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Tsirkin" To: Yang Dongshan 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: <20240701144507-mutt-send-email-mst@kernel.org> References: <20240701140017.49223-1-east.moutain.yang@gmail.com> <20240701105110-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; 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=unavailable 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 Tue, Jul 02, 2024 at 01:18:15AM +0800, Yang Dongshan wrote: > > Please document what this does. > okay, i will. > > > So this will return false if ring has any available 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); > > } > > No, only when the shadow_avail_idx is changed shall the function return true, what does "changed" mean here? changed compared to what? > compared with the value seen by the host last time, else return false even if > there are some buffers available in the queue, as the total size of the > available > buffers in the queue can't satisfy the request. > > It maybe better to pass only one arg to the function like this: > bool virtio_queue_set_notification_and_check(VirtQueue *vq) > { >        virtio_queue_packed_set_notification(vq, true); >         >        return shadow_avail_idx_changed()? true: false; > } > > Thanks Michael a lot! > > On Mon, Jul 1, 2024 at 11:05 PM Michael S. Tsirkin wrote: > > 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 > >