netdev - Re: [PATCH net-next v2 1/6] net: ethtool: allow symmetric RSS hash for any flow type

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Date: Fri, 6 Oct 2023 17:22:48 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Ahmed Zaki <ahmed.zaki@...el.com>
Cc: netdev@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
 linux-doc@...r.kernel.org, corbet@....net, jesse.brandeburg@...el.com,
 anthony.l.nguyen@...el.com, davem@...emloft.net, edumazet@...gle.com,
 pabeni@...hat.com, vladimir.oltean@....com, andrew@...n.ch,
 horms@...nel.org, mkubecek@...e.cz, Wojciech Drewek
 <wojciech.drewek@...el.com>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net-next v2 1/6] net: ethtool: allow symmetric RSS hash
 for any flow type

On Fri,  6 Oct 2023 16:47:21 -0600 Ahmed Zaki wrote:
> Symmetric RSS hash functions are beneficial in applications that monitor
> both Tx and Rx packets of the same flow (IDS, software firewalls, ..etc).
> Getting all traffic of the same flow on the same RX queue results in
> higher CPU cache efficiency.
> 
> Only fields that has counterparts in the other direction can be
> accepted; IP src/dst and L4 src/dst ports.
> 
> The user may request RSS hash symmetry for a specific flow type, via:
> 
>     # ethtool -N|-U eth0 rx-flow-hash <flow_type> s|d|f|n symmetric
> 
> or turn symmetry off (asymmetric) by:
> 
>     # ethtool -N|-U eth0 rx-flow-hash <flow_type> s|d|f|n

Thanks for the changes, code looks good!

The question left unanswered is whether we should care about the exact
implementation of the symmetry (xor, xor duplicate, sort fields).
Toeplitz-based RSS is very precisely specified, so we may want to carry
that precision into the symmetric behavior. I have a weak preference 
to do so... but no willingness to argue with you, so let me put Willem
on the spot and have him make a decision :)

Please make sure to CC Willem and anyone else who commented on previous
revisions on future versions!