Patch "sched/deadline: Fix migration of SCHED_DEADLINE tasks" has been added to the 4.2-stable tree

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

sched/deadline: Fix migration of SCHED_DEADLINE tasks

to the 4.2-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-deadline-fix-migration-of-sched_deadline-tasks.patch
and it can be found in the queue-4.2 subdirectory.

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

From: Luca Abeni <[email protected]>
Date: Fri, 16 Oct 2015 10:06:21 +0200
Subject: sched/deadline: Fix migration of SCHED_DEADLINE tasks

From: Luca Abeni <[email protected]>

commit 5aa5050787f449e7eaef2c5ec93c7b357aa7dcdc upstream.

Commit:

9d5142624256 ("sched/deadline: Reduce rq lock contention by eliminating locking of non-feasible target")

broke select_task_rq_dl() and find_lock_later_rq(), because it introduced a comparison between the local task's deadline and dl.earliest_dl.curr of the remote queue.

However, if the remote runqueue does not contain any SCHED_DEADLINE task its earliest_dl.curr is 0 (always smaller than the deadline of the local task) and the remote runqueue is not selected for pushing.

As a result, if an application creates multiple SCHED_DEADLINE threads, they will never be pushed to runqueues that do not already contain SCHED_DEADLINE tasks.

This patch fixes the issue by checking if dl.dl_nr_running == 0.

---
 kernel/sched/deadline.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

@@ -1066,8 +1066,9 @@ select_task_rq_dl(struct task_struct *p,
 		int target = find_later_rq(p);
 
 		if (target != -1 &&
-				dl_time_before(p->dl.deadline,
-					cpu_rq(target)->dl.earliest_dl.curr))
+				(dl_time_before(p->dl.deadline,
+					cpu_rq(target)->dl.earliest_dl.curr) ||
+				(cpu_rq(target)->dl.dl_nr_running == 0)))
 			cpu = target;
 	}
 	rcu_read_unlock();

@@ -1417,7 +1418,8 @@ static struct rq *find_lock_later_rq(str
 
 		later_rq = cpu_rq(cpu);
 
-		if (!dl_time_before(task->dl.deadline,
+		if (later_rq->dl.dl_nr_running &&
+		    !dl_time_before(task->dl.deadline,
 					later_rq->dl.earliest_dl.curr)) {
 			/*
 			 * Target rq has tasks of equal or earlier deadline,

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

queue-4.2/sched-deadline-fix-migration-of-sched_deadline-tasks.patch