cfq-iosched: don't check cfqg in choose_service_tree()

When cfq_choose_cfqg() is called in select_queue(), there must be at least one
backlogged CFQ queue waiting for dispatching, hence there must be at least one
backlogged CFQ group on service tree. So we never call choose_service_tree()
with cfqg == NULL.
Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/cfq-iosched.c

@@ -2103,12 +2103,6 @@ static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
 	unsigned group_slice;
 	enum wl_prio_t original_prio = cfqd->serving_prio;
 
-	if (!cfqg) {
-		cfqd->serving_prio = IDLE_WORKLOAD;
-		cfqd->workload_expires = jiffies + 1;
-		return;
-	}
-
 	/* Choose next priority. RT > BE > IDLE */
 	if (cfq_group_busy_queues_wl(RT_WORKLOAD, cfqd, cfqg))
 		cfqd->serving_prio = RT_WORKLOAD;