Subject: Re: [PATCH net v2 2/2] vhost_net: fix high cpu load when sendmsg fails

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 16 Dec 2020 04:23:16 -0500

On Wed, Dec 16, 2020 at 04:20:37PM +0800, wangyunjian wrote:
> From: Yunjian Wang <[email-masked]>
> 
> Currently we break the loop and wake up the vhost_worker when
> sendmsg fails. When the worker wakes up again, we'll meet the
> same error. This will cause high CPU load. To fix this issue,
> we can skip this description by ignoring the error. When we
> exceeds sndbuf, the return value of sendmsg is -EAGAIN. In
> the case we don't skip the description and don't drop packet.

Question: with this patch, what happens if sendmsg is interrupted by a signal?

> 
> Signed-off-by: Yunjian Wang <[email-masked]>
> ---
>  drivers/vhost/net.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index c8784dfafdd7..3d33f3183abe 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -827,16 +827,13 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  				msg.msg_flags &= ~MSG_MORE;
>  		}
>  
> -		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
> -		if (unlikely(err < 0)) {
> +		if (unlikely(err == -EAGAIN)) {
>  			vhost_discard_vq_desc(vq, 1);
>  			vhost_net_enable_vq(net, vq);
>  			break;
> -		}
> -		if (err != len)
> -			pr_debug("Truncated TX packet: len %d != %zd\n",
> -				 err, len);
> +		} else if (unlikely(err != len))
> +			vq_err(vq, "Fail to sending packets err : %d, len : %zd\n", err, len);
>  done:
>  		vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
>  		vq->heads[nvq->done_idx].len = 0;
> @@ -922,7 +919,6 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>  			msg.msg_flags &= ~MSG_MORE;
>  		}
>  
> -		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
>  		if (unlikely(err < 0)) {
>  			if (zcopy_used) {
> @@ -931,13 +927,14 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>  				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
>  					% UIO_MAXIOV;
>  			}
> -			vhost_discard_vq_desc(vq, 1);
> -			vhost_net_enable_vq(net, vq);
> -			break;
> +			if (err == -EAGAIN) {
> +				vhost_discard_vq_desc(vq, 1);
> +				vhost_net_enable_vq(net, vq);
> +				break;
> +			}
>  		}
>  		if (err != len)
> -			pr_debug("Truncated TX packet: "
> -				 " len %d != %zd\n", err, len);
> +			vq_err(vq, "Fail to sending packets err : %d, len : %zd\n", err, len);

I'd rather make the pr_debug -> vq_err a separate change, with proper
commit log describing motivation.

>  		if (!zcopy_used)
>  			vhost_add_used_and_signal(&net->dev, vq, head, 0);
>  		else
> -- 
> 2.23.0