sched/deadline: Fix migration of SCHED_DEADLINE tasks

Commit:

  9d51426242 ("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.

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@linux.intel.com>
Fixes: 9d51426242 ("sched/deadline: Reduce rq lock contention by eliminating locking of non-feasible target")
Link: http://lkml.kernel.org/r/1444982781-15608-1-git-send-email-luca.abeni@unitn.it
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/deadline.c

@@ -1066,8 +1066,9 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
 		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(struct task_struct *task, struct rq *rq)
 
 		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,