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

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Date:   Thu, 2 Aug 2018 17:41:41 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jason Wang <jasowang@...hat.com>,
        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 2018/08/02 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?

>>>> +
>>>> +             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.

-- 
Toshiaki Makita