Patch "sched/rt: Add a missing rescheduling point" has been added to the 4.9-stable tree

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

sched/rt: Add a missing rescheduling point

to the 4.9-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-rt-add-a-missing-rescheduling-point.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Sebastian Andrzej Siewior <[email protected]>
Date: Tue, 24 Jan 2017 15:40:06 +0100
Subject: sched/rt: Add a missing rescheduling point

From: Sebastian Andrzej Siewior <[email protected]>

commit 619bd4a71874a8fd78eb6ccf9f272c5e98bcc7b7 upstream.

Since the change in commit:

fd7a4bed1835 ("sched, rt: Convert switched_{from, to}_rt() / prio_changed_rt() to balance callbacks")

... we don't reschedule a task under certain circumstances:

Lets say task-A, SCHED_OTHER, is running on CPU0 (and it may run only on CPU0) and holds a PI lock. This task is removed from the CPU because it used up its time slice and another SCHED_OTHER task is running. Task-B on CPU1 runs at RT priority and asks for the lock owned by task-A. This results in a priority boost for task-A. Task-B goes to sleep until the lock has been made available. Task-A is already runnable (but not active), so it receives no wake up.

The reality now is that task-A gets on the CPU once the scheduler decides to remove the current task despite the fact that a high priority task is enqueued and waiting. This may take a long time.

The desired behaviour is that CPU0 immediately reschedules after the priority boost which made task-A the task with the lowest priority.

---
 kernel/sched/deadline.c |    3 +--
 kernel/sched/rt.c       |    3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

@@ -1729,12 +1729,11 @@ static void switched_to_dl(struct rq *rq
 #ifdef CONFIG_SMP
 		if (tsk_nr_cpus_allowed(p) > 1 && rq->dl.overloaded)
 			queue_push_tasks(rq);
-#else
+#endif
 		if (dl_task(rq->curr))
 			check_preempt_curr_dl(rq, p, 0);
 		else
 			resched_curr(rq);
-#endif
 	}
 }
 

@@ -2198,10 +2198,9 @@ static void switched_to_rt(struct rq *rq
 #ifdef CONFIG_SMP
 		if (tsk_nr_cpus_allowed(p) > 1 && rq->rt.overloaded)
 			queue_push_tasks(rq);
-#else
+#endif /* CONFIG_SMP */
 		if (p->prio < rq->curr->prio)
 			resched_curr(rq);
-#endif /* CONFIG_SMP */
 	}
 }
 

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

queue-4.9/sched-rt-add-a-missing-rescheduling-point.patch