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

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Message-ID: <20150713082609.GU19282@twins.programming.kicks-ass.net>
Date:	Mon, 13 Jul 2015 10:26:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	byungchul.park@....com
Cc:	mingo@...nel.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);
>  
>  	for_each_sched_entity(se) {
>  		struct load_weight *load;

This really doesn't make sense; look at what that
for_each_sched_entity() loop does below this.

I agree that sched_slice() is a difficult proposition in the face of
cgroup, but everything is, cgroups suck arse, they make everything hard.
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