[BLOCK] Implement elv_drain_elevator for improved switch error detection

This patch adds request_queue->nr_sorted which keeps the number of
requests in the iosched and implement elv_drain_elevator which
performs forced dispatching.  elv_drain_elevator checks whether
iosched actually dispatches all requests it has and prints error
message if it doesn't.  As buggy forced dispatching can result in
wrong barrier operations, I think this extra check is worthwhile.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

block/elevator.c

@@ -226,6 +226,7 @@ void elv_dispatch_sort(request_queue_t *q, struct request *rq)
 
 	if (q->last_merge == rq)
 		q->last_merge = NULL;
+	q->nr_sorted--;
 
 	boundary = q->end_sector;
 
@@ -284,6 +285,7 @@ void elv_merge_requests(request_queue_t *q, struct request *rq,
 
 	if (e->ops->elevator_merge_req_fn)
 		e->ops->elevator_merge_req_fn(q, rq, next);
+	q->nr_sorted--;
 
 	q->last_merge = rq;
 }
@@ -315,6 +317,20 @@ void elv_requeue_request(request_queue_t *q, struct request *rq)
 	__elv_add_request(q, rq, ELEVATOR_INSERT_FRONT, 0);
 }
 
+static void elv_drain_elevator(request_queue_t *q)
+{
+	static int printed;
+	while (q->elevator->ops->elevator_dispatch_fn(q, 1))
+		;
+	if (q->nr_sorted == 0)
+		return;
+	if (printed++ < 10) {
+		printk(KERN_ERR "%s: forced dispatching is broken "
+		       "(nr_sorted=%u), please report this\n",
+		       q->elevator->elevator_type->elevator_name, q->nr_sorted);
+	}
+}
+
 void __elv_add_request(request_queue_t *q, struct request *rq, int where,
 		       int plug)
 {
@@ -349,9 +365,7 @@ void __elv_add_request(request_queue_t *q, struct request *rq, int where,
 
 	case ELEVATOR_INSERT_BACK:
 		rq->flags |= REQ_SOFTBARRIER;
-
-		while (q->elevator->ops->elevator_dispatch_fn(q, 1))
-			;
+		elv_drain_elevator(q);
 		list_add_tail(&rq->queuelist, &q->queue_head);
 		/*
 		 * We kick the queue here for the following reasons.
@@ -370,6 +384,7 @@ void __elv_add_request(request_queue_t *q, struct request *rq, int where,
 	case ELEVATOR_INSERT_SORT:
 		BUG_ON(!blk_fs_request(rq));
 		rq->flags |= REQ_SORTED;
+		q->nr_sorted++;
 		if (q->last_merge == NULL && rq_mergeable(rq))
 			q->last_merge = rq;
 		/*
@@ -692,8 +707,7 @@ static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
 
 	set_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
 
-	while (q->elevator->ops->elevator_dispatch_fn(q, 1))
-		;
+	elv_drain_elevator(q);
 
 	while (q->rq.elvpriv) {
 		blk_remove_plug(q);
@@ -701,6 +715,7 @@ static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
 		spin_unlock_irq(q->queue_lock);
 		msleep(10);
 		spin_lock_irq(q->queue_lock);
+		elv_drain_elevator(q);
 	}
 
 	spin_unlock_irq(q->queue_lock);

include/linux/blkdev.h

@@ -406,6 +406,7 @@ struct request_queue
 
 	atomic_t		refcnt;
 
+	unsigned int		nr_sorted;
 	unsigned int		in_flight;
 
 	/*
@@ -631,6 +632,7 @@ static inline void elv_dispatch_add_tail(struct request_queue *q,
 {
 	if (q->last_merge == rq)
 		q->last_merge = NULL;
+	q->nr_sorted--;
 
 	q->end_sector = rq_end_sector(rq);
 	q->boundary_rq = rq;