Re: [PATCH v4 3/5] sched/fair: Switch to task based throttle model

[Aaron Lu <ziqianlu@bytedance.com>]

On Thu, Sep 04, 2025 at 03:21:06PM +0530, K Prateek Nayak wrote:
> Hello Aaron,
> 
> On 9/4/2025 1:46 PM, Aaron Lu wrote:
> > @@ -8722,15 +8730,6 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
> >  	if (unlikely(se == pse))
> >  		return;
> >  
> > -	/*
> > -	 * This is possible from callers such as attach_tasks(), in which we
> > -	 * unconditionally wakeup_preempt() after an enqueue (which may have
> > -	 * lead to a throttle).  This both saves work and prevents false
> > -	 * next-buddy nomination below.
> > -	 */
> > -	if (unlikely(throttled_hierarchy(cfs_rq_of(pse))))
> > -		return;
> 
> I think we should have a:
> 
> 	if (task_is_throttled(p))
> 		return;
> 
> here. I can see at least one possibility via prio_changed_fair()

Ah right. I didn't realize wakeup_preempt() can be called for a throttled
task, I think it is not expected. What about forbid that :)
(not tested in anyway, just to show the idea and get feedback)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cb93e74a850e8..f1383aede764f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13135,7 +13135,11 @@ static void task_fork_fair(struct task_struct *p)
 static void
 prio_changed_fair(struct rq *rq, struct task_struct *p, int oldprio)
 {
-	if (!task_on_rq_queued(p))
+	/*
+	 * p->on_rq can be set for throttled task but there is no need to
+	 * check wakeup preempt for throttled task, so use p->se.on_rq instead.
+	 */
+	if (!p->se.on_rq)
 		return;
 
 	if (rq->cfs.nr_queued == 1)

> where a throttled task might reach here. Rest looks good. I'll
> still wait on Ben for the update_cfs_group() bits :)
> 
> > -
> >  	if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK) && !pse->sched_delayed) {
> >  		set_next_buddy(pse);
> >  	}

Best regards,
Aaron