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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 9 Dec 2020 07:49:52 -0500

On Wed, Dec 09, 2020 at 07:48:24PM +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.
> 
> Signed-off-by: Yunjian Wang <[email-masked]>
> ---
>  drivers/vhost/net.c | 24 +++++-------------------
>  1 file changed, 5 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 531a00d703cd..ac950b1120f5 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -829,14 +829,8 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  
>  		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
> -		if (unlikely(err < 0)) {
> -			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);
> +		if (unlikely(err < 0 || 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;

One of the reasons for sendmsg to fail is ENOBUFS.
In that case for sure we don't want to drop packet.
There could be other transient errors.
Which error did you encounter, specifically?

> @@ -925,19 +919,11 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
>  
>  		/* TODO: Check specific error and bomb out unless ENOBUFS? */
>  		err = sock->ops->sendmsg(sock, &msg, len);
> -		if (unlikely(err < 0)) {
> -			if (zcopy_used) {
> +		if (unlikely(err < 0 || err != len)) {
> +			if (zcopy_used && err < 0)
>  				vhost_net_ubuf_put(ubufs);
> -				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
> -					% UIO_MAXIOV;
> -			}
> -			vhost_discard_vq_desc(vq, 1);
> -			vhost_net_enable_vq(net, vq);
> -			break;
> +			vq_err(vq, "Fail to sending packets err : %d, len : %zd\n", err, len);
>  		}
> -		if (err != len)
> -			pr_debug("Truncated TX packet: "
> -				 " len %d != %zd\n", err, len);
>  		if (!zcopy_used)
>  			vhost_add_used_and_signal(&net->dev, vq, head, 0);
>  		else
> -- 
> 2.23.0