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

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Message-ID: <YH1+zpzUdJeTk0Z3@lorien.usersys.redhat.com>
Date:   Mon, 19 Apr 2021 08:59:58 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Valentin Schneider <valentin.schneider@....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 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:
> >> Consider the following topology:
> >>
> >> Long story short, preempted misfit tasks are affected by task_hot(),
> >> while
> >> currently running misfit tasks are intentionally preempted by the
> >> stopper
> >> task to migrate them over to a higher-capacity CPU.
> >>
> >> Align detach_tasks() with the active-balance logic and let it pick a
> >> cache-hot misfit task when the destination CPU can provide a capacity
> >> uplift.
> >>
> >> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> >
> > Reviewed-by: Rik van Riel <riel@...riel.com>
> >
> 
> Thanks!
> 
> >
> > This patch looks good, but...
> >
> >> @@ -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 :) 


Cheers,
Phil

> 
> > --
> > All Rights Reversed.
> 

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