netdev - Re: [PATCH vhost] vhost_net: Fix too many vring kick on busypoll

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Date:   Mon, 2 Jul 2018 17:05:59 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jason Wang <jasowang@...hat.com>,
        "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 2018/07/02 16:52, Jason Wang wrote:
> On 2018年07月02日 15:11, Toshiaki Makita wrote:
>> On 2018/07/02 15:17, Jason Wang wrote:
>>> On 2018年07月02日 12:37, Toshiaki Makita wrote:
>>>> On 2018/07/02 11:54, Jason Wang wrote:
>>>>> 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.
>>>> Consider this kind of scenario:
>>>> 0. Set 100us busypoll for example.
>>>> 1. handle_tx() runs busypoll.
>>>> 2. Something like zerocopy queues tx_work within 100us.
>>>> 3. busypoll exits and call handle_tx() again.
>>>> 4. Repeat 1-3.
>>>>
>>>> In this case handle_tx() does not process packets but busypoll
>>>> essentially runs beyond 100us without endtime memorized. This may be
>>>> just a theoretical problem, but I was worried that more code to poll tx
>>>> queue can be added in the future and it becomes realistic.
>>> Yes, but consider zerocopy tends to batch 16 used packets and we will
>>> finally finish all processing of packets. The above won't be endless, so
>>> it was probably tolerable.
>> Right. So endtime memorization is more like a future-proof thing.
>> Would you like to keep it or change something?
> 
> I think we'd better introduce it only when we meet real bugs.

I'll change it to a flag to indicate the previous busypoll is interrupted.

>>>>>>>> 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.
>>>> I think I understand when driver XDP can be used. What I'm not sure and
>>>> was going to narrow down is why zerocopy is mostly not applied.
>>>>
>>> I see, any touch to the zerocopy packet (clone, header expansion or
>>> segmentation) that lead a userspace copy will increase the error counter
>>> in vhost_net. Then vhost_net_tx_select_zcopy() may choose not to use
>>> zerocopy. So it was probably something in your setup or a bug somewhere.
>> Thanks for the hint!

Seems zerocopy packets are always nonlinear and
netif_receive_generic_xdp() calls skb_linearize() in which
__pskb_pull_tail() calls skb_zcopy_clear(). Looks like tx_zcopy_err is
always counted when zerocopy is used with XDP in my env.

-- 
Toshiaki Makita