Re: [PATCH] sched/rt: Push RT tasks when preempted by a deadline task

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Yangsonghua, 

On 6/24/2026 8:34 AM, yangsonghua wrote:
> @@ -1736,14 +1737,24 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct task_s
> 
>         update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 1);
> 
> -       if (task_is_blocked(p))
> -               return;
>         /*
>          * The previous task needs to be made eligible for pushing
> -        * if it is still active
> +        * if it is still active and migratable.
>          */
> -       if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
> +       if (!task_is_blocked(p) && on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
>                 enqueue_pushable_task(rq, p);
> +
> +       /*
> +        * When a deadline task takes over this CPU, try to push any queued
> +        * RT tasks to CPUs running lower-priority work. This is independent
> +        * of whether p itself is pushable: even if p is pinned or blocked,
> +        * there may be other migratable RT tasks already in pushable_tasks.
> +        *
> +        * rt_queue_push_tasks() guards on has_pushable_tasks() internally,
> +        * so this is a no-op if nothing is queued.
> +        */
> +       if (dl_task(next))
> +               rt_queue_push_tasks(rq);

next can be NULL if we are are coming here from sched_change_begin()
for the current task:

  sched_change_begin()
    put_prev_task()
      put_prev_task_rt(rq, prev, next = NULL)
        dl_task(next)
          dl_prio(next->prio) !!! NULL pointer dereference !!!

Do we avoid this in some way?

I don't know if this push was intentionally avoided or not. If there
are short running deadline task, we'll unnecessarily push a task and
pull it back later. Is that not a concern?

-- 
Thanks and Regards,
Prateek