linux-kernel - Re: [PATCH v3 04/22] sched: don't need go to smaller sched domain

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Date:	Wed, 9 Jan 2013 17:38:07 +0000
From:	Morten Rasmussen <Morten.Rasmussen@....com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"bp@...en8.de" <bp@...en8.de>, "pjt@...gle.com" <pjt@...gle.com>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	"efault@....de" <efault@....de>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 04/22] sched: don't need go to smaller sched domain

On Sat, Jan 05, 2013 at 08:37:33AM +0000, Alex Shi wrote:
> If parent sched domain has no task allowed cpu find. neither find in
> it's child. So, go out to save useless checking.
> 
> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  kernel/sched/fair.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3c7b09a..ecfbf8e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3378,10 +3378,8 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  			load_idx = sd->wake_idx;
>  
>  		group = find_idlest_group(sd, p, cpu, load_idx);

The previous patch changed the behavior of find_idlest_group() to
returning the local group if it is suitable. This effectively means that
you remove the recursive search for a suitable idle sched group. You
could as well merge find_idlest_group() and find_idlest_cpu() to avoid
iterating through the cpus of the same sched group twice.

Morten

> -		if (!group) {
> -			sd = sd->child;
> -			continue;
> -		}
> +		if (!group)
> +			goto unlock;
>  
>  		new_cpu = find_idlest_cpu(group, p, cpu);
>  
> -- 
> 1.7.12
> 
> --
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