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: <CAMDZJNWZ+WH3JhCy81h1VSka7-z2zF1Tw-EnLTr26J_5SkQZTg@mail.gmail.com>
Date:   Fri, 3 Aug 2018 11:24:12 +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 Fri, Aug 3, 2018 at 11:07 AM Jason Wang <jasowang@...hat.com> wrote:
>
>
>
> On 2018年08月03日 10:51, Tonghao Zhang wrote:
> > 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 ?
>
> Yes. We can add a boolean to record whether or not we've called
> vhost_poll_queue() for rvq. And avoid calling vhost_net_enable_vq() if
> it was true.
so, the commit be294a51a "vhost_net: Avoid rx queue wake-ups during busypoll"
may not consider the case: work is tx work in the dev work list.

> So here's the needed changes:
>
> 1) Split the wakeup disabling to another patch
> 2) Squash the vhost_net_busy_poll_try_queue() and
> vhost_net_busy_poll_check() into vhost_net_busy_poll() and reduce
> duplicated checks.
> 3) If possible, rename the boolean rx in vhost_net_busy_poll() to
> poll_rx, this makes code more readable.
OK
> Thanks
>
> >> Thanks
>