/maintainer-tip.html#patch-subject

Regards,
Tiwei