netdev - Re: [PATCH net-next v2 2/5] virtio-net: transmit napi

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Date:   Fri, 21 Apr 2017 11:53:17 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 2/5] virtio-net: transmit napi



On 2017年04月20日 21:58, Willem de Bruijn wrote:
> On Thu, Apr 20, 2017 at 2:27 AM, Jason Wang <jasowang@...hat.com> wrote:
>>
>> On 2017年04月19日 04:21, Willem de Bruijn wrote:
>>> +static void virtnet_napi_tx_enable(struct virtnet_info *vi,
>>> +                                  struct virtqueue *vq,
>>> +                                  struct napi_struct *napi)
>>> +{
>>> +       if (!napi->weight)
>>> +               return;
>>> +
>>> +       if (!vi->affinity_hint_set) {
>>> +               napi->weight = 0;
>>> +               return;
>>> +       }
>>> +
>>> +       return virtnet_napi_enable(vq, napi);
>>> +}
>>> +
>>>    static void refill_work(struct work_struct *work)
>>
>> Maybe I was wrong, but according to Michael's comment it looks like he want
>> check affinity_hint_set just for speculative tx polling on rx napi instead
>> of disabling it at all.
>>
>> And I'm not convinced this is really needed, driver only provide affinity
>> hint instead of affinity, so it's not guaranteed that tx and rx interrupt
>> are in the same vcpus.
> You're right. I made the restriction broader than the request, to really err
> on the side of caution for the initial merge of napi tx. And enabling
> the optimization is always a win over keeping it off, even without irq
> affinity.
>
> The cycle cost is significant without affinity regardless of whether the
> optimization is used.

Yes, I noticed this in the past too.

> Though this is not limited to napi-tx, it is more
> pronounced in that mode than without napi.
>
> 1x TCP_RR for affinity configuration {process, rx_irq, tx_irq}:
>
> upstream:
>
> 1,1,1: 28985 Mbps, 278 Gcyc
> 1,0,2: 30067 Mbps, 402 Gcyc
>
> napi tx:
>
> 1,1,1: 34492 Mbps, 269 Gcyc
> 1,0,2: 36527 Mbps, 537 Gcyc (!)
> 1,0,1: 36269 Mbps, 394 Gcyc
> 1,0,0: 34674 Mbps, 402 Gcyc
>
> This is a particularly strong example. It is also representative
> of most RR tests. It is less pronounced in other streaming tests.
> 10x TCP_RR, for instance:
>
> upstream:
>
> 1,1,1: 42267 Mbps, 301 Gcyc
> 1,0,2: 40663 Mbps, 445 Gcyc
>
> napi tx:
>
> 1,1,1: 42420 Mbps, 303 Gcyc
> 1,0,2:  42267 Mbps, 431 Gcyc
>
> These numbers were obtained with the virtqueue_enable_cb_delayed
> optimization after xmit_skb, btw. It turns out that moving that before
> increases 1x TCP_RR further to ~39 Gbps, at the cost of reducing
> 100x TCP_RR a bit.

I see, so I think we can leave the affinity hint optimization/check for 
future investigation:

- to avoid endless optimization (e.g we may want to share a single 
vector/napi for tx/rx queue pairs in the future) for this series.
- tx napi is disabled by default which means we can do optimization on top.

Thanks