linux-kernel - Re: [PATCH 2/2] sched/fair: Relax task

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Date:   Mon, 19 Apr 2021 18:17:47 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Phil Auld <pauld@...hat.com>
Cc:     Rik van Riel <riel@...riel.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>
Subject: Re: [PATCH 2/2] sched/fair: Relax task_hot() for misfit tasks

On 19/04/21 08:59, Phil Auld wrote:
> On Fri, Apr 16, 2021 at 10:43:38AM +0100 Valentin Schneider wrote:
>> On 15/04/21 16:39, Rik van Riel wrote:
>> > On Thu, 2021-04-15 at 18:58 +0100, Valentin Schneider wrote:
>> >> @@ -7672,6 +7698,15 @@ int can_migrate_task(struct task_struct *p,
>> >> struct lb_env *env)
>> >>      if (tsk_cache_hot == -1)
>> >>              tsk_cache_hot = task_hot(p, env);
>> >>
>> >> +	/*
>> >> +	 * On a (sane) asymmetric CPU capacity system, the increase in
>> >> compute
>> >> +	 * capacity should offset any potential performance hit caused
>> >> by a
>> >> +	 * migration.
>> >> +	 */
>> >> +	if ((env->dst_grp_type == group_has_spare) &&
>> >> +	    !migrate_degrades_capacity(p, env))
>> >> +		tsk_cache_hot = 0;
>> >
>> > ... I'm starting to wonder if we should not rename the
>> > tsk_cache_hot variable to something else to make this
>> > code more readable. Probably in another patch :)
>> >
>>
>> I'd tend to agree, but naming is hard. "migration_harmful" ?
>
> I thought Rik meant tsk_cache_hot, for which I'd suggest at least
> buying a vowel and putting an 'a' in there :)
>

That's the one I was eyeing: s/tsk_cache_hot/migration_harmful/ or
somesuch. Right now we're feeding it:

o migrate_degrades_locality()
o task_hot()

and I'm adding another one, so that's 2/3 which don't actually care about
cache hotness, but rather "does doing this migration degrade/improve
$criterion?"