linux-kernel - Re: [RFC PATCH] sched/fair: Prevent from cpufreq not being updated when delayed-task is iowait

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Message-ID: <7e3b656e-ea65-4441-b08f-062eb0501355@arm.com>
Date: Wed, 26 Feb 2025 12:08:52 +0000
From: Christian Loehle <christian.loehle@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, peterz@...radead.org,
 mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, Hongyan Xia <Hongyan.Xia2@....com>,
 Pierre Gondois <pierre.gondois@....com>, Luis Machado <luis.machado@....com>
Cc: rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, ke.wang@...soc.com, di.shen@...soc.com,
 xuewen.yan94@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/fair: Prevent from cpufreq not being updated
 when delayed-task is iowait

On 2/26/25 11:43, Xuewen Yan wrote:
> Because the sched-delayed task maybe in io-wait state,
> so we should place the requeue_delayed_entity() after the
> cpufreq_update_util(), to prevent not boosting iowait cpufreq
> before return.
> 
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>  kernel/sched/fair.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2d6d5582c3e9..040674734128 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6931,11 +6931,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & ENQUEUE_RESTORE))))
>  		util_est_enqueue(&rq->cfs, p);
>  
> -	if (flags & ENQUEUE_DELAYED) {
> -		requeue_delayed_entity(se);
> -		return;
> -	}
> -
>  	/*
>  	 * If in_iowait is set, the code below may not trigger any cpufreq
>  	 * utilization updates, so do it here explicitly with the IOWAIT flag
> @@ -6944,6 +6939,11 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	if (p->in_iowait)
>  		cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT);
>  
> +	if (flags & ENQUEUE_DELAYED) {
> +		requeue_delayed_entity(se);
> +		return;
> +	}
> +
>  	if (task_new && se->sched_delayed)
>  		h_nr_runnable = 0;
>  

I understand that iowait cpufreq update isn't happening now (and that's a bug),
but if we reorder we may call cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT)
followed by the cpufreq_update_util() in update_load_avg() of 
requeue_delayed_entity()
	update_load_avg()
		cpufreq_update_util()

and the latter will likely be dropped by the governor, so the update
won't include util of the (re)-enqueuing task, right?

I'll give it some more thought.