linux-kernel - Re: [PATCH 09/10 v2] sched/fair: Fix sched_can_stop

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Message-ID: <c82ed217-cfe4-41a4-b39a-e7356231835f@amd.com>
Date: Fri, 29 Nov 2024 23:56:12 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, <mingo@...hat.com>,
	<peterz@...radead.org>, <juri.lelli@...hat.com>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<vschneid@...hat.com>, <linux-kernel@...r.kernel.org>
CC: <pauld@...hat.com>, <efault@....de>, <luis.machado@....com>, Tejun Heo
	<tj@...nel.org>, David Vernet <void@...ifault.com>
Subject: Re: [PATCH 09/10 v2] sched/fair: Fix sched_can_stop_tick() for fair
 tasks

(+ Tejun, David)

Hello Vincent,

On 11/29/2024 9:47 PM, Vincent Guittot wrote:
> We can't stop the tick of a rq if there are at least 2 tasks enqueued in
> the whole hierarchy and not only at the root cfs rq.
> 
> rq->cfs.nr_queued tracks the number of sched_entity at one level
> whereas rq->cfs.h_nr_enqueued tracks all enqueued tasks in the
> hierarchy.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>   kernel/sched/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3571f91d4b0d..866a1605656c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1341,7 +1341,7 @@ bool sched_can_stop_tick(struct rq *rq)
>   	if (scx_enabled() && !scx_can_stop_tick(rq))
>   		return false;
>   
> -	if (rq->cfs.nr_queued > 1)
> +	if (rq->cfs.h_nr_queued > 1)

Perhaps we can move this fix to the beginning of the series and add:

Fixes: 11cc374f4643b ("sched_ext: Simplify scx_can_stop_tick() invocation in sched_can_stop_tick()")

before converting the h_nr_running to h_nr_queued  since prior to that
commit, sched_can_stop_tick() used to check "rq->nr_running" and since
we check the count of DL, RR, and FIFO tasks up above, it would have
captured number of fair tasks running before sched-ext. That way the fix
can be backported easily to LTS too. Thoughts?

>   		return false;
>   
>   	/*

-- 
Thanks and Regards,
Prateek