netdev - Re: [PATCH net-next v5 3/4] net: vhost: factor out busy polling logic to vhost_net_busy

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Date:   Wed, 4 Jul 2018 19:59:41 +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>,
        Tonghao Zhang <zhangtonghao@...ichuxing.com>
Subject: Re: [PATCH net-next v5 3/4] net: vhost: factor out busy polling logic
 to vhost_net_busy_poll()



On 2018年07月04日 17:46, Tonghao Zhang wrote:
> On Wed, Jul 4, 2018 at 5:18 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>>
>> On 2018年07月04日 15:59, Toshiaki Makita wrote:
>>> On 2018/07/04 13:31, xiangxia.m.yue@...il.com wrote:
>>> ...
>>>> +static void vhost_net_busy_poll(struct vhost_net *net,
>>>> +                            struct vhost_virtqueue *rvq,
>>>> +                            struct vhost_virtqueue *tvq,
>>>> +                            bool rx)
>>>> +{
>>>> +    unsigned long uninitialized_var(endtime);
>>>> +    unsigned long busyloop_timeout;
>>>> +    struct socket *sock;
>>>> +    struct vhost_virtqueue *vq = rx ? tvq : rvq;
>>>> +
>>>> +    mutex_lock_nested(&vq->mutex, rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX);
>>>> +
>>>> +    vhost_disable_notify(&net->dev, vq);
>>>> +    sock = rvq->private_data;
>>>> +    busyloop_timeout = rx ? rvq->busyloop_timeout : tvq->busyloop_timeout;
>>>> +
>>>> +    preempt_disable();
>>>> +    endtime = busy_clock() + busyloop_timeout;
>>>> +    while (vhost_can_busy_poll(tvq->dev, endtime) &&
>>>> +           !(sock && sk_has_rx_data(sock->sk)) &&
>>>> +           vhost_vq_avail_empty(tvq->dev, tvq))
>>>> +            cpu_relax();
>>>> +    preempt_enable();
>>>> +
>>>> +    if ((rx && !vhost_vq_avail_empty(&net->dev, vq)) ||
>>>> +        (!rx && (sock && sk_has_rx_data(sock->sk)))) {
>>>> +            vhost_poll_queue(&vq->poll);
>>>> +    } else if (vhost_enable_notify(&net->dev, vq) && rx) {
>>> Hmm... on tx here sock has no rx data, so you are waiting for sock
>>> wakeup for rx and vhost_enable_notify() seems not needed. Do you want
>>> this actually?
>>>
>>> } else if (rx && vhost_enable_notify(&net->dev, vq)) {
>> Right, rx need to be checked first here.
> thanks， if we dont call the vhost_enable_notify for tx. so we dont
> need to call vhost_disable_notify for tx?
>
> @@ -451,7 +451,9 @@ static void vhost_net_busy_poll(struct vhost_net *net,
>                                                tvq->busyloop_timeout;
>
>          mutex_lock_nested(&vq->mutex, rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX);
> -       vhost_disable_notify(&net->dev, vq);
> +
> +       if (rx)
> +               vhost_disable_notify(&net->dev, vq);
>
>          preempt_disable();
>          endtime = busy_clock() + busyloop_timeout;

Sorry for being unclear. We need enable tx notification at end end of 
the loop.

I meant we need replace

+    } else if (vhost_enable_notify(&net->dev, vq) && rx) {

with

+    } else if (rx && vhost_enable_notify(&net->dev, vq)) {

We only need rx notification when there's no avail buffers. This means 
we need only enable notification for tx.

And maybe we can simplify the logic as

if (rx) {
......
} else {
......
}

here. (not a must).

Thanks


>
>> Thanks
>>
>>>> +            vhost_disable_notify(&net->dev, vq);
>>>> +            vhost_poll_queue(&vq->poll);
>>>> +    }