linux-kernel - [PATCH net-next] vsock/virtio: a better comment on credit update

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Message-ID: <20190903073748.25214-1-mst@redhat.com>
Date:   Tue, 3 Sep 2019 03:38:16 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Stefano Garzarella <sgarzare@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: [PATCH net-next] vsock/virtio: a better comment on credit update

The comment we have is just repeating what the code does.
Include the *reason* for the condition instead.

Cc: Stefano Garzarella <sgarzare@...hat.com>
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 net/vmw_vsock/virtio_transport_common.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 94cc0fa3e848..5bb70c692b1e 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -307,8 +307,13 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
 
 	spin_unlock_bh(&vvs->rx_lock);
 
-	/* We send a credit update only when the space available seen
-	 * by the transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE
+	/* To reduce the number of credit update messages,
+	 * don't update credits as long as lots of space is available.
+	 * Note: the limit chosen here is arbitrary. Setting the limit
+	 * too high causes extra messages. Too low causes transmitter
+	 * stalls. As stalls are in theory more expensive than extra
+	 * messages, we set the limit to a high value. TODO: experiment
+	 * with different values.
 	 */
 	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) {
 		virtio_transport_send_credit_update(vsk,
-- 
MST