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Date:   Wed, 16 Dec 2020 00:56:30 -0500 (EST)
From:   Jason Wang <jasowang@...hat.com>
To:     wangyunjian <wangyunjian@...wei.com>
Cc:     netdev@...r.kernel.org, mst@...hat.com,
        willemdebruijn kernel <willemdebruijn.kernel@...il.com>,
        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 -----
> 
> 
> > -----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.

It's the next head not the one that contains the buggy packet?

Thanks

> 
> 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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