linux-kernel - Re: [PATCH] sched/fair: Reschedule the cfs

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Date: Sat, 25 May 2024 19:57:41 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Mike Galbraith <efault@....de>
CC: Chunxin Zang <spring.cxz@...il.com>, <mingo@...hat.com>,
	<peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, <linux-kernel@...r.kernel.org>,
	<yangchen11@...iang.com>, <zhouchunhua@...iang.com>,
	<zangchunxin@...iang.com>
Subject: Re: [PATCH] sched/fair: Reschedule the cfs_rq when current is
 ineligible

On 2024-05-25 at 08:41:28 +0200, Mike Galbraith wrote:
> On Fri, 2024-05-24 at 21:40 +0800, Chunxin Zang wrote:
> > I found that some tasks have been running for a long enough time and
> > have become illegal, but they are still not releasing the CPU. This
> > will increase the scheduling delay of other processes. Therefore, I
> > tried checking the current process in wakeup_preempt and entity_tick,
> > and if it is illegal, reschedule that cfs queue.
> 
> My box gave making the XXX below reality a two thumbs up when fiddling
> with the original unfettered and a bit harsh RUN_TO_PARITY.
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8a5b1ae0aa55..922834f172b0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8413,12 +8413,13 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>  	update_curr(cfs_rq);
> 
>  	/*
> -	 * XXX pick_eevdf(cfs_rq) != se ?
> +	 * Run @curr until it is no longer our best option.  Basing the preempt
> +	 * decision on @curr reselection puts any previous decisions back on the
> +	 * table in context "now", including granularity preservation decisions
> +	 * by RUN_TO_PARITY.
>  	 */
> -	if (pick_eevdf(cfs_rq) == pse)
> -		goto preempt;
> -
> -	return;
> +	if (pick_eevdf(cfs_rq) == se)
> +		return;
>

I suppose this change benefits the overloaded scenario:
neither current nor the wakee is the best one.

before: current continues to run.
after: best se in the tree preempts current.

hackbench -g 12 -l 1000000000 & (480 tasks, 2x of the CPUs)

cyclictest --mlockall -D 1m -q
before:
T: 0 (15983) P: 0 I:1000 C:  43054 Min:     11 Act:  144 Avg:  627 Max:   11446

after:
T: 0 (16473) P: 0 I:1000 C:  49822 Min:      7 Act:  160 Avg:  388 Max:   10190

Min, Avg, Max latency all decreased.

thanks,
Chenyu