linux-kernel - Re: [PATCH v4 08/10] sched/fair: Steal work from an overloaded CPU when CPU goes idle

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Date:   Fri, 7 Dec 2018 20:21:28 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Steve Sistare <steven.sistare@...cle.com>, mingo@...hat.com,
        peterz@...radead.org
Cc:     subhra.mazumdar@...cle.com, dhaval.giani@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, quentin.perret@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 08/10] sched/fair: Steal work from an overloaded CPU
 when CPU goes idle

Hi Steve,

On 06/12/2018 21:28, Steve Sistare wrote:
[...]
> @@ -6778,20 +6791,22 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>  	update_misfit_status(NULL, rq);
>  
>  	/*
> -	 * We must set idle_stamp _before_ calling idle_balance(), such that we
> -	 * measure the duration of idle_balance() as idle time.
> +	 * We must set idle_stamp _before_ calling try_steal() or
> +	 * idle_balance(), such that we measure the duration as idle time.
>  	 */
>  	rq_idle_stamp_update(rq);
>  

idle_balance() has a

	/*
	 * Do not pull tasks towards !active CPUs...
	 */
	if (!cpu_active(this_cpu))
		return 0;

check which we probably want for stealing too, so we could hoist it up here
to cover both idle_balance() and try_steal().

>  	new_tasks = idle_balance(rq, rf);
> +	if (new_tasks == 0)
> +		new_tasks = try_steal(rq, rf);
>  
>  	if (new_tasks)
>  		rq_idle_stamp_clear(rq);
>  
>  	/*
> -	 * Because idle_balance() releases (and re-acquires) rq->lock, it is
> -	 * possible for any higher priority task to appear. In that case we
> -	 * must re-start the pick_next_entity() loop.
> +	 * Because try_steal() and idle_balance() release (and re-acquire)
> +	 * rq->lock, it is possible for any higher priority task to appear.
> +	 * In that case we must re-start the pick_next_entity() loop.
>  	 */
>  	if (new_tasks < 0)
>  		return RETRY_TASK;
[...]