netdev - Re: [PATCH net-next v6 3/4] net: vhost: factor out busy polling logic to vhost_net_busy

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Date:   Tue, 24 Jul 2018 11:53:08 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     toshiaki.makita1@...il.com, jasowang@...hat.com, mst@...hat.com,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v6 3/4] net: vhost: factor out busy polling logic
 to vhost_net_busy_poll()

On 2018/07/24 2:31, Tonghao Zhang wrote:
> On Mon, Jul 23, 2018 at 10:20 PM Toshiaki Makita
> <toshiaki.makita1@...il.com> wrote:
>>
>> On 18/07/23 (月) 21:43, Tonghao Zhang wrote:
>>> On Mon, Jul 23, 2018 at 5:58 PM Toshiaki Makita
>>> <makita.toshiaki@....ntt.co.jp> wrote:
>>>>
>>>> On 2018/07/22 3:04, xiangxia.m.yue@...il.com wrote:
>>>>> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
>>>>>
>>>>> Factor out generic busy polling logic and will be
>>>>> used for in tx path in the next patch. And with the patch,
>>>>> qemu can set differently the busyloop_timeout for rx queue.
>>>>>
>>>>> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
>>>>> ---
>>>> ...
>>>>> +static void vhost_net_busy_poll_vq_check(struct vhost_net *net,
>>>>> +                                      struct vhost_virtqueue *rvq,
>>>>> +                                      struct vhost_virtqueue *tvq,
>>>>> +                                      bool rx)
>>>>> +{
>>>>> +     struct socket *sock = rvq->private_data;
>>>>> +
>>>>> +     if (rx) {
>>>>> +             if (!vhost_vq_avail_empty(&net->dev, tvq)) {
>>>>> +                     vhost_poll_queue(&tvq->poll);
>>>>> +             } else if (unlikely(vhost_enable_notify(&net->dev, tvq))) {
>>>>> +                     vhost_disable_notify(&net->dev, tvq);
>>>>> +                     vhost_poll_queue(&tvq->poll);
>>>>> +             }
>>>>> +     } else if ((sock && sk_has_rx_data(sock->sk)) &&
>>>>> +                 !vhost_vq_avail_empty(&net->dev, rvq)) {
>>>>> +             vhost_poll_queue(&rvq->poll);
>>>>
>>>> Now we wait for vq_avail for rx as well, I think you cannot skip
>>>> vhost_enable_notify() on tx. Probably you might want to do:
>>> I think vhost_enable_notify is needed.
>>>
>>>> } else if (sock && sk_has_rx_data(sock->sk)) {
>>>>          if (!vhost_vq_avail_empty(&net->dev, rvq)) {
>>>>                  vhost_poll_queue(&rvq->poll);
>>>>          } else if (unlikely(vhost_enable_notify(&net->dev, rvq))) {
>>>>                  vhost_disable_notify(&net->dev, rvq);
>>>>                  vhost_poll_queue(&rvq->poll);
>>>>          }
>>>> }
>>> As Jason review as before, we only want rx kick when packet is pending at
>>> socket but we're out of available buffers. So we just enable notify,
>>> but not poll it ?
>>>
>>>          } else if ((sock && sk_has_rx_data(sock->sk)) &&
>>>                      !vhost_vq_avail_empty(&net->dev, rvq)) {
>>>                  vhost_poll_queue(&rvq->poll);
>>>          else {
>>>                  vhost_enable_notify(&net->dev, rvq);
>>>          }
>>
>> When vhost_enable_notify() returns true the avail becomes non-empty
>> while we are enabling notify. We may delay the rx process if we don't
>> check the return value of vhost_enable_notify().
> I got it thanks.
>>>> Also it's better to care vhost_net_disable_vq()/vhost_net_enable_vq() on tx?
>>> I cant find why it is better, if necessary, we can do it.
>>
>> The reason is pretty simple... we are busypolling the socket so we don't
>> need rx wakeups during it?
> OK, but one question, how about rx? do we use the
> vhost_net_disable_vq/vhost_net_ensable_vq on rx ?

If we are busypolling the sock tx buf? I'm not sure if polling it
improves the performance.

-- 
Toshiaki Makita