[PATCH] cfq-iosched: don't allow sync merges across queues

Currently we allow any merge, even if the io originates from different
processes. This can cause really bad starvation and unfairness, if those
ios happen to be synchronous (reads or direct writes).

So add a allow_merge hook to the io scheduler ops, so an io scheduler can
help decide whether a bio/process combination may be merged with an
existing request.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -568,6 +568,38 @@ cfq_merged_requests(request_queue_t *q, struct request *rq,
 	cfq_remove_request(next);
 }
 
+static int cfq_allow_merge(request_queue_t *q, struct request *rq,
+			   struct bio *bio)
+{
+	struct cfq_data *cfqd = q->elevator->elevator_data;
+	const int rw = bio_data_dir(bio);
+	struct cfq_queue *cfqq;
+	pid_t key;
+
+	/*
+	 * If bio is async or a write, always allow merge
+	 */
+	if (!bio_sync(bio) || rw == WRITE)
+		return 1;
+
+	/*
+	 * bio is sync. if request is not, disallow.
+	 */
+	if (!rq_is_sync(rq))
+		return 0;
+
+	/*
+	 * Ok, both bio and request are sync. Allow merge if they are
+	 * from the same queue.
+	 */
+	key = cfq_queue_pid(current, rw, 1);
+	cfqq = cfq_find_cfq_hash(cfqd, key, current->ioprio);
+	if (cfqq != RQ_CFQQ(rq))
+		return 0;
+
+	return 1;
+}
+
 static inline void
 __cfq_set_active_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 {
@@ -2125,6 +2157,7 @@ static struct elevator_type iosched_cfq = {
 		.elevator_merge_fn = 		cfq_merge,
 		.elevator_merged_fn =		cfq_merged_request,
 		.elevator_merge_req_fn =	cfq_merged_requests,
+		.elevator_allow_merge_fn =	cfq_allow_merge,
 		.elevator_dispatch_fn =		cfq_dispatch_requests,
 		.elevator_add_req_fn =		cfq_insert_request,
 		.elevator_activate_req_fn =	cfq_activate_request,

block/elevator.c

@@ -50,6 +50,21 @@ static const int elv_hash_shift = 6;
 #define rq_hash_key(rq)		((rq)->sector + (rq)->nr_sectors)
 #define ELV_ON_HASH(rq)		(!hlist_unhashed(&(rq)->hash))
 
+/*
+ * Query io scheduler to see if the current process issuing bio may be
+ * merged with rq.
+ */
+static int elv_iosched_allow_merge(struct request *rq, struct bio *bio)
+{
+	request_queue_t *q = rq->q;
+	elevator_t *e = q->elevator;
+
+	if (e->ops->elevator_allow_merge_fn)
+		return e->ops->elevator_allow_merge_fn(q, rq, bio);
+
+	return 1;
+}
+
 /*
  * can we safely merge with this request?
  */
@@ -65,12 +80,15 @@ inline int elv_rq_merge_ok(struct request *rq, struct bio *bio)
 		return 0;
 
 	/*
-	 * same device and no special stuff set, merge is ok
+	 * must be same device and not a special request
 	 */
-	if (rq->rq_disk == bio->bi_bdev->bd_disk && !rq->special)
-		return 1;
+	if (rq->rq_disk != bio->bi_bdev->bd_disk || !rq->special)
+		return 0;
 
+	if (!elv_iosched_allow_merge(rq, bio))
 		return 0;
+
+	return 1;
 }
 EXPORT_SYMBOL(elv_rq_merge_ok);
 

include/linux/elevator.h

@@ -12,6 +12,8 @@ typedef void (elevator_merge_req_fn) (request_queue_t *, struct request *, struc
 
 typedef void (elevator_merged_fn) (request_queue_t *, struct request *, int);
 
+typedef int (elevator_allow_merge_fn) (request_queue_t *, struct request *, struct bio *);
+
 typedef int (elevator_dispatch_fn) (request_queue_t *, int);
 
 typedef void (elevator_add_req_fn) (request_queue_t *, struct request *);
@@ -33,6 +35,7 @@ struct elevator_ops
 	elevator_merge_fn *elevator_merge_fn;
 	elevator_merged_fn *elevator_merged_fn;
 	elevator_merge_req_fn *elevator_merge_req_fn;
+	elevator_allow_merge_fn *elevator_allow_merge_fn;
 
 	elevator_dispatch_fn *elevator_dispatch_fn;
 	elevator_add_req_fn *elevator_add_req_fn;