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: <fc4dc137-98de-2cee-4f41-e23ab4eb2492@redhat.com>
Date:   Fri, 3 Aug 2018 11:07:18 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.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 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 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.

Thanks

>> Thanks