| Message ID | 20240625094851.5252-1-east.moutain.yang@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v4] virtio-net: Notify the guest with the latest available idx | expand |
On Tue, Jun 25, 2024 at 05:48:51PM +0800, Wencheng Yang wrote: > From: thomas <east.moutain.yang@gmail.com> > > Patch 06b12970174 ("virtio-net: fix network stall under load") > added double-check to test whether the available buffer size > can satisfy the request or not, in case the guest has added > some buffers to the avail ring simultaneously after the first > check. > > It will be lucky if the available buffer size becomes okay > after the double-check, then the host can send the packet to > the guest. If the buffer size still can't satisfy the request, > even if the guest has added some buffers, notify the guest > with the latest available idx seen by the host, similiar to > the action taken by the host after the first check, else > viritio-net would stall at the host side forever. > > The case below can reproduce the stall. > > Guest 0 > +--------+ > | iperf | > ---------------> | server | > Host | +--------+ > +--------+ | ... > | iperf |---- > | client |---- Guest n > +--------+ | +--------+ > | | iperf | > ---------------> | server | > +--------+ > > Boot many guests from qemu with virtio network: > qemu ... -netdev tap,id=net_x \ > -device virtio-net-pci-non-transitional,\ > iommu_platform=on,mac=xx:xx:xx:xx:xx:xx,netdev=net_x > > Each guest acts as iperf server with commands below: > iperf3 -s -D -i 10 -p 8001 > iperf3 -s -D -i 10 -p 8002 > > The host as iperf client: > iperf3 -c guest_IP -p 8001 -i 30 -w 256k -P 20 -t 40000 > iperf3 -c guest_IP -p 8002 -i 30 -w 256k -P 20 -t 40000 > > After some time, the host loses connection to the guest, > the guest can send packet to the host, but can't receive > packet from host. > > It's more likely to happen if SWIOTLB is enabled in the guest, > allocating and freeing bounce buffer takes some CPU ticks, > copying from/to bounce buffer takes more CPU ticks, compared > with that there is no bounce buffer in the guest. > Once the rate of producing packets from the host approximates > the rate of receiveing packets in the guest, the guest would > loop in NAPI. > > receive packets --- > | | > v | > free buf virtnet_poll > | | > v | > add buf to avail ring --- > | > | need kick the host? > | NAPI continues > v > receive packets --- > | | > v | > free buf virtnet_poll > | | > v | > add buf to avail ring --- > | > v > ... ... > > On the other hand, the host fetches free buf from avail > ring, if the buf in the avail ring is not enough, the > host notifies the guest the event by writing the avail > idx read from avail ring to the event idx of used ring, > then the host goes to sleep, waiting for the kick signal > from the guest. > > Once the guest finds the host is waiting for kick singal > (in virtqueue_kick_prepare_split()), it kicks the host. > > The host may stall forever at the sequences below: > > Host Guest > ------------ ----------- > fetch buf, send packet receive packet --- > ... ... | > fetch buf, send packet add buf | > ... add buf virtnet_poll > buf not enough avail idx-> add buf | > read avail idx add buf | > add buf --- > receive packet --- > write event idx ... | > waiting for kick add buf virtnet_poll > ... | > --- > no more packet, exit NAPI > > In the first loop of NAPI above, indicated in the range of > virtnet_poll above, the host is sending packets while the > guest is receiving packets and adding buf. > step 1: The buf is not enough, for example, a big packet > needs 5 buf, but the available buf count is 3. > The host read current avail idx. > step 2: The guest adds some buf, then checks whether the > host is waiting for kick signal, not at this time. > The used ring is not empty, the guest continues > the second loop of NAPI. > step 3: The host write the avail idx readed from avail > ring to used ring as event idx via > virtio_queue_set_notification(q->rx_vq, 1). > step 4: At the end of the second loop of NAPI, recheck > whether kick is needed, as the event idx in the > used ring written by the host is beyound the > range of kick condition, the guest will not > send kick signal to the host. > > The patch notifies the guest with the latest avail idx seen > by the host in the double-check, which increases the > probability of hitting kick condition, but logically > speaking, it can't resolve the issue. It's kind of > optimization agianst patch 06b12970174. So let's try to resolve the issue instead. > Changelog: > v4: > - Correct spelling mistake in the subject > - Describe the issue that virtio-net is blocked at host side > > v3: > - Add virtio-net tag in the subject > - Refine commit log > > v2: > - Add SOB tag at the end of the commit message > - Place Fixes tag at the end of the commit message > > v1: > - Initial patch > > Fixes: 06b12970174 ("virtio-net: fix network stall under load") > Signed-off-by: Wencheng Yang <east.moutain.yang@gmail.com> > --- > hw/net/virtio-net.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > index 9c7e85caea..23c6c8c898 100644 > --- a/hw/net/virtio-net.c > +++ b/hw/net/virtio-net.c > @@ -1654,6 +1654,7 @@ static int virtio_net_has_buffers(VirtIONetQueue *q, int bufsize) > if (virtio_queue_empty(q->rx_vq) || > (n->mergeable_rx_bufs && > !virtqueue_avail_bytes(q->rx_vq, bufsize, 0))) { > + virtio_queue_set_notification(q->rx_vq, 1); > return 0; > } Fundamentally, this is why e.g. vhost_enable_notify is a bool and callers do things like: ... if (unlikely(vhost_enable_notify(&net->dev, vq))) { vhost_disable_notify(&net->dev, vq); continue; } break; > } > -- > 2.39.0
diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c index 9c7e85caea..23c6c8c898 100644 --- a/hw/net/virtio-net.c +++ b/hw/net/virtio-net.c @@ -1654,6 +1654,7 @@ static int virtio_net_has_buffers(VirtIONetQueue *q, int bufsize) if (virtio_queue_empty(q->rx_vq) || (n->mergeable_rx_bufs && !virtqueue_avail_bytes(q->rx_vq, bufsize, 0))) { + virtio_queue_set_notification(q->rx_vq, 1); return 0; } }