From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net] net: core: orphan frags before queuing to slow qdisc Date: Sun, 19 Jan 2014 11:21:40 +0200 Message-ID: <20140119092140.GA2984@redhat.com> References: <1389951734-13234-1-git-send-email-jasowang@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: davem@davemloft.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <1389951734-13234-1-git-send-email-jasowang@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Fri, Jan 17, 2014 at 05:42:14PM +0800, Jason Wang wrote: > Many qdiscs can queue a packet for a long time, this will lead an issue > with zerocopy skb. It means the frags will not be orphaned in an expected > short time, this breaks the assumption that virtio-net will transmit the > packet in time. > > So if guest packets were queued through such kind of qdisc and hit the > limitation of the max pending packets for virtio/vhost. All packets that > go to another destination from guest will also be blocked. > > A case for reproducing the issue: > > - Boot two VMs and connect them to the same bridge kvmbr. > - Setup tbf with a very low rate/burst on eth0 which is a port of kvmbr. > - Let VM1 send lots of packets thorugh eth0 > - After a while, VM1 is unable to send any packets out since the number of > pending packets (queued to tbf) were exceeds the limitation of vhost/virito > > Solve this issue by orphaning the frags before queuing it to a slow qdisc (the > one without TCQ_F_CAN_BYPASS). This seems too aggressive to me. The issue is that packet can stay queued indefinitely long. So I think we should only do this for tbf. > > Cc: Michael S. Tsirkin > Signed-off-by: Jason Wang > --- > net/core/dev.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/net/core/dev.c b/net/core/dev.c > index 0ce469e..1209774 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -2700,6 +2700,12 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, > contended = qdisc_is_running(q); > if (unlikely(contended)) > spin_lock(&q->busylock); > + if (!(q->flags & TCQ_F_CAN_BYPASS) && > + unlikely(skb_orphan_frags(skb, GFP_ATOMIC))) { > + kfree_skb(skb); > + rc = NET_XMIT_DROP; > + goto out; > + } > > spin_lock(root_lock); > if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) { > @@ -2739,6 +2745,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, > } > } > spin_unlock(root_lock); > +out: > if (unlikely(contended)) > spin_unlock(&q->busylock); > return rc; > -- > 1.8.3.2