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

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Date:   Tue, 15 Dec 2020 12:09:49 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     wangyunjian <wangyunjian@...wei.com>, netdev@...r.kernel.org,
        mst@...hat.com, willemdebruijn.kernel@...il.com
Cc:     virtualization@...ts.linux-foundation.org,
        jerry.lilijun@...wei.com, chenchanghu@...wei.com,
        xudingke@...wei.com, brian.huangbin@...wei.com
Subject: Re: [PATCH net 2/2] vhost_net: fix high cpu load when sendmsg fails


On 2020/12/15 上午9:48, wangyunjian wrote:
> From: Yunjian Wang <wangyunjian@...wei.com>
>
> 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.
>
> Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
> ---
>   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..f966592d8900 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;
> -		}


As I've pointed out in last version. If you don't discard descriptor, 
you probably need to add the head to used ring. Otherwise this 
descriptor will be always inflight that may confuse drivers.


> -		if (err != len)
> -			pr_debug("Truncated TX packet: len %d != %zd\n",
> -				 err, len);
> +		} else if (unlikely(err < 0 || err != len))


It looks to me err != len covers err < 0.

Thanks


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