netdev - Re: [PATCH net-next v7 3/4] net: vhost: factor out busy polling logic to vhost_net_busy

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Date:   Fri, 3 Aug 2018 13:07:46 +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 12:04, Tonghao Zhang wrote:
> On Fri, Aug 3, 2018 at 11:43 AM Jason Wang<jasowang@...hat.com>  wrote:
>>
>> 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.
> And in the handle_rx but not busy polling, the tx can wakeup anytime
> and the tx work will be added to dev work list. In that case, if we
> add
> the rx poll to the queue, it is necessary ？ the commit be294a51a may
> check whether the rx work is in the dev work list.

I think the point this we don't poll rx during tx at that time. So if rx 
poll is interrupted, we should reschedule handle_rx(). After we poll rx 
on handle_tx(), we can try to optimize this on top.

Thanks