If two processes do I/O close to each other, then BFQ merges the
bfq_queues associated with these processes, to get a more sequential
I/O, and thus a higher throughput.  In this respect, to detect whether
two processes are doing I/O close to each other, BFQ keeps a list of
the head-of-line I/O requests of all active bfq_queues.  The list is
ordered by initial sectors, and implemented through a red-black tree
(rq_pos_tree).

Unfortunately, the update of the rq_pos_tree was incomplete, because
the tree was not updated on the removal of the head-of-line I/O
request of a bfq_queue, in case the queue did not remain empty. This
commit adds the missing update.

Signed-off-by: Paolo Valente <redacted>
Signed-off-by: Angelo Ruocco <redacted>
---
 block/bfq-iosched.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a9e06217e64d..c6f0b93a769c 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c

@@ -1627,6 +1627,8 @@ static void bfq_remove_request(struct request_queue *q,
 			rb_erase(&bfqq->pos_node, bfqq->pos_root);
 			bfqq->pos_root = NULL;
 		}
+	} else {
+		bfq_pos_tree_add_move(bfqd, bfqq);
 	}
 
 	if (rq->cmd_flags & REQ_META)

-- 
2.15.1