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@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 94cc0fa..5bb70c6 100644 (file)
--- 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,