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Subject: Re: [patch] sched: improve tick time missed wakeup preempt
 protection
From: Peter Zijlstra <peterz@infradead.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
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References: <1258891682.14325.31.camel@marge.simson.net>
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Date: Sun, 22 Nov 2009 14:16:07 +0100
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On Sun, 2009-11-22 at 13:08 +0100, Mike Galbraith wrote:
> sched: improve tick time missed wakeup preempt protection
> 
> f685ceac provides protection from tasks just missing wakeup preemption, and then
> having to wait a full slice.  However, it offers this protection to tasks which
> have no business receiving the benefit, namely SCHED_BATCH and SCHED_IDLE.  It
> also treats all tasks equally, which obviously isn't true.  Exclude tasks of
> other than SCHED_NORMAL class, and scale minimum runtime before a tick time
> preemption by the difference in task weights, after which, we can just use the
> standard wakeup preempt vruntime delta test, sysctl_sched_wakeup_granularity.
> 
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> LKML-Reference: <new-submission>
> 
> ---
>  kernel/sched_fair.c |   20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -830,17 +830,23 @@ check_preempt_tick(struct cfs_rq *cfs_rq
>  	 * narrow margin doesn't have to wait for a full slice.
>  	 * This also mitigates buddy induced latencies under load.
>  	 */
> -	if (!sched_feat(WAKEUP_PREEMPT))
> +	if (!sched_feat(WAKEUP_PREEMPT) || cfs_rq->nr_running < 2)
>  		return;
> -
> -	if (delta_exec < sysctl_sched_min_granularity)
> -		return;
> -
> -	if (cfs_rq->nr_running > 1) {
> +	else  {
>  		struct sched_entity *se = __pick_next_entity(cfs_rq);
> +		unsigned long min = sysctl_sched_min_granularity;
>  		s64 delta = curr->vruntime - se->vruntime;
>  
> -		if (delta > ideal_runtime)
> +		if (task_of(se)->policy != SCHED_NORMAL)
> +			return;
> +		if (delta < 0)
> +			return;
> +		if (curr->load.weight != se->load.weight)
> +			min = calc_delta_mine(min, curr->load.weight, &se->load);
> +		if (delta_exec < min)
> +			return;
> +
> +		if (delta > sysctl_sched_wakeup_granularity)
>  			resched_task(rq_of(cfs_rq)->curr);
>  	}
>  }

You can loose the else, the if branch does an unconditional return,
there's no other way to get below there than 'else' ;-)