sched: Deadline has dynamic priority

While FIFO/RR have static priority, DEADLINE is a dynamic priority
scheme. Notably it has static priority -1. Do not assume the priority
doesn't change for deadline tasks just because the static priority
doesn't change.

This ensures DL always sees {DE,EN}QUEUE_MOVE where appropriate.

Fixes: ff77e46 ("sched/rt: Fix PI handling vs. sched_setscheduler()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Pierre Gondois <pierre.gondois@arm.com>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://patch.msgid.link/20260114130528.GB831285@noisy.programming.kicks-ass.net

Author: Peter Zijlstra

kernel/sched/core.c

@@ -7320,7 +7320,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 	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;

kernel/sched/syscalls.c

@@ -639,7 +639,7 @@ int __sched_setscheduler(struct task_struct *p,
 		 * itself.
 		 */
 		newprio = rt_effective_prio(p, newprio);
-		if (newprio == oldprio)
+		if (newprio == oldprio && !dl_prio(newprio))
 			queue_flags &= ~DEQUEUE_MOVE;
 	}