Date:   Mon, 2 Jul 2018 10:54:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org,
        Tonghao Zhang <zhangtonghao@...ichuxing.com>
Subject: Re: [PATCH vhost] vhost_net: Fix too many vring kick on busypoll



On 2018年07月02日 10:45, Toshiaki Makita wrote:
> Hi Jason,
>
> On 2018/06/29 18:30, Jason Wang wrote:
>> On 2018年06月29日 16:09, Toshiaki Makita wrote:
> ...
>>> To fix this, poll the work instead of enabling notification when
>>> busypoll is interrupted by something. IMHO signal_pending() and
>>> vhost_has_work() are kind of interruptions rather than signals to
>>> completely cancel the busypoll, so let's run busypoll after the
>>> necessary work is done. In order to avoid too long busyloop due to
>>> interruption, save the endtime in vq field and use it when reentering
>>> the work function.
>> I think we don't care long busyloop unless e.g tx can starve rx?
> I just want to keep it user-controllable. Unless memorizing it busypoll
> can run unexpectedly long.

I think the total amount of time for busy polling is bounded. If I was 
wrong, it should be a bug somewhere.

>
>>> I observed this problem makes tx performance poor but does not rx. I
>>> guess it is because rx notification from socket is not that heavy so
>>> did not impact the performance even when we failed to mask the
>>> notification.
>> I think the reason is:
>>
>> For tx, we may only process used ring under handle_tx(). Busy polling
>> code can not recognize this case.
>> For rx, we call peek_head_len() after exiting busy loop, so if the work
>> is for rx, it can be processed in handle_rx() immediately.
> Sorry but I don't see the difference. Tx busypoll calls
> vhost_get_vq_desc() immediately after busypoll exits in
> vhost_net_tx_get_vq_desc().

Yes, so for the problem of zerocopy callback issue is we don't poll used 
(done_idx != upend_idx). Maybe we can try to add this and avoid the 
check of vhost_has_worker().

>
> The scenario I described above (in 2nd paragraph) is a bit simplified.
> To be clearer what I think is happening is:
>
> 1. handle_tx() runs busypoll with notification enabled (due to zerocopy
>     callback or something).

I think it was due to we enable notification when we exit handle_tx().

> 2. Guest produces a packet in vring.
> 3. handle_tx() busypoll detects the produced packet and get it.
> 4. While vhost is processing the packet, guest kicks vring because it
>     produced the packet. Vring notification is disabled automatically by
>     event_idx and tx work is queued.
> 5. After processing the packet vhost enters busyloop again, but detects
>     tx work and exits busyloop immediately. Then handle_tx() exits after
>     enabling the notification.
> 6. Because the queued work was tx, handle_tx() is called immediately
>     again. handle_tx() runs busypoll with notification enabled.
> 7. Repeat 2-6.

Exactly what I understand.

>
>>> Anyway for consistency I fixed rx routine as well as tx.
>>>
>>> Performance numbers:
>>>
>>> - Bulk transfer from guest to external physical server.
>>>       [Guest]->vhost_net->tap--(XDP_REDIRECT)-->i40e --(wire)--> [Server]
>> Just to confirm in this case since zerocopy is enabled, we are in fact
>> use the generic XDP datapath?
> For some reason zerocopy was not applied for most packets, so in most
> cases driver XDP was used. I was going to dig into it but not yet.

Right, just to confirm this. This is expected.

In tuntap, we do native XDP only for small and non zerocopy packets. See 
tun_can_build_skb(). The reason is XDP may adjust packet header which is 
not supported by zercopy. We can only use XDP generic for zerocopy in 
this case.

>
>>> - Set 10us busypoll.
>>> - Guest disables checksum and TSO because of host XDP.
>>> - Measured single flow Mbps by netperf, and kicks by perf kvm stat
>>>     (EPT_MISCONFIG event).
>>>
>>>                               Before              After
>>>                             Mbps  kicks/s      Mbps  kicks/s
>>> UDP_STREAM 1472byte              247758                 27
>>>                   Send   3645.37            6958.10
>>>                   Recv   3588.56            6958.10
>>>                 1byte                9865                 37
>>>                   Send      4.34               5.43
>>>                   Recv      4.17               5.26
>>> TCP_STREAM             8801.03    45794   9592.77     2884
>> Impressive numbers.
>>
>>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>>> ---
> ...
>>> +static bool vhost_busy_poll_interrupted(struct vhost_dev *dev)
>>> +{
>>> +    return unlikely(signal_pending(current)) || vhost_has_work(dev);
>>>    }
>>>      static void vhost_net_disable_vq(struct vhost_net *n,
>>> @@ -437,10 +438,21 @@ static int vhost_net_tx_get_vq_desc(struct
>>> vhost_net *net,
>>>          if (r == vq->num && vq->busyloop_timeout) {
>>>            preempt_disable();
>>> -        endtime = busy_clock() + vq->busyloop_timeout;
>>> -        while (vhost_can_busy_poll(vq->dev, endtime) &&
>>> -               vhost_vq_avail_empty(vq->dev, vq))
>>> +        if (vq->busyloop_endtime) {
>>> +            endtime = vq->busyloop_endtime;
>>> +            vq->busyloop_endtime = 0;
>> Looks like endtime may be before current time. So we skip busy loop in
>> this case.
> Sure, I'll add a condition.
>
>>> +        } else {
>>> +            endtime = busy_clock() + vq->busyloop_timeout;
>>> +        }
>>> +        while (vhost_can_busy_poll(endtime)) {
>>> +            if (vhost_busy_poll_interrupted(vq->dev)) {
>>> +                vq->busyloop_endtime = endtime;
>>> +                break;
>>> +            }
>>> +            if (!vhost_vq_avail_empty(vq->dev, vq))
>>> +                break;
>>>                cpu_relax();
>>> +        }
>>>            preempt_enable();
>>>            r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
>>>                          out_num, in_num, NULL, NULL);
> ...
>>> @@ -642,39 +658,51 @@ static void vhost_rx_signal_used(struct
>>> vhost_net_virtqueue *nvq)
>>>      static int vhost_net_rx_peek_head_len(struct vhost_net *net,
>>> struct sock *sk)
>>>    {
>>> -    struct vhost_net_virtqueue *rvq = &net->vqs[VHOST_NET_VQ_RX];
>>> -    struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
>>> -    struct vhost_virtqueue *vq = &nvq->vq;
>>> +    struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
>>> +    struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
>>> +    struct vhost_virtqueue *rvq = &rnvq->vq;
>>> +    struct vhost_virtqueue *tvq = &tnvq->vq;
>> NIt: I admit the variable name is confusing. It would be better to do
>> the renaming in a separate patch to ease review.
> OK.
>
>>>        unsigned long uninitialized_var(endtime);
>>> -    int len = peek_head_len(rvq, sk);
>>> +    int len = peek_head_len(rnvq, sk);
>>>    -    if (!len && vq->busyloop_timeout) {
>>> +    if (!len && tvq->busyloop_timeout) {
>>>            /* Flush batched heads first */
>>> -        vhost_rx_signal_used(rvq);
>>> +        vhost_rx_signal_used(rnvq);
>>>            /* Both tx vq and rx socket were polled here */
>>> -        mutex_lock_nested(&vq->mutex, 1);
>>> -        vhost_disable_notify(&net->dev, vq);
>>> +        mutex_lock_nested(&tvq->mutex, 1);
>>> +        vhost_disable_notify(&net->dev, tvq);
>>>              preempt_disable();
>>> -        endtime = busy_clock() + vq->busyloop_timeout;
>>> +        if (rvq->busyloop_endtime) {
>>> +            endtime = rvq->busyloop_endtime;
>>> +            rvq->busyloop_endtime = 0;
>>> +        } else {
>>> +            endtime = busy_clock() + tvq->busyloop_timeout;
>>> +        }
>>>    -        while (vhost_can_busy_poll(&net->dev, endtime) &&
>>> -               !sk_has_rx_data(sk) &&
>>> -               vhost_vq_avail_empty(&net->dev, vq))
>>> +        while (vhost_can_busy_poll(endtime)) {
>>> +            if (vhost_busy_poll_interrupted(&net->dev)) {
>>> +                rvq->busyloop_endtime = endtime;
>>> +                break;
>>> +            }
>>> +            if (sk_has_rx_data(sk) ||
>>> +                !vhost_vq_avail_empty(&net->dev, tvq))
>>> +                break;
>>>                cpu_relax();
>> Actually, I plan to poll guest rx refilling as well here to avoid rx
>> kicks. Want to draft another patch to do it as well? It's as simple as
>> add a vhost_vq_avail_empty for rvq above.
> OK. will try it.
>
>>> +        }
>>>              preempt_enable();
>>>    -        if (!vhost_vq_avail_empty(&net->dev, vq))
>>> -            vhost_poll_queue(&vq->poll);
>>> -        else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>>> -            vhost_disable_notify(&net->dev, vq);
>>> -            vhost_poll_queue(&vq->poll);
>>> +        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);
>>>            }
>>>    -        mutex_unlock(&vq->mutex);
>>> +        mutex_unlock(&tvq->mutex);
>>>    -        len = peek_head_len(rvq, sk);
>>> +        len = peek_head_len(rnvq, sk);
>>>        }
>>>          return len;
> ...
>>> @@ -889,7 +918,12 @@ static void handle_rx(struct vhost_net *net)
>>>                goto out;
>>>            }
>>>        }
>>> -    vhost_net_enable_vq(net, vq);
>>> +    if (unlikely(vq->busyloop_endtime)) {
>>> +        /* Busy poll is interrupted. */
>>> +        vhost_poll_queue(&vq->poll);
>>> +    } else {
>>> +        vhost_net_enable_vq(net, vq);
>>> +    }
>> This is to reduce the rx wake ups. Better with another patch.
>>
>> So I suggest to split the patches into:
>>
>> 1 better naming of variable of vhost_net_rx_peek_head_len().
>> 2 avoid tx kicks (most of what this patch did)
>> 3 avoid rx wakeups (exactly the above 6 lines did)
>> 4 avoid rx kicks
> OK, I'll reorganize the patch.
> Thank you for your feedback!
>
>> Btw, tonghao is doing some refactoring of busy polling as well. Depends
>> on the order of being merged, one of you may need rebasing.
> Thanks for sharing. I'll take a look.
>

Thanks.

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