drbd: add explicit plugging when submitting batches

When submitting batches of requests which had been queued on the
submitter thread, typically because they needed to wait for an
activity log transactions, use explicit plugging to help potential
merging of requests in the backend io-scheduler.
Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_req.c

@@ -1291,6 +1291,7 @@ static void drbd_unplug(struct blk_plug_cb *cb, bool from_schedule)
 	struct drbd_resource *resource = plug->cb.data;
 	struct drbd_request *req = plug->most_recent_req;
 
+	kfree(cb);
 	if (!req)
 		return;
 
@@ -1300,8 +1301,8 @@ static void drbd_unplug(struct blk_plug_cb *cb, bool from_schedule)
 	req->rq_state |= RQ_UNPLUG;
 	/* but also queue a generic unplug */
 	drbd_queue_unplug(req->device);
-	spin_unlock_irq(&resource->req_lock);
 	kref_put(&req->kref, drbd_req_destroy);
+	spin_unlock_irq(&resource->req_lock);
 }
 
 static struct drbd_plug_cb* drbd_check_plugged(struct drbd_resource *resource)
@@ -1337,8 +1338,6 @@ static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request
 	bool no_remote = false;
 	bool submit_private_bio = false;
 
-	struct drbd_plug_cb *plug = drbd_check_plugged(resource);
-
 	spin_lock_irq(&resource->req_lock);
 	if (rw == WRITE) {
 		/* This may temporarily give up the req_lock,
@@ -1403,8 +1402,11 @@ static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request
 			no_remote = true;
 	}
 
-	if (plug != NULL && no_remote == false)
-		drbd_update_plug(plug, req);
+	if (no_remote == false) {
+		struct drbd_plug_cb *plug = drbd_check_plugged(resource);
+		if (plug)
+			drbd_update_plug(plug, req);
+	}
 
 	/* If it took the fast path in drbd_request_prepare, add it here.
 	 * The slow path has added it already. */
@@ -1454,7 +1456,10 @@ void __drbd_make_request(struct drbd_device *device, struct bio *bio, unsigned l
 
 static void submit_fast_path(struct drbd_device *device, struct list_head *incoming)
 {
+	struct blk_plug plug;
 	struct drbd_request *req, *tmp;
+
+	blk_start_plug(&plug);
 	list_for_each_entry_safe(req, tmp, incoming, tl_requests) {
 		const int rw = bio_data_dir(req->master_bio);
 
@@ -1472,6 +1477,7 @@ static void submit_fast_path(struct drbd_device *device, struct list_head *incom
 		list_del_init(&req->tl_requests);
 		drbd_send_and_submit(device, req);
 	}
+	blk_finish_plug(&plug);
 }
 
 static bool prepare_al_transaction_nonblock(struct drbd_device *device,
@@ -1501,10 +1507,12 @@ static bool prepare_al_transaction_nonblock(struct drbd_device *device,
 	return !list_empty(pending);
 }
 
-void send_and_submit_pending(struct drbd_device *device, struct list_head *pending)
+static void send_and_submit_pending(struct drbd_device *device, struct list_head *pending)
 {
+	struct blk_plug plug;
 	struct drbd_request *req;
 
+	blk_start_plug(&plug);
 	while ((req = list_first_entry_or_null(pending, struct drbd_request, tl_requests))) {
 		req->rq_state |= RQ_IN_ACT_LOG;
 		req->in_actlog_jif = jiffies;
@@ -1512,6 +1520,7 @@ void send_and_submit_pending(struct drbd_device *device, struct list_head *pendi
 		list_del_init(&req->tl_requests);
 		drbd_send_and_submit(device, req);
 	}
+	blk_finish_plug(&plug);
 }
 
 void do_submit(struct work_struct *ws)