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 11:28:57 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     makita.toshiaki@....ntt.co.jp
Cc:     toshiaki.makita1@...il.com, 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 Tue, Jul 24, 2018 at 10:53 AM Toshiaki Makita
<makita.toshiaki@....ntt.co.jp> wrote:
>
> On 2018/07/24 2:31, Tonghao Zhang wrote:
> > 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 ?
>
> If we are busypolling the sock tx buf? I'm not sure if polling it
> improves the performance.
Not the sock tx buff, when we are busypolling in handle_rx, we will
check the tx vring via  vhost_vq_avail_empty.
So, should we the disable tvq, e.g. vhost_net_disable_vq(net, tvq)?> --
> Toshiaki Makita
>