netdev - RE: [PATCH net] vhost_net: fix high cpu load when sendmsg fails

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Date:   Fri, 11 Dec 2020 07:37:42 +0000
From:   wangyunjian <wangyunjian@...wei.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
CC:     "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>,
        "huangbin (J)" <brian.huangbin@...wei.com>
Subject: RE: [PATCH net] vhost_net: fix high cpu load when sendmsg fails

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Friday, December 11, 2020 10:53 AM
> To: wangyunjian <wangyunjian@...wei.com>; Michael S. Tsirkin
> <mst@...hat.com>
> Cc: 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 2020/12/9 下午9:27, wangyunjian wrote:
> >> -----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.
> 
> 
> I think not, it can happen if we exceeds sndbuf. E.g see tun_alloc_skb().

This patch 'net: add alloc_skb_with_frags() helper' modifys the return value
of sock_alloc_send_pskb() from -ENOBUFS to -EAGAIN when we exceeds sndbuf.
So the return value of tun_alloc_skb has been changed.

We don't drop packet if the reasons for sendmsg to fail is EAGAIN.
How about this?

Thanks

> 
> Thanks
> 
> 
> >
> >> 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