|
| From: | Yang Dongshan |
| Subject: | Re: [PATCH v7] virtio-net: Fix network stall at the host side waiting for kick |
| Date: | Tue, 2 Jul 2024 07:45:31 +0800 |
vq->shadow_avail_idx != vring_avail_idx(vq);
For packed queue, the logic is similar, if vq->shadow_avail_idxbecomes available, it means the guest has added buf at the slot.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;
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 <mst@redhat.com> wrote:
>
> On Mon, Jul 01, 2024 at 10:00:17PM +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, 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 <east.moutain.yang@gmail.com>
> > ---
> >
> > 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
>
>
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