linux-kernel - Re: [PATCH 04/12] sched: Cleanup sched_delayed handling for class switches

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Message-ID: <CAKfTPtAY5Py=rmzbT+e7p5Em2eU0Do25ZKfcKZ2ZTjf6jFFOCg@mail.gmail.com>
Date: Tue, 7 Oct 2025 17:22:05 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tj@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, longman@...hat.com, 
	hannes@...xchg.org, mkoutny@...e.com, void@...ifault.com, arighi@...dia.com, 
	changwoo@...lia.com, cgroups@...r.kernel.org, sched-ext@...ts.linux.dev, 
	liuwenfang@...or.com, tglx@...utronix.de
Subject: Re: [PATCH 04/12] sched: Cleanup sched_delayed handling for class switches

On Mon, 6 Oct 2025 at 12:45, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Use the new sched_class::switching_from() method to dequeue delayed
> tasks before switching to another class.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Acked-by: Tejun Heo <tj@...nel.org>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/core.c     |   12 ++++++++----
>  kernel/sched/ext.c      |    6 ------
>  kernel/sched/fair.c     |    7 +++++++
>  kernel/sched/syscalls.c |    3 ---
>  4 files changed, 15 insertions(+), 13 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7368,9 +7368,6 @@ void rt_mutex_setprio(struct task_struct
>                 queue_flag &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>         }
>
> -       if (prev_class != next_class && p->se.sched_delayed)
> -               dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
> -
>         scoped_guard (sched_change, p, queue_flag) {
>                 /*
>                  * Boosting condition are:
> @@ -10845,8 +10842,15 @@ struct sched_change_ctx *sched_change_be
>                 if (WARN_ON_ONCE(flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)))
>                         flags &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>
> -               if (p->sched_class->switching_from)
> +               if (p->sched_class->switching_from) {
> +                       /*
> +                        * switching_from_fair() assumes CLASS implies NOCLOCK;
> +                        * fixing this assumption would mean switching_from()
> +                        * would need to be able to change flags.
> +                        */
> +                       WARN_ON(!(flags & DEQUEUE_NOCLOCK));
>                         p->sched_class->switching_from(rq, p);
> +               }
>         }
>
>         *ctx = (struct sched_change_ctx){
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3924,9 +3924,6 @@ static void scx_disable_workfn(struct kt
>                         queue_flags &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>                 }
>
> -               if (old_class != new_class && p->se.sched_delayed)
> -                       dequeue_task(task_rq(p), p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
> -
>                 scoped_guard (sched_change, p, queue_flags) {
>                         p->sched_class = new_class;
>                 }
> @@ -4677,9 +4674,6 @@ static int scx_enable(struct sched_ext_o
>                         queue_flags &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>                 }
>
> -               if (old_class != new_class && p->se.sched_delayed)
> -                       dequeue_task(task_rq(p), p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
> -
>                 scoped_guard (sched_change, p, queue_flags) {
>                         p->scx.slice = SCX_SLICE_DFL;
>                         p->sched_class = new_class;
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13237,6 +13237,12 @@ static void attach_task_cfs_rq(struct ta
>         attach_entity_cfs_rq(se);
>  }
>
> +static void switching_from_fair(struct rq *rq, struct task_struct *p)
> +{
> +       if (p->se.sched_delayed)
> +               dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
> +}
> +
>  static void switched_from_fair(struct rq *rq, struct task_struct *p)
>  {
>         detach_task_cfs_rq(p);
> @@ -13638,6 +13644,7 @@ DEFINE_SCHED_CLASS(fair) = {
>
>         .reweight_task          = reweight_task_fair,
>         .prio_changed           = prio_changed_fair,
> +       .switching_from         = switching_from_fair,
>         .switched_from          = switched_from_fair,
>         .switched_to            = switched_to_fair,
>
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -689,9 +689,6 @@ int __sched_setscheduler(struct task_str
>                 queue_flags &= ~(DEQUEUE_SAVE | DEQUEUE_MOVE);
>         }
>
> -       if (prev_class != next_class && p->se.sched_delayed)
> -               dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
> -
>         scoped_guard (sched_change, p, queue_flags) {
>
>                 if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
>
>