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

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Date:   Mon, 2 Jul 2018 13:37:24 +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 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.

>>>> 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.

-- 
Toshiaki Makita