sched: fix __load_balance_iterator() for cfq with only one task

The __load_balance_iterator() returns a NULL when there's only one
sched_entity which is a task. It is caused by the following code-path.

	/* Skip over entities that are not tasks */
	do {
		se = list_entry(next, struct sched_entity, group_node);
		next = next->next;
	} while (next != &cfs_rq->tasks && !entity_is_task(se));

	if (next == &cfs_rq->tasks)
		return NULL;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      This will return NULL even when se is a task.

As a side-effect, there was a regression in sched_mc behavior since 2.6.25,
since iter_move_one_task() when it calls load_balance_start_fair(),
would not get any tasks to move!

Fix this by checking if the last entity was a task or not.

Signed-off-by: Gautham R Shenoy <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched_fair.c

@@ -1458,7 +1458,7 @@ __load_balance_iterator(struct cfs_rq *cfs_rq, struct list_head *next)
 		next = next->next;
 	} while (next != &cfs_rq->tasks && !entity_is_task(se));
 
-	if (next == &cfs_rq->tasks)
+	if (next == &cfs_rq->tasks && !entity_is_task(se))
 		return NULL;
 
 	cfs_rq->balance_iterator = next;