linux-kernel - Re: [PATCH RFC] sched,numa: decay wakee

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Date:	Fri, 16 May 2014 15:22:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, chegu_vinod@...com, mingo@...nel.org,
	umgwanakikbuti@...il.com
Subject: Re: [PATCH RFC] sched,numa: decay wakee_flips instead of zeroing

On Fri, May 16, 2014 at 12:13:32AM -0400, Rik van Riel wrote:
> Affine wakeups have the potential to interfere with NUMA placement.
> If a task wakes up too many other tasks, affine wakeups will get
> disabled.
> 
> However, regardless of how many other tasks it wakes up, it gets
> re-enabled once a second, potentially interfering with NUMA
> placement of other tasks.
> 
> By decaying wakee_wakes in half instead of zeroing it, we can avoid
> that problem for some workloads.

See https://lkml.org/lkml/2013/7/2/110 and further

> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4f01e2f1..0381b11 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4009,7 +4009,7 @@ static void record_wakee(struct task_struct *p)
>  	 * about the loss.
>  	 */
>  	if (jiffies > current->wakee_flip_decay_ts + HZ) {
> -		current->wakee_flips = 0;
> +		current->wakee_flips >>= 1;
>  		current->wakee_flip_decay_ts = jiffies;
>  	}

Would it make sense to do something like:

	now = jiffies;
	while (current->wakee_flips && now > current->wakee_flip_decay_ts + HZ) {
		current->wakee_flips >>= 1;
		current->wakee_flip_decay_ts += HZ;
	}
	if (unlikely(now > current->wakee_flip_decay_ts + HZ))
		current->wakee_flip_decay_ts = now;

Or is that over engineering things?



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