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: <a900e9f3-3aad-2cfa-da4c-226bf8981059@redhat.com>
Date:   Fri, 3 Aug 2018 11:43:13 +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 11:24, Tonghao Zhang wrote:
> On Fri, Aug 3, 2018 at 11:07 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>>
>> 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, the commit be294a51a "vhost_net: Avoid rx queue wake-ups during busypoll"
> may not consider the case: work is tx work in the dev work list.

So two kinds of work, tx kick or tx wakeup.

For tx kick, we check vhost_vq_avail_empty() and avoid unnecessary kicks 
by not enabling kick if we found something is pending on txq. For tx 
wakeup, yes, the commit does not consider it. And that's why we want to 
disable tx wakeups during busy polling.

Thanks

>
>> 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.
> OK
>> Thanks
>>
>>>> Thanks