Re: [PATCH 2/2] sched/fair: Relax task_hot() for misfit tasks

From: Phil Auld <pauld@redhat.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Rik van Riel <riel@surriel.com>,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Quentin Perret <qperret@google.com>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	Lingutla Chandrasekhar <clingutla@codeaurora.org>
Subject: Re: [PATCH 2/2] sched/fair: Relax task_hot() for misfit tasks
Date: Mon, 19 Apr 2021 08:59:58 -0400	[thread overview]
Message-ID: <YH1+zpzUdJeTk0Z3@lorien.usersys.redhat.com> (raw)
In-Reply-To: <87tuo6lg39.mognet@arm.com>

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@arm.com>
> >
> > Reviewed-by: Rik van Riel <riel@surriel.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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