Re: [PATCH v7] virtio-net: Fix network stall at the host side waiting for kick

[Yang Dongshan]

Hi, Michael

> My suggestion:
> 
> 
> change virtqueue_get_avail_bytes to return the shadow
> in an opaque unsigned value.
> 
> 
> add virtqueue_poll that gets this opaque and tells us whether any new
> buffers became available in the queue since that value> 
> was returned.


> accordingly, virtio_queue_set_notification_and_check
> will accept this opaque value and check avail buffers
> against it.

According to your suggestion, it's able to handle the case where the
queue is not empty, when the queue is empty, should I add an API to
get the shadow idx as virtio_queue_set_notification_and_check()
needs the opaque arg.

What value should return from virtqueue_get_avail_bytes() in case of
error branch in the function? 

On 2024/7/2, 19:27, "Michael S. Tsirkin" <mst@redhat.com 
<mailto:mst@redhat.com>> wrote:


On Tue, Jul 02, 2024 at 07:45:31AM +0800, Yang Dongshan wrote:
> > what does "changed" mean here? changed compared to what?
> For a split queue, if the shadow_avail_idx synced from avail ring idx
> by vring_avail_idx(vq) last time doesn't equal the current value of avail ring
> idx.
> 
> vq->shadow_avail_idx != vring_avail_idx(vq);
> 
> For packed queue, the logic is similar, if vq->shadow_avail_idx
> 
> becomes 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;
> 
> 


This answer does not make sense from API POV.


My suggestion:


change virtqueue_get_avail_bytes to return the shadow
in an opaque unsigned value.


add virtqueue_poll that gets this opaque and tells us whether any new
buffers became available in the queue since that value
was returned.


accordingly, virtio_queue_set_notification_and_check
will accept this opaque value and check avail buffers
against it.






> On Tue, Jul 2, 2024 at 2:46 AM Michael S. Tsirkin <mst@redhat.com 
> <mailto:mst@redhat.com>> wrote:
> 
> 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 
> > <mailto:mst@redhat.com>>
> wrote:
> >
> > On Mon, Jul 01, 2024 at 10:00:17PM +0800, Wencheng Yang wrote:
> > > From: thomas <east.moutain.yang@gmail.com 
> > > <mailto: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 
> > > <mailto: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
> >
> >
> 
>