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Date:   Fri, 3 May 2019 11:14:21 -0400
From:   lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [Intel-wired-lan] i40e X722 RSS problem with NAT-Traversal IPsec
 packets

On Thu, May 02, 2019 at 01:59:46PM -0700, Alexander Duyck wrote:
> If I recall correctly RSS is only using something like the lower 9
> bits (indirection table size of 512) of the resultant hash on the
> X722, even fewer if you have fewer queues that are a power of 2 and
> happen to program the indirection table in a round robin fashion. So
> for example on my system setup with 32 queues it is technically only
> using the lower 5 bits of the hash.
> 
> One issue as a result of that is that you can end up with swaths of
> bits that don't really seem to impact the hash all that much since it
> will never actually change those bits of the resultant hash. In order
> to guarantee that every bit in the input impacts the hash you have to
> make certain you have to gaps in the key wider than the bits you
> examine in the final hash.
> 
> A quick and dirty way to verify that the hash key is part of the issue
> would be to use something like a simple repeating value such as AA:55
> as your hash key. With something like that every bit you change in the
> UDP port number should result in a change in the final RSS hash for
> queue counts of 3 or greater. The downside is the upper 16 bits of the
> hash are identical to the lower 16 so the actual hash value itself
> isn't as useful.

OK I set the hkey to
aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55:aa:55
and still only see queue 0 and 2 getting hit with a couple of dozen
different UDP port numbers I picked.  Changing the hash with ethtool to
that didn't even move where the tcp packets for my ssh connection are
going (they are always on queue 2 it seems).

Does it just not hash UDP packets correctly?  Is it even doing RSS?
(the register I checked claimed it is).

This system has 40 queues assigned by default since that is how many
CPUs there are.  Changing it to a lower number didn't make a difference
(I tried 32 and 8).

-- 
Len Sorensen

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