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Tsirkin" To: Wencheng Yang Cc: qemu-devel@nongnu.org, jasowang@redhat.com, stefanha@redhat.com, kwolf@redhat.com, hreitz@redhat.com, qemu-stable@nongnu.org Subject: Re: [PATCH v7] virtio-net: Fix network stall at the host side waiting for kick Message-ID: <20240701105110-mutt-send-email-mst@kernel.org> References: <20240701140017.49223-1-east.moutain.yang@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240701140017.49223-1-east.moutain.yang@gmail.com> Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Jul 01, 2024 at 10:00:17PM +0800, Wencheng Yang wrote: > From: thomas > > 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, viritio-net would > stall at the host side forever. > > The patch checks whether the guest has added some buffers > after last check of avail idx when the available buffers are > not sufficient, if so then recheck the available buffers > in the loop. > > The patch also reverts patch "06b12970174". > > 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 ... | > wait 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 buffers. > 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 writes the avail idx read 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. > > Fixes: 06b12970174 ("virtio-net: fix network stall under load") > Signed-off-by: Wencheng Yang > --- > > Changelog: > v7: > - Add function virtio_queue_set_notification_and_check() > - Restore the function sequence introduce in v6 > > v6: > - Take packed packed queue into cosideration > - Adjust function sequence to fix compilation issue > > v5: > - Modify return type of virtio_queue_set_notification() to > bool to indicate whether the guest has added some buffers > after last check of avail idx > - Loop in virtio_net_has_buffers() if the available buffers > are not sufficient and the guest has added some buffers. > - Revert patch "06b12970174" > - Update the subject > > 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 > > hw/net/virtio-net.c | 19 ++++++++----------- > hw/virtio/virtio.c | 39 ++++++++++++++++++++++++++++++++++++++ > include/hw/virtio/virtio.h | 1 + > 3 files changed, 48 insertions(+), 11 deletions(-) > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c > index 9c7e85caea..357c651d9b 100644 > --- a/hw/net/virtio-net.c > +++ b/hw/net/virtio-net.c > @@ -1642,18 +1642,15 @@ static bool virtio_net_can_receive(NetClientState *nc) > static int virtio_net_has_buffers(VirtIONetQueue *q, int bufsize) > { > VirtIONet *n = q->n; > - 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); > - > - /* To avoid a race condition where the guest has made some buffers > - * available after the above check but before notification was > - * enabled, check for available buffers again. > - */ > - if (virtio_queue_empty(q->rx_vq) || > + > + while (virtio_queue_empty(q->rx_vq) || > (n->mergeable_rx_bufs && > - !virtqueue_avail_bytes(q->rx_vq, bufsize, 0))) { > + !virtqueue_avail_bytes(q->rx_vq, bufsize, 0))) { > + /* guest may have made some buf, try again */ > + if (virtio_queue_set_notification_and_check(q->rx_vq, 1)) { > + virtio_queue_set_notification(q->rx_vq, 0); > + continue; > + } else { > return 0; > } > } > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c > index 893a072c9d..b165764017 100644 > --- a/hw/virtio/virtio.c > +++ b/hw/virtio/virtio.c > @@ -668,6 +668,45 @@ static inline bool is_desc_avail(uint16_t flags, bool wrap_counter) > return (avail != used) && (avail == wrap_counter); > } > > +bool virtio_queue_set_notification_and_check(VirtQueue *vq, int enable) > +{ > + uint16_t shadow_idx; > + VRingPackedDesc desc; > + VRingMemoryRegionCaches *caches; > + > + vq->notification = enable; > + > + if (!vq->vring.desc) { > + return false; > + } > + > + if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) { > + virtio_queue_packed_set_notification(vq, enable); > + > + if (enable) { > + caches = vring_get_region_caches(vq); > + if (!caches) { > + return false; > + } > + > + vring_packed_desc_read(vq->vdev, &desc, &caches->desc, > + vq->shadow_avail_idx, true); > + if (is_desc_avail(desc.flags, vq->shadow_avail_wrap_counter)) { > + return true; > + } > + } > + } else { > + shadow_idx = vq->shadow_avail_idx; > + virtio_queue_split_set_notification(vq, enable); > + > + if (enable) { > + return shadow_idx != vring_avail_idx(vq); > + } > + } > + > + return false; > +} Please document what this does. So this will return false if ring has any availabe buffers? Equivalent to: bool virtio_queue_set_notification_and_check(VirtQueue *vq, int enable) { virtio_queue_packed_set_notification(vq, enable); return virtio_queue_empty(vq); } or am I missing something? > + > /* Fetch avail_idx from VQ memory only when we really need to know if > * guest has added some buffers. > * Called within rcu_read_lock(). */ > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h > index 7d5ffdc145..ed85023b87 100644 > --- a/include/hw/virtio/virtio.h > +++ b/include/hw/virtio/virtio.h > @@ -302,6 +302,7 @@ void virtio_notify_config(VirtIODevice *vdev); > > bool virtio_queue_get_notification(VirtQueue *vq); > void virtio_queue_set_notification(VirtQueue *vq, int enable); > +bool virtio_queue_set_notification_and_check(VirtQueue *vq, int enable); > > int virtio_queue_ready(VirtQueue *vq); > > -- > 2.39.0