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 14:25:56 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jason Wang <jasowang@...hat.com>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     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 2018/08/03 14:07, Jason Wang wrote:
> 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.

That's true. We may be able to skip poll_queue in handle_rx/tx after
rx/tx busypolling is unified by this patch set.

-- 
Toshiaki Makita