linux-kernel - Re: [PATCH v2] sched: let __sched_period() use rq's nr

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Date:	Fri, 10 Jul 2015 14:31:10 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	byungchul.park@....com
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, pjt@...gle.com
Subject: Re: [PATCH v2] sched: let __sched_period() use rq's nr_running

On Fri, Jul 10, 2015 at 05:11:30PM +0900, byungchul.park@....com wrote:
> From: Byungchul Park <byungchul.park@....com>
> 
> __sched_period() returns a period which a rq can have. the period has to be
> stretched by the number of task *the rq has*, when nr_running > nr_latency.
> otherwise, task slice can be very smaller than sysctl_sched_min_granularity
> depending on the position of tg hierarchy when CONFIG_FAIR_GROUP_SCHED.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....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 09456fc..8ae7aeb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -635,7 +635,7 @@ static u64 __sched_period(unsigned long nr_running)
>   */
>  static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -	u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
> +	u64 slice = __sched_period(rq_of(cfs_rq)->nr_running + !se->on_rq);

This would stretch the period to fit rq->cfs.h_nr_running (which is
equal to rq.nr_running), but I still think that the slice may be smaller
than sched_min_granularity for low priority tasks since the slice is
scaled by priority.

Also, I'm not sure if we want to enforce sched_slice >=
sched_min_granularity, it would mean that tasks inside task groups can
stretch the overall period and increase latency for non-grouped tasks.
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