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

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Date:	Mon, 01 Jun 2015 22:42:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Josef Bacik <jbacik@...com>
Cc:	riel@...hat.com, mingo@...hat.com, linux-kernel@...r.kernel.org,
	umgwanakikbuti@...il.com, morten.rasmussen@....com,
	kernel-team <Kernel-team@...com>
Subject: Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for
 BALANCE_WAKE

On Mon, 2015-06-01 at 15:38 -0400, Josef Bacik wrote:

> Ok I got this patch to give me the same performance as all our other 
> crap, just need to apply this incremental
> 
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b71eb2b..e11cfec 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4761,13 +4761,10 @@ select_task_rq_fair(struct task_struct *p, int 
> prev_cpu, int sd_flag, int wake_f
> 
>   		if (tmp->flags & sd_flag)
>   			sd = tmp;
> -		else if (!want_affine || (want_affine && affine_sd))
> -			break;
>   	}

That bit worries me a bit, because that causes us to have a weird
definition for what sd is.

Without WAKE_AFFINE, sd is the biggest domain with BALANCE_WAKE (or any
other sd_flag) set.

But with WAKE_AFFINE, its the first domain that satisfies the wake
affine constraint of covering both the previous and waking cpu. It
basically reduces sd to affine_sd.


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