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Commit 4c9a4bc8 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
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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: default avatarMike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: default avatarPeter 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent dbc7f069

kernel/sched/core.c

+22 −3
Original line number Diff line number Diff line
@@ -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 @@ static int __sched_setscheduler(struct task_struct *p, 
	}



	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;

}