Re: [PATCH 05/12] sched: Move sched_class::prio_changed() into the change pattern

[luca abeni <luca.abeni@santannapisa.it>]

Hi Peter,

On Wed, 14 Jan 2026 14:05:28 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Jan 14, 2026 at 11:23:36AM +0100, Peter Zijlstra wrote:
> 
> > Juri, Luca, I'm tempted to suggest to simply remove the replenish on
> > RESTORE entirely -- that would allow the task to continue as it had
> > been, irrespective of it being 'late'.
> > 
> > Something like so -- what would this break?
> > 
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -2214,10 +2214,6 @@ enqueue_dl_entity(struct sched_dl_entity
> >  		update_dl_entity(dl_se);
> >  	} else if (flags & ENQUEUE_REPLENISH) {
> >  		replenish_dl_entity(dl_se);
> > -	} else if ((flags & ENQUEUE_RESTORE) &&
> > -		   !is_dl_boosted(dl_se) &&
> > -		   dl_time_before(dl_se->deadline,
> > rq_clock(rq_of_dl_se(dl_se)))) {
> > -		setup_new_dl_entity(dl_se);
> >  	}
> >  
> >  	/*  
> 
> Ah, this is de-boost, right? Boosting allows one to break the CBS
> rules and then we have to rein in the excesses.

Sorry, I am missing a little bit of context (I am trying to catch up
reading the mailing list archives)... But I agree that the call to
setup_new_dl_entity() mentioned above does not make too much sense.

I suspect the hunk above could be directly removed, as you originally
suggested (on de-boosting(), the task returns to its original deadline,
which is larger than the inherited one, so I am not sure whether we
should generate a new deadline or just leave it as it is, even if it
has been missed).



				Luca
> 
> But we have {DE,EN}QUEUE_MOVE for this, that explicitly allows
> priority to change and is set for rt_mutex_setprio() (among others).
> 
> So doing s/RESTORE/MOVE/ above.
> 
> The corollary to all this is that everybody that sets MOVE must be
> able to deal with balance callbacks, so audit that too.
> 
> This then gives something like so.. which builds and boots for me, but
> clearly I haven't been able to trigger these funny cases.
> 
> ---
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4969,9 +4969,13 @@ struct balance_callback *splice_balance_
>  	return __splice_balance_callbacks(rq, true);
>  }
>  
> -static void __balance_callbacks(struct rq *rq)
> +void __balance_callbacks(struct rq *rq, struct rq_flags *rf)
>  {
> +	if (rf)
> +		rq_unpin_lock(rq, rf);
>  	do_balance_callbacks(rq, __splice_balance_callbacks(rq,
> false));
> +	if (rf)
> +		rq_repin_lock(rq, rf);
>  }
>  
>  void balance_callbacks(struct rq *rq, struct balance_callback *head)
> @@ -5018,7 +5022,7 @@ static inline void finish_lock_switch(st
>  	 * prev into current:
>  	 */
>  	spin_acquire(&__rq_lockp(rq)->dep_map, 0, 0, _THIS_IP_);
> -	__balance_callbacks(rq);
> +	__balance_callbacks(rq, NULL);
>  	raw_spin_rq_unlock_irq(rq);
>  }
>  
> @@ -6901,7 +6905,7 @@ static void __sched notrace __schedule(i
>  			proxy_tag_curr(rq, next);
>  
>  		rq_unpin_lock(rq, &rf);
> -		__balance_callbacks(rq);
> +		__balance_callbacks(rq, NULL);
>  		raw_spin_rq_unlock_irq(rq);
>  	}
>  	trace_sched_exit_tp(is_switch);
> @@ -7350,7 +7354,7 @@ void rt_mutex_setprio(struct task_struct
>  	trace_sched_pi_setprio(p, pi_task);
>  	oldprio = p->prio;
>  
> -	if (oldprio == prio)
> +	if (oldprio == prio && !dl_prio(prio))
>  		queue_flag &= ~DEQUEUE_MOVE;
>  
>  	prev_class = p->sched_class;
> @@ -7396,9 +7400,7 @@ void rt_mutex_setprio(struct task_struct
>  out_unlock:
>  	/* Caller holds task_struct::pi_lock, IRQs are still
> disabled */ 
> -	rq_unpin_lock(rq, &rf);
> -	__balance_callbacks(rq);
> -	rq_repin_lock(rq, &rf);
> +	__balance_callbacks(rq, &rf);
>  	__task_rq_unlock(rq, p, &rf);
>  }
>  #endif /* CONFIG_RT_MUTEXES */
> @@ -9167,6 +9169,8 @@ void sched_move_task(struct task_struct
>  
>  	if (resched)
>  		resched_curr(rq);
> +
> +	__balance_callbacks(rq, &rq_guard.rf);
>  }
>  
>  static struct cgroup_subsys_state *
> @@ -10891,6 +10895,9 @@ void sched_change_end(struct sched_chang
>  				resched_curr(rq);
>  		}
>  	} else {
> +		/*
> +		 * XXX validate prio only really changed when
> ENQUEUE_MOVE is set.
> +		 */
>  		p->sched_class->prio_changed(rq, p, ctx->prio);
>  	}
>  }
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2214,9 +2214,14 @@ enqueue_dl_entity(struct sched_dl_entity
>  		update_dl_entity(dl_se);
>  	} else if (flags & ENQUEUE_REPLENISH) {
>  		replenish_dl_entity(dl_se);
> -	} else if ((flags & ENQUEUE_RESTORE) &&
> +	} else if ((flags & ENQUEUE_MOVE) &&
>  		   !is_dl_boosted(dl_se) &&
>  		   dl_time_before(dl_se->deadline,
> rq_clock(rq_of_dl_se(dl_se)))) {
> +		/*
> +		 * Deals with the de-boost case, and ENQUEUE_MOVE
> explicitly
> +		 * allows us to change priority. Callers are
> expected to deal
> +		 * with balance_callbacks.
> +		 */
>  		setup_new_dl_entity(dl_se);
>  	}
>  
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -545,6 +545,7 @@ static void scx_task_iter_start(struct s
>  static void __scx_task_iter_rq_unlock(struct scx_task_iter *iter)
>  {
>  	if (iter->locked_task) {
> +		__balance_callbacks(iter->rq, &iter->rf);
>  		task_rq_unlock(iter->rq, iter->locked_task,
> &iter->rf); iter->locked_task = NULL;
>  	}
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2430,7 +2430,8 @@ extern const u32
> sched_prio_to_wmult[40
>   *                should preserve as much state as possible.
>   *
>   * MOVE - paired with SAVE/RESTORE, explicitly does not preserve the
> location
> - *        in the runqueue.
> + *        in the runqueue. IOW the priority is allowed to change.
> Callers
> + *        must expect to deal with balance callbacks.
>   *
>   * NOCLOCK - skip the update_rq_clock() (avoids double updates)
>   *
> @@ -4019,6 +4020,8 @@ extern void enqueue_task(struct rq *rq,
>  extern bool dequeue_task(struct rq *rq, struct task_struct *p, int
> flags); 
>  extern struct balance_callback *splice_balance_callbacks(struct rq
> *rq); +
> +extern void __balance_callbacks(struct rq *rq, struct rq_flags *rf);
>  extern void balance_callbacks(struct rq *rq, struct balance_callback
> *head); 
>  /*
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -639,7 +639,7 @@ int __sched_setscheduler(struct task_str
>  		 * itself.
>  		 */
>  		newprio = rt_effective_prio(p, newprio);
> -		if (newprio == oldprio)
> +		if (newprio == oldprio && !dl_prio(newprio))
>  			queue_flags &= ~DEQUEUE_MOVE;
>  	}
>