linux-kernel - Re: [PATCH] sched: Get rid of unnecessary checks from select_idle

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Date:	Wed, 09 Jan 2013 13:51:00 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Alex Shi <alex.shi@...el.com>
Subject: Re: [PATCH] sched: Get rid of unnecessary checks from select_idle_sibling

On 01/09/2013 12:20 PM, Namhyung Kim wrote:
> From: Namhyung Kim <namhyung.kim@....com>
> 
> AFAICS @target cpu of select_idle_sibling() is always either prev_cpu
> or this_cpu.  So no need to check it again and the conditionals can be
> consolidated.
> 
> Cc: Mike Galbraith <efault@....de>
> Cc: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Alex Shi <alex.shi@...el.com>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> ---
>  kernel/sched/fair.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5eea8707234a..af665814c216 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3254,25 +3254,16 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>   */
>  static int select_idle_sibling(struct task_struct *p, int target)
>  {
> -	int cpu = smp_processor_id();
> -	int prev_cpu = task_cpu(p);
>  	struct sched_domain *sd;
>  	struct sched_group *sg;
>  	int i;
> 
>  	/*
> -	 * If the task is going to be woken-up on this cpu and if it is
> -	 * already idle, then it is the right target.
> -	 */
> -	if (target == cpu && idle_cpu(cpu))
> -		return cpu;
> -
> -	/*
> -	 * If the task is going to be woken-up on the cpu where it previously
> -	 * ran and if it is currently idle, then it the right target.
> +	 * If the task is going to be woken-up on this cpu or the cpu where it
> +	 * previously ran and it is already idle, then it is the right target.
>  	 */
> -	if (target == prev_cpu && idle_cpu(prev_cpu))
> -		return prev_cpu;
> +	if (idle_cpu(target))
> +		return target;
> 
>  	/*
>  	 * Otherwise, iterate the domains and find an elegible idle cpu.
> 
If NUMA_TTWU_BIAS or NUMA_TTWU_TO is true(it is false by
default),cpu/prev_cpu can be changed to be a random node_cpu(the node
that 'this_cpu' is on). In which case even if the node cpu is idle,it
would not be a viable target,looks like.Maybe that is why
select_idle_sibling() makes the check if the target is prev_cpu/this cpu.

Regards
Preeti U Murthy

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