Re: [BUG] Apparent lost wakeups

[Peter Zijlstra]

On Tue, Oct 13, 2026 at 03:59:36PM +0530, K Prateek Nayak wrote:

> Essentially:
> 
>   /* rq->next_class starts as fair_sched_class */
> 
>   pick_next_task()
>     /* Finishes ->pick_task() for stop, dl, rt */
>     pick_task_fair()
>       sched_balance_newidle()
>         raw_spin_rq_unlock()
>                                            activate_task(rq0, p) /* fair */ 
>                                              /* p->on_rq but not curr */
>                                              rq->next_class = fair_sched_class;
> 
>                                            __sched_setscheduler()
>                                              /* Upgrades p to FIFO */
>                                              sched_change_end()
>                                                /* Does not change rq->next_class */
>                                              rq_unlock()
> 
>         raw_spin_rq_lock()
> 
>         if (this_rq->cfs.h_nr_queued)
>           /* Skipped! No fair task */
> 
>         if (rq_modified_above(this_rq, &fair_sched_class))
>           /* Skipped: rq->nex_class is still fair */
> 
>         return 0;
>       return NULL; 
> 
>   pick_task_idle()
>     return rq->idle;
> 
>   /* In __schedule() */
>   clear_tsk_need_resched();
> 
>   /* Goes idle with rq->nr_running = 1 and no need_resched() */

Excellent Changelog material that ;-)

> 
> > Does something like so make sense?
> 
> Couple nits below.
> 
> > 
> > That is, any task on the rq cares about promotion, only the current
> > running task cares about demotion.
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 047fbe8ef298..2a8bf562a769 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -11261,7 +11261,7 @@ void sched_change_end(struct sched_change_ctx *ctx)
> >  		if (p->sched_class->switched_to)
> >  			p->sched_class->switched_to(rq, p);
> >  
> > -		if (ctx->running) {
> > +		if (ctx->queued) {
> 
> I think this branch should just call wakeup_preempt() to account
> for p->sched_class == rq->next_class in case p is upgraded to
> same class as rq->curr.

Yes, that'll do.

> >  			/*
> >  			 * If this was a class promotion; let the old class
> >  			 * know it got preempted. Note that none of the
> > @@ -11272,6 +11272,9 @@ void sched_change_end(struct sched_change_ctx *ctx)
> >  				rq->next_class->wakeup_preempt(rq, p, 0);
> >  				rq->next_class = p->sched_class;
> >  			}
> > +		}
> > +
> > +		if (ctx->running) {
> >  			/*
> >  			 * If this was a degradation in class; make sure to
> >  			 * reschedule.
> 
> This looks correct. Upgrade keeps the task running and degrade
> re-evaluates the selection.

Can you turn all this into a nice patch -- you've done all the hard work
on it anyway.

Thanks!