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

[Hongyan Xia]

On 26/02/2025 12:08, Christian Loehle wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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.

True, but I think the code was like this before anyway. On the 
non-delayed path, the problem you mentioned still exists. Not saying 
this is the right thing, but just saying this is what it has always been 
like.