linux-kernel - Re: [PATCH net-next V2 2/2] virtio-net: sleep instead of busy waiting for cvq command

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Date:   Wed, 17 May 2023 08:50:29 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, maxime.coquelin@...hat.com,
        alvaro.karsz@...id-run.com, eperezma@...hat.com,
        xuanzhuo@...ux.alibaba.com, david.marchand@...hat.com
Subject: Re: [PATCH net-next V2 2/2] virtio-net: sleep instead of busy waiting
 for cvq command

On Wed, May 17, 2023 at 4:54 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Thu, Apr 13, 2023 at 02:40:27PM +0800, Jason Wang wrote:
> > We used to busy waiting on the cvq command this tends to be
> > problematic since there no way for to schedule another process which
> > may serve for the control virtqueue. This might be the case when the
> > control virtqueue is emulated by software. This patch switches to use
> > completion to allow the CPU to sleep instead of busy waiting for the
> > cvq command.
> >
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> > Changes since V1:
> > - use completion for simplicity
> > - don't try to harden the CVQ command which requires more thought
> > Changes since RFC:
> > - break the device when timeout
> > - get buffer manually since the virtio core check more_used() instead
> > ---
> >  drivers/net/virtio_net.c | 21 ++++++++++++++-------
> >  1 file changed, 14 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 2e56bbf86894..d3eb8fd6c9dc 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/average.h>
> >  #include <linux/filter.h>
> >  #include <linux/kernel.h>
> > +#include <linux/completion.h>
> >  #include <net/route.h>
> >  #include <net/xdp.h>
> >  #include <net/net_failover.h>
> > @@ -295,6 +296,8 @@ struct virtnet_info {
> >
> >       /* failover when STANDBY feature enabled */
> >       struct failover *failover;
> > +
> > +     struct completion completion;
> >  };
> >
> >  struct padded_vnet_hdr {
> > @@ -1709,6 +1712,13 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq,
> >       return !oom;
> >  }
> >
> > +static void virtnet_cvq_done(struct virtqueue *cvq)
> > +{
> > +     struct virtnet_info *vi = cvq->vdev->priv;
> > +
> > +     complete(&vi->completion);
> > +}
> > +
> >  static void skb_recv_done(struct virtqueue *rvq)
> >  {
> >       struct virtnet_info *vi = rvq->vdev->priv;
> > @@ -2169,12 +2179,8 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
> >       if (unlikely(!virtqueue_kick(vi->cvq)))
> >               return vi->ctrl->status == VIRTIO_NET_OK;
> >
> > -     /* Spin for a response, the kick causes an ioport write, trapping
> > -      * into the hypervisor, so the request should be handled immediately.
> > -      */
> > -     while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > -            !virtqueue_is_broken(vi->cvq))
> > -             cpu_relax();
> > +     wait_for_completion(&vi->completion);
> > +     virtqueue_get_buf(vi->cvq, &tmp);
> >
> >       return vi->ctrl->status == VIRTIO_NET_OK;
>
> This seems to break surprise removal and other
> situations where vq gets broken since callbacks
> aren't usually invoked then.

Yes, so I think I can go back to the original idea by simply adding
cond_resched() here.

>
>
> >  }
> > @@ -3672,7 +3678,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >
> >       /* Parameters for control virtqueue, if any */
> >       if (vi->has_cvq) {
> > -             callbacks[total_vqs - 1] = NULL;
> > +             callbacks[total_vqs - 1] = virtnet_cvq_done;
> >               names[total_vqs - 1] = "control";
> >       }
> >
>
> There is a cost to this, in that we are burning an extra MSI vector
> for the slow path cvq. if device has 3 vectors, suddenly we can't
> allocate vectors for rx and tx, big problem.
>
> So I'm afraid we need to pass a new flag that will share
> the config changed interrupt and cvq.

See above, it looks to me a simple cond_resched() is sufficient, then
we don't need a new vector.

Thanks

>
>
>
> > @@ -4122,6 +4128,7 @@ static int virtnet_probe(struct virtio_device *vdev)
> >       if (vi->has_rss || vi->has_rss_hash_report)
> >               virtnet_init_default_rss(vi);
> >
> > +     init_completion(&vi->completion);
> >       enable_rx_mode_work(vi);
> >
> >       /* serialize netdev register + virtio_device_ready() with ndo_open() */
> > --
> > 2.25.1
>