sched: Use fair:prio_changed() instead of ad-hoc implementation

set_user_nice() implements its own version of fair::prio_changed() and
therefore misses a specific optimization towards nohz_full CPUs that
avoid sending an resched IPI to a reniced task running alone. Use the
proper callback instead.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20191203160106.18806-3-frederic@kernel.org

kernel/sched/core.c

@@ -4540,17 +4540,17 @@ void set_user_nice(struct task_struct *p, long nice)
 	p->prio = effective_prio(p);
 	delta = p->prio - old_prio;
 
-	if (queued) {
+	if (queued)
 		enqueue_task(rq, p, ENQUEUE_RESTORE | ENQUEUE_NOCLOCK);
+	if (running)
+		set_next_task(rq, p);
+
 	/*
 	 * If the task increased its priority or is running and
 	 * lowered its priority, then reschedule its CPU:
 	 */
-		if (delta < 0 || (delta > 0 && task_running(rq, p)))
-			resched_curr(rq);
-	}
-	if (running)
-		set_next_task(rq, p);
+	p->sched_class->prio_changed(rq, p, old_prio);
+
 out_unlock:
 	task_rq_unlock(rq, p, &rf);
 }