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: <3272c3b4-a44c-8554-329e-8a5e1a59aafd@redhat.com>
Date:   Thu, 2 Aug 2018 17:23:51 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     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 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.

Thanks