linux-kernel - Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE

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Date:	Fri, 12 Jun 2015 07:35:05 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Josef Bacik <jbacik@...com>, riel@...hat.com, mingo@...hat.com,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com
Subject: Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for
 BALANCE_WAKE

On Thu, 2015-05-28 at 13:05 +0200, Peter Zijlstra wrote:

> @@ -5022,22 +5026,28 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>  		 * If both cpu and prev_cpu are part of this domain,
>  		 * cpu is a valid SD_WAKE_AFFINE target.
>  		 */
> -		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
> -		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
> +		if (want_affine && !affine_sd &&
> +		    (tmp->flags & SD_WAKE_AFFINE) &&
> +		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
>  			affine_sd = tmp;
> -			break;
> -		}
>  
>  		if (tmp->flags & sd_flag)
>  			sd = tmp;
> +		else if (!want_affine || (want_affine && affine_sd))
> +			break;
>  	}

Hm, new_cpu == cpu.
 
> -	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> +	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync)) {
>  		prev_cpu = cpu;
> +		sd = NULL; /* WAKE_AFFINE trumps BALANCE_WAKE */
> +	}

If branch above is not taken, new_cpu remains cpu.
 
>  	if (sd_flag & SD_BALANCE_WAKE) {
> -		new_cpu = select_idle_sibling(p, prev_cpu);
> -		goto unlock;
> +		int tmp = select_idle_sibling(p, prev_cpu);
> +		if (tmp >= 0) {
> +			new_cpu = tmp;
> +			goto unlock;
> +		}
>  	}

If select_idle_sibling() returns -1, new_cpu remains cpu.

>  
>  	while (sd) {

If sd == NULL, we fall through and try to pull wakee despite nacked-by
tsk_cpus_allowed() or wake_affine().

	-Mike

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