linux-kernel - Re: sched: remove redundant hierarchy walk in check_preempt

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Message-Id: <1239256853.13185.1131.camel@twins>
Date:	Thu, 09 Apr 2009 08:00:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: sched: remove redundant hierarchy walk in check_preempt_wakeup

On Wed, 2009-04-08 at 15:29 -0700, Paul Turner wrote:
> Hi Ingo, Peter,
> 
> In check_preempt_wakeup we walk up until both entities are queued on the
> same cfs_rq.  After the iteration of vruntime vs wall-clock pre-emption 
> changes the current check now checks at every level of the tree above 
> this.  However, unless I'm missing something at this point they should
> have a common parent.

Yes, you're right, and them having a common parent means the rest of the
preemption checks will be against itself.

> --
> 
> sched: remove redundant hierarchy walk in check_preempt_wakeup
> 
> Under group scheduling we traverse up until we are at common siblings to make
> the wakeup comparison on.
> 
> At this point however, they should have the same parent so continuing to check
> up the tree is redundant.
> 
> Signed-off-by: Paul Turner <pjt@...gle.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

> ---
>  kernel/sched_fair.c |   13 +++----------
>  1 files changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 3816f21..5f9650e 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1487,17 +1487,10 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
>  
>  	find_matching_se(&se, &pse);
>  
> -	while (se) {
> -		BUG_ON(!pse);
> +	BUG_ON(!pse);
>  
> -		if (wakeup_preempt_entity(se, pse) == 1) {
> -			resched_task(curr);
> -			break;
> -		}
> -
> -		se = parent_entity(se);
> -		pse = parent_entity(pse);
> -	}
> +	if (wakeup_preempt_entity(se, pse) == 1)
> +		resched_task(curr);
>  }
>  
>  static struct task_struct *pick_next_task_fair(struct rq *rq)
--
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