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Date:   Wed, 9 Dec 2020 13:27:57 +0000
From:   wangyunjian <wangyunjian@...wei.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     "jasowang@...hat.com" <jasowang@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Lilijun (Jerry)" <jerry.lilijun@...wei.com>,
        chenchanghu <chenchanghu@...wei.com>,
        xudingke <xudingke@...wei.com>
Subject: RE: [PATCH net] vhost_net: fix high cpu load when sendmsg fails

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:mst@...hat.com]
> Sent: Wednesday, December 9, 2020 8:50 PM
> To: wangyunjian <wangyunjian@...wei.com>
> Cc: jasowang@...hat.com; virtualization@...ts.linux-foundation.org;
> netdev@...r.kernel.org; Lilijun (Jerry) <jerry.lilijun@...wei.com>;
> chenchanghu <chenchanghu@...wei.com>; xudingke <xudingke@...wei.com>
> Subject: Re: [PATCH net] vhost_net: fix high cpu load when sendmsg fails
> 
> On Wed, Dec 09, 2020 at 07:48:24PM +0800, 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.
> >
> > Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
> > ---
> >  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.

Now the function tap_sendmsg()/tun_sendmsg() don't return ENOBUFS.

> There could be other transient errors.
> Which error did you encounter, specifically?

Currently a guest vm send a skb which length is more than 64k.
If virtio hdr is wrong, the problem will also be triggered.

Thanks

> 
> > @@ -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

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