cfq-iosched: Don't clear queue stats when preempt.

For v2, I added back lines to cfq_preempt_queue() that were removed
during updates for accounting unaccounted_time. Thanks for pointing out
that I'd missed these, Vivek.

Previous commit "cfq-iosched: Don't set active queue in preempt" wrongly
cleared stats for preempting queues when it shouldn't have, because when
we choose a queue to preempt, it still isn't necessarily scheduled next.

Thanks to Vivek Goyal for figuring this out and understanding how the
preemption code works.

Signed-off-by: Justin TerAvest <teravest@google.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/cfq-iosched.c

@@ -1620,33 +1620,27 @@ static inline void cfq_del_timer(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	cfq_blkiocg_update_idle_time_stats(&cfqq->cfqg->blkg);
 }
 
-static void cfq_clear_queue_stats(struct cfq_data *cfqd,
-				  struct cfq_queue *cfqq)
-{
-	cfq_blkiocg_update_avg_queue_size_stats(&cfqq->cfqg->blkg);
-	cfqq->slice_start = 0;
-	cfqq->dispatch_start = jiffies;
-	cfqq->allocated_slice = 0;
-	cfqq->slice_end = 0;
-	cfqq->slice_dispatch = 0;
-	cfqq->nr_sectors = 0;
-
-	cfq_clear_cfqq_wait_request(cfqq);
-	cfq_clear_cfqq_must_dispatch(cfqq);
-	cfq_clear_cfqq_must_alloc_slice(cfqq);
-	cfq_clear_cfqq_fifo_expire(cfqq);
-	cfq_mark_cfqq_slice_new(cfqq);
-
-	cfq_del_timer(cfqd, cfqq);
-}
-
 static void __cfq_set_active_queue(struct cfq_data *cfqd,
 				   struct cfq_queue *cfqq)
 {
 	if (cfqq) {
 		cfq_log_cfqq(cfqd, cfqq, "set_active wl_prio:%d wl_type:%d",
 				cfqd->serving_prio, cfqd->serving_type);
-		cfq_clear_queue_stats(cfqd, cfqq);
+		cfq_blkiocg_update_avg_queue_size_stats(&cfqq->cfqg->blkg);
+		cfqq->slice_start = 0;
+		cfqq->dispatch_start = jiffies;
+		cfqq->allocated_slice = 0;
+		cfqq->slice_end = 0;
+		cfqq->slice_dispatch = 0;
+		cfqq->nr_sectors = 0;
+
+		cfq_clear_cfqq_wait_request(cfqq);
+		cfq_clear_cfqq_must_dispatch(cfqq);
+		cfq_clear_cfqq_must_alloc_slice(cfqq);
+		cfq_clear_cfqq_fifo_expire(cfqq);
+		cfq_mark_cfqq_slice_new(cfqq);
+
+		cfq_del_timer(cfqd, cfqq);
 	}
 
 	cfqd->active_queue = cfqq;
@@ -3339,7 +3333,8 @@ static void cfq_preempt_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 
 	cfq_service_tree_add(cfqd, cfqq, 1);
 
-	cfq_clear_queue_stats(cfqd, cfqq);
+	cfqq->slice_end = 0;
+	cfq_mark_cfqq_slice_new(cfqq);
 }
 
 /*