sched: Allow balance callbacks for check_class_changed()

In order to remove dropping rq->lock from the
switched_{to,from}()/prio_changed() sched_class methods, run the
balance callbacks after it.

We need to remove dropping rq->lock because its buggy,
suppose using sched_setattr()/sched_setscheduler() to change a running
task from FIFO to OTHER.

By the time we get to switched_from_rt() the task is already enqueued
on the cfs runqueues. If switched_from_rt() does pull_rt_task() and
drops rq->lock, load-balancing can come in and move our task @p to
another rq.

The subsequent switched_to_fair() still assumes @p is on @rq and bad
things will happen.

By using balance callbacks we delay the load-balancing operations
{rt,dl}x{push,pull} until we've done all the important work and the
task is fully set up.

Furthermore, the balance callbacks do not know about @p, therefore
they cannot get confused like this.

Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: ktkhai@parallels.com
Cc: rostedt@goodmis.org
Cc: juri.lelli@gmail.com
Cc: pang.xunlei@linaro.org
Cc: oleg@redhat.com
Cc: wanpeng.li@linux.intel.com
Link: http://lkml.kernel.org/r/20150611124742.615343911@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched/core.c

@@ -1001,7 +1001,11 @@ inline int task_curr(const struct task_struct *p)
 }
 
 /*
- * Can drop rq->lock because from sched_class::switched_from() methods drop it.
+ * switched_from, switched_to and prio_changed must _NOT_ drop rq->lock,
+ * use the balance_callback list if you want balancing.
+ *
+ * this means any call to check_class_changed() must be followed by a call to
+ * balance_callback().
  */
 static inline void check_class_changed(struct rq *rq, struct task_struct *p,
 				       const struct sched_class *prev_class,
@@ -1010,7 +1014,7 @@ static inline void check_class_changed(struct rq *rq, struct task_struct *p,
 	if (prev_class != p->sched_class) {
 		if (prev_class->switched_from)
 			prev_class->switched_from(rq, p);
-		/* Possble rq->lock 'hole'.  */
+
 		p->sched_class->switched_to(rq, p);
 	} else if (oldprio != p->prio || dl_task(p))
 		p->sched_class->prio_changed(rq, p, oldprio);
@@ -1491,8 +1495,12 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
 
 	p->state = TASK_RUNNING;
 #ifdef CONFIG_SMP
-	if (p->sched_class->task_woken)
+	if (p->sched_class->task_woken) {
+		/*
+		 * XXX can drop rq->lock; most likely ok.
+		 */
 		p->sched_class->task_woken(rq, p);
+	}
 
 	if (rq->idle_stamp) {
 		u64 delta = rq_clock(rq) - rq->idle_stamp;
@@ -3100,7 +3108,11 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 
 	check_class_changed(rq, p, prev_class, oldprio);
 out_unlock:
+	preempt_disable(); /* avoid rq from going away on us */
 	__task_rq_unlock(rq);
+
+	balance_callback(rq);
+	preempt_enable();
 }
 #endif
 
@@ -3661,11 +3673,18 @@ change:
 	}
 
 	check_class_changed(rq, p, prev_class, oldprio);
+	preempt_disable(); /* avoid rq from going away on us */
 	task_rq_unlock(rq, p, &flags);
 
 	if (pi)
 		rt_mutex_adjust_pi(p);
 
+	/*
+	 * Run balance callbacks after we've adjusted the PI chain.
+	 */
+	balance_callback(rq);
+	preempt_enable();
+
 	return 0;
 }