Patch "sched: Allow balance callbacks for check_class_changed()" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

sched: Allow balance callbacks for check_class_changed()

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-allow-balance-callbacks-for-check_class_changed.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Zijlstra <[email protected]>
Date: Thu, 11 Jun 2015 14:46:39 +0200
Subject: sched: Allow balance callbacks for check_class_changed()

From: Peter Zijlstra <[email protected]>

commit 4c9a4bc89a9cca8128bce67d6bc8870d6b7ee0b2 upstream.

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.

---
 kernel/sched/core.c |   24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

@@ -937,6 +937,13 @@ inline int task_curr(const struct task_s
 	return cpu_curr(task_cpu(p)) == p;
 }
 
+/*
+ * 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,
 				       int oldprio)

@@ -1423,8 +1430,12 @@ ttwu_do_wakeup(struct rq *rq, struct tas
 
 	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;

@@ -3006,7 +3017,11 @@ void rt_mutex_setprio(struct task_struct
 
 	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
 

@@ -3512,10 +3527,17 @@ change:
 		enqueue_task(rq, p, 0);
 
 	check_class_changed(rq, p, prev_class, oldprio);
+	preempt_disable(); /* avoid rq from going away on us */
 	task_rq_unlock(rq, p, &flags);
 
 	rt_mutex_adjust_pi(p);
 
+	/*
+	 * Run balance callbacks after we've adjusted the PI chain.
+	 */
+	balance_callback(rq);
+	preempt_enable();
+
 	return 0;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/sched-rt-remove-return-value-from-pull_rt_task.patch queue-3.14/sched-replace-post_schedule-with-a-balance-callback-list.patch queue-3.14/sched-clean-up-idle-task-smp-logic.patch queue-3.14/sched-dl-convert-switched_-from-to-_dl-prio_changed_dl-to-balance-callbacks.patch queue-3.14/sched-dl-remove-return-value-from-pull_dl_task.patch queue-3.14/sched-allow-balance-callbacks-for-check_class_changed.patch queue-3.14/sched-rt-convert-switched_-from-to-_rt-prio_changed_rt-to-balance-callbacks.patch