Re: [RFC PATCH] sched: Reduce overestimating avg_idle

[Rik van Riel]

On 07/31/2013 05:37 AM, Jason Low wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e8b3350..62b484b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1341,12 +1341,12 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
>
>   	if (rq->idle_stamp) {
>   		u64 delta = rq->clock - rq->idle_stamp;
> -		u64 max = 2*sysctl_sched_migration_cost;
> +		u64 max = (sysctl_sched_migration_cost * 3) / 2;
>
> -		if (delta > max)
> +		update_avg(&rq->avg_idle, delta);
> +
> +		if (rq->avg_idle > max)
>   			rq->avg_idle = max;
> -		else
> -			update_avg(&rq->avg_idle, delta);
>   		rq->idle_stamp = 0;
>   	}

I wonder if we could get even more conservative values
of avg_idle by clamping delta to max, before calling
update_avg...

Or rather, I wonder if that would matter enough to make
a difference, and in what direction that difference would
be.

In other words:

	if (rq->idle_stamp) {
		u64 delta = rq->clock - rq->idle_stamp;
		u64 max = (sysctl_sched_migration_cost * 3) / 2;

		if (delta > max)
			delta = max;

		update_avg(&rq->avg_idle, delta);
		rq->idle_stamp = 0;
	}

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