netdev - Re: [PATCH net-next v6 3/4] net: vhost: factor out busy polling logic to vhost_net_busy

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Date:   Tue, 24 Jul 2018 01:31:10 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     toshiaki.makita1@...il.com
Cc:     makita.toshiaki@....ntt.co.jp, jasowang@...hat.com, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v6 3/4] net: vhost: factor out busy polling logic
 to vhost_net_busy_poll()

On Mon, Jul 23, 2018 at 10:20 PM Toshiaki Makita
<toshiaki.makita1@...il.com> wrote:
>
> On 18/07/23 (月) 21:43, Tonghao Zhang wrote:
> > On Mon, Jul 23, 2018 at 5:58 PM Toshiaki Makita
> > <makita.toshiaki@....ntt.co.jp> wrote:
> >>
> >> On 2018/07/22 3:04, xiangxia.m.yue@...il.com wrote:
> >>> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> >>>
> >>> Factor out generic busy polling logic and will be
> >>> used for in tx path in the next patch. And with the patch,
> >>> qemu can set differently the busyloop_timeout for rx queue.
> >>>
> >>> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
> >>> ---
> >> ...
> >>> +static void vhost_net_busy_poll_vq_check(struct vhost_net *net,
> >>> +                                      struct vhost_virtqueue *rvq,
> >>> +                                      struct vhost_virtqueue *tvq,
> >>> +                                      bool rx)
> >>> +{
> >>> +     struct socket *sock = rvq->private_data;
> >>> +
> >>> +     if (rx) {
> >>> +             if (!vhost_vq_avail_empty(&net->dev, tvq)) {
> >>> +                     vhost_poll_queue(&tvq->poll);
> >>> +             } else if (unlikely(vhost_enable_notify(&net->dev, tvq))) {
> >>> +                     vhost_disable_notify(&net->dev, tvq);
> >>> +                     vhost_poll_queue(&tvq->poll);
> >>> +             }
> >>> +     } else if ((sock && sk_has_rx_data(sock->sk)) &&
> >>> +                 !vhost_vq_avail_empty(&net->dev, rvq)) {
> >>> +             vhost_poll_queue(&rvq->poll);
> >>
> >> Now we wait for vq_avail for rx as well, I think you cannot skip
> >> vhost_enable_notify() on tx. Probably you might want to do:
> > I think vhost_enable_notify is needed.
> >
> >> } else if (sock && sk_has_rx_data(sock->sk)) {
> >>          if (!vhost_vq_avail_empty(&net->dev, rvq)) {
> >>                  vhost_poll_queue(&rvq->poll);
> >>          } else if (unlikely(vhost_enable_notify(&net->dev, rvq))) {
> >>                  vhost_disable_notify(&net->dev, rvq);
> >>                  vhost_poll_queue(&rvq->poll);
> >>          }
> >> }
> > As Jason review as before, we only want rx kick when packet is pending at
> > socket but we're out of available buffers. So we just enable notify,
> > but not poll it ?
> >
> >          } else if ((sock && sk_has_rx_data(sock->sk)) &&
> >                      !vhost_vq_avail_empty(&net->dev, rvq)) {
> >                  vhost_poll_queue(&rvq->poll);
> >          else {
> >                  vhost_enable_notify(&net->dev, rvq);
> >          }
>
> When vhost_enable_notify() returns true the avail becomes non-empty
> while we are enabling notify. We may delay the rx process if we don't
> check the return value of vhost_enable_notify().
I got it thanks.
> >> Also it's better to care vhost_net_disable_vq()/vhost_net_enable_vq() on tx?
> > I cant find why it is better, if necessary, we can do it.
>
> The reason is pretty simple... we are busypolling the socket so we don't
> need rx wakeups during it?
OK, but one question, how about rx? do we use the
vhost_net_disable_vq/vhost_net_ensable_vq on rx ?
> --
> Toshiaki Makita