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Date:   Tue, 15 Dec 2020 08:03:45 +0000
From:   wangyunjian <wangyunjian@...wei.com>
To:     Jason Wang <jasowang@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>
CC:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Lilijun (Jerry)" <jerry.lilijun@...wei.com>,
        chenchanghu <chenchanghu@...wei.com>,
        xudingke <xudingke@...wei.com>,
        "huangbin (J)" <brian.huangbin@...wei.com>
Subject: RE: [PATCH net 2/2] vhost_net: fix high cpu load when sendmsg fails



> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Tuesday, December 15, 2020 12:10 PM
> To: wangyunjian <wangyunjian@...wei.com>; netdev@...r.kernel.org;
> mst@...hat.com; willemdebruijn.kernel@...il.com
> Cc: virtualization@...ts.linux-foundation.org; Lilijun (Jerry)
> <jerry.lilijun@...wei.com>; chenchanghu <chenchanghu@...wei.com>;
> xudingke <xudingke@...wei.com>; huangbin (J)
> <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.

Sorry for missing the comment.

After deleting discard descriptor and break, the next processing will be the same
as the normal success of sendmsg(), and vhost_zerocopy_signal_used() or
vhost_add_used_and_signal() method will be called to add the head to used ring.

Thanks
> 
> 
> > -		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.

OK

> 
> 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

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