blk-throttle: Move service tree validation out of the throtl_rb_first()

The throtl_schedule_next_dispatch() will validate if the service queue
is empty before calling update_min_dispatch_time(), and the
update_min_dispatch_time() will call throtl_rb_first(), which will
validate service queue again.

Thus we can move the service queue validation out of the
throtl_rb_first() to remove the redundant validation in the fast path.
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-throttle.c

@@ -638,9 +638,6 @@ static struct throtl_grp *
 throtl_rb_first(struct throtl_service_queue *parent_sq)
 {
 	struct rb_node *n;
-	/* Service tree is empty */
-	if (!parent_sq->nr_pending)
-		return NULL;
 
 	n = rb_first_cached(&parent_sq->pending_tree);
 	WARN_ON_ONCE(!n);
@@ -1224,9 +1221,13 @@ static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
 	unsigned int nr_disp = 0;
 
 	while (1) {
-		struct throtl_grp *tg = throtl_rb_first(parent_sq);
+		struct throtl_grp *tg;
 		struct throtl_service_queue *sq;
 
+		if (!parent_sq->nr_pending)
+			break;
+
+		tg = throtl_rb_first(parent_sq);
 		if (!tg)
 			break;