netdev - Re: [PATCH net-next v7 3/4] net: vhost: factor out busy polling logic to vhost_net_busy

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Message-ID: <CAMDZJNUMBYOVh+pzaykRq23ePqNR7BOA8wP_=i85-PXvwaxuBA@mail.gmail.com>
Date:   Fri, 3 Aug 2018 10:51:08 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     jasowang@...hat.com
Cc:     makita.toshiaki@....ntt.co.jp, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v7 3/4] net: vhost: factor out busy polling logic
 to vhost_net_busy_poll()

On Thu, Aug 2, 2018 at 5:23 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
>
> On 2018年08月02日 16:41, Toshiaki Makita wrote:
> > On 2018/08/02 17:18, Jason Wang wrote:
> >> On 2018年08月01日 17:52, Tonghao Zhang wrote:
> >>>> +static void vhost_net_busy_poll_check(struct vhost_net *net,
> >>>> +                                   struct vhost_virtqueue *rvq,
> >>>> +                                   struct vhost_virtqueue *tvq,
> >>>> +                                   bool rx)
> >>>> +{
> >>>> +     struct socket *sock = rvq->private_data;
> >>>> +
> >>>> +     if (rx)
> >>>> +             vhost_net_busy_poll_try_queue(net, tvq);
> >>>> +     else if (sock && sk_has_rx_data(sock->sk))
> >>>> +             vhost_net_busy_poll_try_queue(net, rvq);
> >>>> +     else {
> >>>> +             /* On tx here, sock has no rx data, so we
> >>>> +              * will wait for sock wakeup for rx, and
> >>>> +              * vhost_enable_notify() is not needed. */
> >>> A possible case is we do have rx data but guest does not refill the rx
> >>> queue. In this case we may lose notifications from guest.
> >> Yes, should consider this case. thanks.
> > I'm a bit confused. Isn't this covered by the previous
> > "else if (sock && sk_has_rx_data(...))" block?
>
> The problem is it does nothing if vhost_vq_avail_empty() is true and
> vhost_enble_notify() is false.
>
> >
> >>>>> +
> >>>>> +             cpu_relax();
> >>>>> +     }
> >>>>> +
> >>>>> +     preempt_enable();
> >>>>> +
> >>>>> +     if (!rx)
> >>>>> +             vhost_net_enable_vq(net, vq);
> >>>> No need to enable rx virtqueue, if we are sure handle_rx() will be
> >>>> called soon.
> >>> If we disable rx virtqueue in handle_tx and don't send packets from
> >>> guest anymore(handle_tx is not called), so we can wake up for sock rx.
> >>> so the network is broken.
> >> Not sure I understand here. I mean is we schedule work for handle_rx(),
> >> there's no need to enable it since handle_rx() will do this for us.
> > Looks like in the last "else" block in vhost_net_busy_poll_check() we
> > need to enable vq since in that case we have no rx data and handle_rx()
> > is not scheduled.
> >
>
> Yes.
So we will use the vhost_has_work() to check whether or not the
handle_rx is scheduled ?
If we use the vhost_has_work(), the work in the dev work_list may be
rx work, or tx work, right ?

> Thanks