drbd: introduce drbd_recv_header_maybe_unplug

Recently, drbd_recv_header() was changed to potentially
implicitly "unplug" the backend device(s), in case there
is currently nothing to receive.

Be more explicit about it: re-introduce the original drbd_recv_header(),
and introduce a new drbd_recv_header_maybe_unplug() for use by the
receiver "main loop".

Using explicit plugging via blk_start_plug(); blk_finish_plug();
really helps the io-scheduler of the backend with merging requests.

Wrap the receiver "main loop" with such a plug.
Also catch unplug events on the Primary,
and try to propagate.

This is performance relevant.  Without this, if the receiving side does
not merge requests, number of IOPS on the peer can me significantly
higher than IOPS on the Primary, and can easily become the bottleneck.

Together, both changes should help to reduce the number of IOPS
as seen on the backend of the receiving side, by increasing
the chance of merging mergable requests, without trading latency
for more throughput.

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>

1 files changed, 17 insertions, 5 deletions

diff --git a/drivers/block/drbd/drbd_worker.c b/drivers/block/drbd/drbd_worker.c
index c268d886c4f0..2745db2255ed 100644
--- a/drivers/block/drbd/drbd_worker.c
+++ b/drivers/block/drbd/drbd_worker.c
@@ -1382,18 +1382,22 @@ static int drbd_send_barrier(struct drbd_connection *connection)
 	return conn_send_command(connection, sock, P_BARRIER, sizeof(*p), NULL, 0);
 }
 
+static int pd_send_unplug_remote(struct drbd_peer_device *pd)
+{
+	struct drbd_socket *sock = &pd->connection->data;
+	if (!drbd_prepare_command(pd, sock))
+		return -EIO;
+	return drbd_send_command(pd, sock, P_UNPLUG_REMOTE, 0, NULL, 0);
+}
+
 int w_send_write_hint(struct drbd_work *w, int cancel)
 {
 	struct drbd_device *device =
 		container_of(w, struct drbd_device, unplug_work);
-	struct drbd_socket *sock;
 
 	if (cancel)
 		return 0;
-	sock = &first_peer_device(device)->connection->data;
-	if (!drbd_prepare_command(first_peer_device(device), sock))
-		return -EIO;
-	return drbd_send_command(first_peer_device(device), sock, P_UNPLUG_REMOTE, 0, NULL, 0);
+	return pd_send_unplug_remote(first_peer_device(device));
 }
 
 static void re_init_if_first_write(struct drbd_connection *connection, unsigned int epoch)
@@ -1455,6 +1459,7 @@ int w_send_dblock(struct drbd_work *w, int cancel)
 	struct drbd_device *device = req->device;
 	struct drbd_peer_device *const peer_device = first_peer_device(device);
 	struct drbd_connection *connection = peer_device->connection;
+	bool do_send_unplug = req->rq_state & RQ_UNPLUG;
 	int err;
 
 	if (unlikely(cancel)) {
@@ -1470,6 +1475,9 @@ int w_send_dblock(struct drbd_work *w, int cancel)
 	err = drbd_send_dblock(peer_device, req);
 	req_mod(req, err ? SEND_FAILED : HANDED_OVER_TO_NETWORK);
 
+	if (do_send_unplug && !err)
+		pd_send_unplug_remote(peer_device);
+
 	return err;
 }
 
@@ -1484,6 +1492,7 @@ int w_send_read_req(struct drbd_work *w, int cancel)
 	struct drbd_device *device = req->device;
 	struct drbd_peer_device *const peer_device = first_peer_device(device);
 	struct drbd_connection *connection = peer_device->connection;
+	bool do_send_unplug = req->rq_state & RQ_UNPLUG;
 	int err;
 
 	if (unlikely(cancel)) {
@@ -1501,6 +1510,9 @@ int w_send_read_req(struct drbd_work *w, int cancel)
 
 	req_mod(req, err ? SEND_FAILED : HANDED_OVER_TO_NETWORK);
 
+	if (do_send_unplug && !err)
+		pd_send_unplug_remote(peer_device);
+
 	return err;
 }