linux-kernel - Re: [PATCH 1/3] sched_ext: Split schedule_deferred() into locked and unlocked variants

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Message-ID: <CABFh=a4gb7GcbMP-gB0tWXtsFtgYkpVz9NtBA1f-Ne5VWzSpmw@mail.gmail.com>
Date: Sat, 25 Oct 2025 19:17:03 -0400
From: Emil Tsalapatis <linux-lists@...alapatis.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Andrea Righi <andrea.righi@...ux.dev>, 
	Changwoo Min <changwoo@...lia.com>, linux-kernel@...r.kernel.org, 
	sched-ext@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>, 
	Wen-Fang Liu <liuwenfang@...or.com>
Subject: Re: [PATCH 1/3] sched_ext: Split schedule_deferred() into locked and
 unlocked variants

On Fri, Oct 24, 2025 at 8:18 PM Tejun Heo <tj@...nel.org> wrote:
>
> schedule_deferred() currently requires the rq lock to be held so that it can
> use scheduler hooks for efficiency when available. However, there are cases
> where deferred actions need to be scheduled from contexts that don't hold the
> rq lock.
>
> Split into schedule_deferred() which can be called from any context and just
> queues irq_work, and schedule_deferred_locked() which requires the rq lock and
> can optimize by using scheduler hooks when available. Update the existing call
> site to use the _locked variant.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---

Reviewed-by: Emil Tsalapatis <emil@...alapatis.com>

I assume we don't really care about ba

>  kernel/sched/ext.c | 33 ++++++++++++++++++++++++---------
>  1 file changed, 24 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 000000000000..111111111111 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -775,12 +775,28 @@ static void deferred_irq_workfn(struct irq_work *irq_work)
>   * schedule_deferred - Schedule execution of deferred actions on an rq
>   * @rq: target rq
>   *
> - * Schedule execution of deferred actions on @rq. Must be called with @rq
> - * locked. Deferred actions are executed with @rq locked but unpinned, and thus
> - * can unlock @rq to e.g. migrate tasks to other rqs.
> + * Schedule execution of deferred actions on @rq. Deferred actions are executed
> + * with @rq locked but unpinned, and thus can unlock @rq to e.g. migrate tasks
> + * to other rqs.
>   */
>  static void schedule_deferred(struct rq *rq)
>  {
> +       /*
> +        * Queue an irq work. They are executed on IRQ re-enable which may take
> +        * a bit longer than the scheduler hook in schedule_deferred_locked().
> +        */
> +       irq_work_queue(&rq->scx.deferred_irq_work);
> +}
> +
> +/**
> + * schedule_deferred_locked - Schedule execution of deferred actions on an rq
> + * @rq: target rq
> + *
> + * Schedule execution of deferred actions on @rq. Equivalent to
> + * schedule_deferred() but requires @rq to be locked and can be more efficient.
> + */
> +static void schedule_deferred_locked(struct rq *rq)
> +{
>         lockdep_assert_rq_held(rq);
>
>         /*
> @@ -812,12 +828,11 @@ static void schedule_deferred(struct rq *rq)
>         }
>
>         /*
> -        * No scheduler hooks available. Queue an irq work. They are executed on
> -        * IRQ re-enable which may take a bit longer than the scheduler hooks.
> -        * The above WAKEUP and BALANCE paths should cover most of the cases and
> -        * the time to IRQ re-enable shouldn't be long.
> +        * No scheduler hooks available. Use the generic irq_work path. The
> +        * above WAKEUP and BALANCE paths should cover most of the cases and the
> +        * time to IRQ re-enable shouldn't be long.
>          */
> -       irq_work_queue(&rq->scx.deferred_irq_work);
> +       schedule_deferred(rq);
>  }
>
>  /**
> @@ -1211,7 +1226,7 @@ static void direct_dispatch(struct scx_sched *sch, struct task_struct *p,
>                 WARN_ON_ONCE(p->scx.dsq || !list_empty(&p->scx.dsq_list.node));
>                 list_add_tail(&p->scx.dsq_list.node,
>                               &rq->scx.ddsp_deferred_locals);
> -               schedule_deferred(rq);
> +               schedule_deferred_locked(rq);
>                 return;
>         }
>
> --
> 2.47.1
>