linux-kernel - Re: [PATCH] sched: Check nr_running before calling pick_next

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Message-ID: <4E0E8107.4090300@google.com>
Date:	Fri, 01 Jul 2011 19:23:03 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Rakib Mullick <rakib.mullick@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Check nr_running before calling pick_next_task
 in schedule().

Hi Rakib,

This doesn't strike me as a very good trade.

It adds a branch to the case where we actually have work to save 
branches in the case when we're idle anyway?

- Paul

On 07/01/11 11:41, Rakib Mullick wrote:
> Currently at schedule(), when we call pick_next_task we don't check whether current rq is empty or not. Since idle_balance can fail,
> its nice to check whether we really have any task on rq or not. If not, we can call idle_sched_class.pick_next_task straight.
>
> Signed-off-by: Rakib Mullick<rakib.mullick@...il.com>
> ---
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 5925275..a4f4f58 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4273,7 +4273,14 @@ need_resched:
>   		idle_balance(cpu, rq);
>
>   	put_prev_task(rq, prev);
> -	next = pick_next_task(rq);
> +	/*  Since idle_balance can fail, its better to check rq->nr_running.
> +	 *  Otherwise we can call idle_sched_class.pick_next_task straight,
> +	 *  cause we need to do some accounting.
> +	 */
> +	if (likely(rq->nr_running))
> +		next = pick_next_task(rq);
> +	else
> +		next = idle_sched_class.pick_next_task(rq);
>   	clear_tsk_need_resched(prev);
>   	rq->skip_clock_update = 0;
>
>
>

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