RE: [PATCH] vhost: vsock: don't send pkt when vq is not started

[Justin He <Justin.He@arm.com>]

Hi Stefano

> -----Original Message-----
> From: Stefano Garzarella <sgarzare@redhat.com>
> Sent: Thursday, April 30, 2020 4:26 PM
> To: Justin He <Justin.He@arm.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>; Michael S. Tsirkin
> <mst@redhat.com>; Jason Wang <jasowang@redhat.com>;
> kvm@vger.kernel.org; virtualization@lists.linux-foundation.org;
> netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Kaly Xin
> <Kaly.Xin@arm.com>
> Subject: Re: [PATCH] vhost: vsock: don't send pkt when vq is not started
>
> Hi Jia,
> thanks for the patch, some comments below:
>
> On Thu, Apr 30, 2020 at 10:13:14AM +0800, Jia He wrote:
> > Ning Bo reported an abnormal 2-second gap when booting Kata container
> [1].
> > The unconditional timeout is caused by
> VSOCK_DEFAULT_CONNECT_TIMEOUT of
> > connect at client side. The vhost vsock client tries to connect an
> > initlizing virtio vsock server.
> >
> > The abnormal flow looks like:
> > host-userspace           vhost vsock                       guest vsock
> > ==============           ===========                       ============
> > connect()     -------->  vhost_transport_send_pkt_work()   initializing
> >    |                     vq->private_data==NULL
> >    |                     will not be queued
> >    V
> > schedule_timeout(2s)
> >                          vhost_vsock_start()  <---------   device ready
> >                          set vq->private_data
> >
> > wait for 2s and failed
> >
> > connect() again          vq->private_data!=NULL          recv connecting pkt
> >
> > 1. host userspace sends a connect pkt, at that time, guest vsock is under
> > initializing, hence the vhost_vsock_start has not been called. So
> > vq->private_data==NULL, and the pkt is not been queued to send to guest.
> > 2. then it sleeps for 2s
> > 3. after guest vsock finishes initializing, vq->private_data is set.
> > 4. When host userspace wakes up after 2s, send connecting pkt again,
> > everything is fine.
> >
> > This fixes it by checking vq->private_data in vhost_transport_send_pkt,
> > and return at once if !vq->private_data. This makes user connect()
> > be returned with ECONNREFUSED.
> >
> > After this patch, kata-runtime (with vsock enabled) boottime reduces from
> > 3s to 1s on ThunderX2 arm64 server.
> >
> > [1] https://github.com/kata-containers/runtime/issues/1917
> >
> > Reported-by: Ning Bo <n.b@live.com>
> > Signed-off-by: Jia He <justin.he@arm.com>
> > ---
> >  drivers/vhost/vsock.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> > index e36aaf9ba7bd..67474334dd88 100644
> > --- a/drivers/vhost/vsock.c
> > +++ b/drivers/vhost/vsock.c
> > @@ -241,6 +241,7 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt
> *pkt)
> >  {
> >  struct vhost_vsock *vsock;
> >  int len = pkt->len;
> > +struct vhost_virtqueue *vq;
> >
> >  rcu_read_lock();
> >
> > @@ -252,6 +253,13 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt
> *pkt)
> >  return -ENODEV;
> >  }
> >
> > +vq = &vsock->vqs[VSOCK_VQ_RX];
> > +if (!vq->private_data) {
>
> I think is better to use vhost_vq_get_backend():
>
> if (!vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])) {
> ...
>
> This function should be called with 'vq->mutex' acquired as explained in
> the comment, but here we can avoid that, because we are not using the vq,
> so it is safe, because in vhost_transport_do_send_pkt() we check it again.
>
> Please add a comment explaining that.
>

Thanks, vhost_vq_get_backend is better. I chose a 5.3 kernel to develop
and missed this helper.
>
> As an alternative to this patch, should we kick the send worker when the
> device is ready?
>
> IIUC we reach the timeout because the send worker (that runs
> vhost_transport_do_send_pkt()) exits immediately since 'vq->private_data'
> is NULL, and no one will requeue it.
>
> Let's do it when we know the device is ready:
>
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index e36aaf9ba7bd..295b5867944f 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -543,6 +543,11 @@ static int vhost_vsock_start(struct vhost_vsock
> *vsock)
>                 mutex_unlock(&vq->mutex);
>         }
>
> +       /* Some packets may have been queued before the device was started,
> +        * let's kick the send worker to send them.
> +        */
> +       vhost_work_queue(&vsock->dev, &vsock->send_pkt_work);
> +
Yes, it works.
But do you think a threshold should be set here to prevent the queue
from being too long? E.g. the client user sends too many connect pkts
in a short time before the server is completely ready.

>         mutex_unlock(&vsock->dev.mutex);
>         return 0;
>
> I didn't test it, can you try if it fixes the issue?
>
> I'm not sure which is better...
I don't know, either. Wait for more comments 😊

--
Cheers,
Justin (Jia He)
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