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

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Date:   Wed, 09 Nov 2016 13:09:18 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     ecree@...arflare.com
Cc:     netdev@...r.kernel.org, linux-net-drivers@...arflare.com
Subject: Re: [PATCH net-next 0/2] sfc: enable 4-tuple UDP RSS hashing

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.