sched: add sched_class->needs_post_schedule() member

We currently run class->post_schedule() outside of the rq->lock, which
means that we need to test for the need to post_schedule outside of
the lock to avoid a forced reacquistion.  This is currently not a problem
as we only look at rq->rt.overloaded.  However, we want to enhance this
going forward to look at more state to reduce the need to post_schedule to
a bare minimum set.  Therefore, we introduce a new member-func called
needs_post_schedule() which tests for the post_schedule condtion without
actually performing the work.  Therefore it is safe to call this
function before the rq->lock is released, because we are guaranteed not
to drop the lock at an intermediate point (such as what post_schedule()
may do).

We will use this later in the series

[ rostedt: removed paranoid BUG_ON ]

Signed-off-by: Gregory Haskins <ghaskins@novell.com>

kernel/sched.c

@@ -2621,6 +2621,12 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
 {
 	struct mm_struct *mm = rq->prev_mm;
 	long prev_state;
+#ifdef CONFIG_SMP
+	int post_schedule = 0;
+
+	if (current->sched_class->needs_post_schedule)
+		post_schedule = current->sched_class->needs_post_schedule(rq);
+#endif
 
 	rq->prev_mm = NULL;
 
@@ -2639,7 +2645,7 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
 	finish_arch_switch(prev);
 	finish_lock_switch(rq, prev);
 #ifdef CONFIG_SMP
-	if (current->sched_class->post_schedule)
+	if (post_schedule)
 		current->sched_class->post_schedule(rq);
 #endif