Re: [PATCH v25 1/9] sched: Make class_schedulers avoid pushing current, and get rid of proxy_tag_curr()

[K Prateek Nayak]

Hello John,

On 3/13/2026 8:00 AM, John Stultz wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b7f77c165a6e0..d86d648a75a4b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6702,23 +6702,6 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  }
>  #endif /* SCHED_PROXY_EXEC */
>  
> -static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
> -{
> -	if (!sched_proxy_exec())
> -		return;
> -	/*
> -	 * pick_next_task() calls set_next_task() on the chosen task
> -	 * at some point, which ensures it is not push/pullable.
> -	 * However, the chosen/donor task *and* the mutex owner form an
> -	 * atomic pair wrt push/pull.
> -	 *
> -	 * Make sure owner we run is not pushable. Unfortunately we can
> -	 * only deal with that by means of a dequeue/enqueue cycle. :-/
> -	 */
> -	dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
> -	enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
> -}
> -
>  /*
>   * __schedule() is the main scheduler function.
>   *
> @@ -6871,9 +6854,6 @@ static void __sched notrace __schedule(int sched_mode)
>  		 */
>  		RCU_INIT_POINTER(rq->curr, next);
>  
> -		if (!task_current_donor(rq, next))
> -			proxy_tag_curr(rq, next);
> -

Back to my concern with the queuing of the balance_callback, and the
deadline and RT folks can keep me honest here, consider the following:

    CPU0
    ====

  ======> Task A (prio: 80)
  ...
  
  mutex_lock(Mutex0)
  ... /* Executing critical section. */

    =====> Interrupt: Wakes up Task B (prio: 50); B->blocked_on = Mutex0;
      resched_curr()
    <===== Interrupt return
  preempt_schedule_irq()
    schedule()
      put_prev_set_next_Task(A, B)
      rq->donor = B
      if (task_is_blocked(B)
        next = find_proxy_task() /* Return Task A */
      rq->curr = A
      queue_balance_callback()
    do_balance_callbacks()
      /* Finds A as task_on_cpu(); Does nothing. */

  ... /* returns from schedule */
  ... /* continues with critical section */

  mutex_unlock(Mutex0)
    mutex_handoff(B /* Task B */)
    preempt_disable()
      try_to_wake_up()
        resched_curr()
    preempt_enable()
      preempt_schedule()
        proxy_force_return()
          /* Returns to same CPU */

        /*
         * put_prev_set_next_task() is skipped since
         * rq->donor context is same. no balance
         * callbacks are queued. Task A still on the
         * push list.
         */
        rq->donor = B
        rq->curr = B

  =======> sched_out: Task A

  !!! No balance callback; Task A still on push list. !!!
  
  <======= sched_in: Task B


So what I'm getting to is, if we find that rq->donor has not changed
with sched_proxy_exec() but rq->curr has changed during schedule(), we
should forcefully do a:

  prev->sched_class->put_prev_task(rq, rq->donor, rq->donor /* or rq->idle / NULL ? */);
  next->sched_class->set_next_task(rq, rq->donor, true /* to queue balance callback. */);

That way, when we do set_nex_task(), we see if we potentially have
tasks in the push list and queue a balance callback since the
task_on_cpu() condition may no longer apply to the tasks left behind
on the list.

Thoughts?

>  		/*
>  		 * The membarrier system call requires each architecture
>  		 * to have a full memory barrier after updating
> @@ -6907,10 +6887,6 @@ static void __sched notrace __schedule(int sched_mode)
>  		/* Also unlocks the rq: */
>  		rq = context_switch(rq, prev, next, &rf);
>  	} else {
> -		/* In case next was already curr but just got blocked_donor */
> -		if (!task_current_donor(rq, next))
> -			proxy_tag_curr(rq, next);
> -
>  		rq_unpin_lock(rq, &rf);
>  		__balance_callbacks(rq, NULL);
>  		raw_spin_rq_unlock_irq(rq);

-- 
Thanks and Regards,
Prateek