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

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Date: Fri, 24 May 2024 23:30:12 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Chunxin Zang <spring.cxz@...il.com>
CC: <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>, <kprateek.nayak@....com>
Subject: Re: [PATCH] sched/fair: Reschedule the cfs_rq when current is
 ineligible

On 2024-05-24 at 21:40:11 +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.
>
> The modification can reduce the scheduling delay by about 30% when
> RUN_TO_PARITY is enabled.
> So far, it has been running well in my test environment, and I have
> pasted some test results below.
> 

Interesting, besides hackbench, I assume that you have workload in
real production environment that is sensitive to wakeup latency?

>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 03be0d1330a6..a0005d240db5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5523,6 +5523,9 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
>  			hrtimer_active(&rq_of(cfs_rq)->hrtick_timer))
>  		return;
>  #endif
> +
> +	if (!entity_eligible(cfs_rq, curr))
> +		resched_curr(rq_of(cfs_rq));
>  }
>

entity_tick() -> update_curr() -> update_deadline():
se->vruntime >= se->deadline ? resched_curr()
only current has expired its slice will it be scheduled out.

So here you want to schedule current out if its lag becomes 0.

In lastest sched/eevdf branch, it is controlled by two sched features:
RESPECT_SLICE: Inhibit preemption until the current task has exhausted it's slice.
RUN_TO_PARITY: Relax RESPECT_SLICE and only protect current until 0-lag.
https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=sched/eevdf&id=e04f5454d68590a239092a700e9bbaf84270397c

Maybe something like this can achieve your goal
	if (sched_feat(RUN_TOPARITY) && !entity_eligible(cfs_rq, curr))
		resched_curr

>  
> @@ -8325,6 +8328,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>  	if (unlikely(p->policy != SCHED_NORMAL) || !sched_feat(WAKEUP_PREEMPTION))
>  		return;
>  
> +	if (!entity_eligible(cfs_rq, se))
> +		goto preempt;
> +

Not sure if this is applicable, later in this function, pick_eevdf() checks
if the current is eligible, !entity_eligible(cfs_rq, curr), if not, curr will
be evicted. And this change does not consider the cgroup hierarchy.

Besides, the check of current eligiblity can get false negative result,
if the enqueued entity has a positive lag. Prateek proposed to
remove the check of current's eligibility in pick_eevdf():
https://lore.kernel.org/lkml/20240325060226.1540-2-kprateek.nayak@amd.com/

If I understand your requirement correctly, you want to reduce the wakeup
latency. There are some codes under developed by Peter, which could
customized task's wakeup latency via setting its slice:
https://lore.kernel.org/lkml/20240405110010.934104715@infradead.org/

thanks,
Chenyu