linux-kernel - Re: [PATCH] sched: call select_idle_sibling when not affine

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Date:	Wed, 14 May 2014 17:45:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	mingo@...nel.org, george.mccollister@...il.com,
	ktkhai@...allels.com, Mel Gorman <mgorman@...e.de>,
	Vinod@...ns.programming.kicks-ass.net
Subject: Re: [PATCH] sched: call select_idle_sibling when not affine_sd

On Wed, May 14, 2014 at 11:40:37AM -0400, Rik van Riel wrote:
> Subject: sched: call select_idle_sibling when not affine_sd
> 
> On smaller systems, the top level sched domain will be an affine
> domain, and select_idle_sibling is invoked for every SD_WAKE_AFFINE
> wakeup. This seems to be working well.
> 
> On larger systems, with the node distance between far away NUMA nodes
> being > RECLAIM_DISTANCE, select_idle_sibling is only called if the
> waker and the wakee are on nodes less than RECLAIM_DISTANCE apart.
> 
> This patch leaves in place the policy of not pulling the task across
> nodes on such systems, while fixing the issue that select_idle_sibling
> is not called at all in certain circumstances.
> 
> The code will look for an idle CPU in the same CPU package as the
> CPU where the task ran previously.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  kernel/sched/fair.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 39b63d0..1e58159 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4423,10 +4423,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>  			sd = tmp;
>  	}
>  
> -	if (affine_sd) {
> -		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> -			prev_cpu = cpu;
> +	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> +		prev_cpu = cpu;
>  
> +	if (sd_flag & SD_WAKE_AFFINE) {

I think you meant SD_BALANCE_WAKE?

>  		new_cpu = select_idle_sibling(p, prev_cpu);
>  		goto unlock;
>  	}


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