linux-kernel - Re: [PATCH sched_ext/for-6.19] sched_ext: Allow forcibly picking an scx task

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Message-ID: <CABFh=a55MGEVxuBg9mtZob5GbacPk_2EjXLDdjKMJmVhF6d7Sw@mail.gmail.com>
Date: Sat, 18 Oct 2025 16:07:29 -0400
From: Emil Tsalapatis <linux-lists@...alapatis.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>, 
	Changwoo Min <changwoo@...lia.com>, sched-ext@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH sched_ext/for-6.19] sched_ext: Allow forcibly picking an
 scx task

On Sat, Oct 18, 2025 at 10:10 AM Andrea Righi <arighi@...dia.com> wrote:
>
> Refactor pick_task_scx() adding a new argument to forcibly pick a
> SCHED_EXT task, ignoring any higher-priority sched class activity.
>
> This refactoring prepares the code for future scenarios, e.g., allowing
> the ext dl_server to force a SCHED_EXT task selection.
>
> No functional changes.
>
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Andrea Righi <arighi@...dia.com>
> ---


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

>  kernel/sched/ext.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index adff739b396ce..35bc37c7ee199 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -2351,7 +2351,8 @@ static struct task_struct *first_local_task(struct rq *rq)
>                                         struct task_struct, scx.dsq_list.node);
>  }
>
> -static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf)
> +static struct task_struct *
> +do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx)
>  {
>         struct task_struct *prev = rq->curr;
>         bool keep_prev, kick_idle = false;
> @@ -2365,7 +2366,15 @@ static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf)
>
>         maybe_queue_balance_callback(rq);
>
> -       if (rq_modified_above(rq, &ext_sched_class))
> +       /*
> +        * If any higher-priority sched class enqueued a runnable task on
> +        * this rq during balance_one(), abort and return RETRY_TASK, so
> +        * that the scheduler loop can restart.
> +        *
> +        * If @force_scx is true, always try to pick a SCHED_EXT task,
> +        * regardless of any higher-priority sched classes activity.
> +        */
> +       if (!force_scx && rq_modified_above(rq, &ext_sched_class))
>                 return RETRY_TASK;
>
>         keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
> @@ -2408,6 +2417,11 @@ static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf)
>         return p;
>  }
>
> +static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf)
> +{
> +       return do_pick_task_scx(rq, rf, false);
> +}
> +
>  #ifdef CONFIG_SCHED_CORE
>  /**
>   * scx_prio_less - Task ordering for core-sched
> --
> 2.51.1.dirty
>
>