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

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Date:   Thu, 15 Apr 2021 16:39:44 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     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 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>


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 :)

-- 
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