drbd: try hard to max out the updates per AL transaction

There may have been more incoming requests while we where preparing
the current transaction. Try to consolidate more updates into this
transaction until we make no more progres.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_req.c

@@ -1226,6 +1226,37 @@ void do_submit(struct work_struct *ws)
 			break;
 
 		wait_event(mdev->al_wait, prepare_al_transaction_nonblock(mdev, &incoming, &pending));
+		/* Maybe more was queued, while we prepared the transaction?
+		 * Try to stuff them into this transaction as well.
+		 * Be strictly non-blocking here, no wait_event, we already
+		 * have something to commit.
+		 * Stop if we don't make any more progres.
+		 */
+		for (;;) {
+			LIST_HEAD(more_pending);
+			LIST_HEAD(more_incoming);
+			bool made_progress;
+
+			/* It is ok to look outside the lock,
+			 * it's only an optimization anyways */
+			if (list_empty(&mdev->submit.writes))
+				break;
+
+			spin_lock(&mdev->submit.lock);
+			list_splice_tail_init(&mdev->submit.writes, &more_incoming);
+			spin_unlock(&mdev->submit.lock);
+
+			if (list_empty(&more_incoming))
+				break;
+
+			made_progress = prepare_al_transaction_nonblock(mdev, &more_incoming, &more_pending);
+
+			list_splice_tail_init(&more_pending, &pending);
+			list_splice_tail_init(&more_incoming, &incoming);
+
+			if (!made_progress)
+				break;
+		}
 		drbd_al_begin_io_commit(mdev, false);
 
 		list_for_each_entry_safe(req, tmp, &pending, tl_requests) {