Subject: Re: [PATCH net-next-2.6] vhost: Restart tx poll when socket send queue is full

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 23 Feb 2010 12:24:37 +0200

On Thu, Feb 18, 2010 at 12:59:11PM -0800, Sridhar Samudrala wrote:
> When running guest to remote host TCP stream test using vhost-net
> via tap/macvtap, i am seeing network transmit hangs. This happens
> when handle_tx() returns because of the socket send queue full 
> condition.
> This patch fixes this by restarting tx poll when hitting this
> condition.
> 
> Signed-off-by: Sridhar Samudrala <[email-masked]>
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 91a324c..82d4bbe 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -113,12 +113,16 @@ static void handle_tx(struct vhost_net *net)
>  	if (!sock)
>  		return;
>  
> -	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> -	if (wmem >= sock->sk->sk_sndbuf)
> -		return;
> -
>  	use_mm(net->dev.mm);
>  	mutex_lock(&vq->mutex);
> +
> +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> +	if (wmem >= sock->sk->sk_sndbuf) {
> +		tx_poll_start(net, sock);
> +		set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
> +		goto unlock;
> +	}
> +
>  	vhost_disable_notify(vq);
>  
>  	if (wmem < sock->sk->sk_sndbuf * 2)
> @@ -178,6 +182,7 @@ static void handle_tx(struct vhost_net *net)
>  		}
>  	}
>  
> +unlock:
>  	mutex_unlock(&vq->mutex);
>  	unuse_mm(net->dev.mm);
>  }

It might be better to avoid use_mm when ring is full.
Does the following fix the tx hang for you?

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 4c89283..f5f6efe 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -113,8 +113,12 @@ static void handle_tx(struct vhost_net *net)
 		return;
 
 	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
-	if (wmem >= sock->sk->sk_sndbuf)
-		return;
+	if (wmem >= sock->sk->sk_sndbuf) {
+		mutex_lock(&vq->mutex);
+		tx_poll_start(net, sock);
+		mutex_unlock(&vq->mutex);
+                return;
+	}
 
 	use_mm(net->dev.mm);
 	mutex_lock(&vq->mutex);