sched: Prevent boosting of idle task on -rt

Idle task boosting is a nono in general. There is one exception, when
NOHZ is active:

The idle task calls get_next_timer_interrupt() and holds the timer
wheel base->lock on the CPU and another CPU wants to access the timer
(probably to cancel it). We can safely ignore the boosting request, as
the idle CPU runs this code with interrupts disabled and will complete
the lock protected section without being interrupted. So there is no
real need to boost.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -6117,6 +6117,25 @@ void task_setprio(struct task_struct *p, int prio)
 	BUG_ON(prio < 0 || prio > MAX_PRIO);
 
 	rq = task_rq_lock(p, &flags);
+
+	/*
+	 * Idle task boosting is a nono in general. There is one
+	 * exception, when NOHZ is active:
+	 *
+	 * The idle task calls get_next_timer_interrupt() and holds
+	 * the timer wheel base->lock on the CPU and another CPU wants
+	 * to access the timer (probably to cancel it). We can safely
+	 * ignore the boosting request, as the idle CPU runs this code
+	 * with interrupts disabled and will complete the lock
+	 * protected section without being interrupted. So there is no
+	 * real need to boost.
+	 */
+	if (unlikely(p == rq->idle)) {
+		WARN_ON(p != rq->curr);
+		WARN_ON(p->pi_blocked_on);
+		goto out_unlock;
+	}
+
 	update_rq_clock(rq);
 
 	oldprio = p->prio;
@@ -6144,6 +6163,7 @@ void task_setprio(struct task_struct *p, int prio)
 		check_class_changed(rq, p, prev_class, oldprio, running);
 	}
 
+out_unlock:
 	task_rq_unlock(rq, &flags);
 }