netdev - Re: [PATCH net-next 0/2] sfc: enable 4-tuple UDP RSS hashing

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Date:   Wed, 9 Nov 2016 18:51:15 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     David Miller <davem@...emloft.net>
CC:     <netdev@...r.kernel.org>, <linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net-next 0/2] sfc: enable 4-tuple UDP RSS hashing

On 09/11/16 18:09, David Miller wrote:
> From: Edward Cree <ecree@...arflare.com>
> Date: Tue, 8 Nov 2016 13:02:05 +0000
>
>> On 07/11/16 18:20, David Miller wrote:
>>> From: Edward Cree <ecree@...arflare.com>
>>> Date: Thu, 3 Nov 2016 22:10:31 +0000
>>>
>>>> EF10 based NICs have configurable RSS hash fields, and can be made to take the
>>>> ports into the hash on UDP (they already do so for TCP).  This patch series
>>>> enables this, in order to improve spreading of UDP traffic.
>>> What does the chip do with fragmented traffic?
>> Only the first fragment will be considered UDP, it will treat the rest as "other
>> IP" and 2-tuple hash them, probably hitting a different queue.
>>
>> My understanding is that while that will reduce performance, that shouldn't be a
>> problem as performance-sensitive users will avoid fragmentation anyway.
>> It could also lead to out-of-order packet delivery, but it's UDP so that's
>> supposed to be OK.
> Our software hashing never tries to inspect the ports for fragmented
> frames.  And I'm pretty sure this is intentional.
>
> We should minimize the difference between what we do in software, which
> we fully control, and what we ask the hardware to offload for us.
>
> If you can't configure the chip to skip the ports for fragmented frames
> than I'm going to ask you to drop this.
I just checked and it turns out I was mistaken, we don't treat the first fragment
differently after all, we skip the ports for all fragments including the first.
Sorry for the misinformation.