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

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Message-ID: <b89a6f43-0ff0-430d-d280-66e0b224e06a@lab.ntt.co.jp>
Date:   Tue, 24 Jul 2018 12:41:15 +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 12:28, Tonghao Zhang wrote:
> On Tue, Jul 24, 2018 at 10:53 AM Toshiaki Makita
> <makita.toshiaki@....ntt.co.jp> wrote:
>>
>> 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.
> Not the sock tx buff, when we are busypolling in handle_rx, we will
> check the tx vring via  vhost_vq_avail_empty.
> So, should we the disable tvq, e.g. vhost_net_disable_vq(net, tvq)?> --

When you want to stop vq kicks from the guest you should call
vhost_disable_notify() and when you want to stop vq wakeups from the
socket you should call vhost_net_disable_vq().

You are polling vq_avail so you want to stop vq kicks thus
vhost_disable_notify() is needed and it is already called.

-- 
Toshiaki Makita