Virtualization - Sep 2019 - [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 at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.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

On Tue, Sep 03, 2019 at 03:38:16AM -0400, Michael S. Tsirkin
wrote:
> The comment we have is just repeating what the code does.
> Include the *reason* for the condition instead.
> 
> Cc: Stefano Garzarella <sgarzare at redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.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,
Reviewed-by: Stefano Garzarella <sgarzare at redhat.com>

From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Tue, 3 Sep 2019 03:38:16 -0400
> The comment we have is just repeating what the code does.
> Include the *reason* for the condition instead.
> 
> Cc: Stefano Garzarella <sgarzare at redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Applied.