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

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Message-ID: <CAF=yD-Kp8-iQtDM3+mgfJ6Ba0vkAeb09VZBa_k6RUequEyjd0w@mail.gmail.com>
Date: Sat, 7 Oct 2023 04:01:30 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Ahmed Zaki <ahmed.zaki@...el.com>, 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>
Subject: Re: [PATCH net-next v2 1/6] net: ethtool: allow symmetric RSS hash
 for any flow type

On Fri, Oct 6, 2023 at 7:22 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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 :)

I do have a stronger willingness to argue, thanks ;-)

Can we give a more precise name, such as symmetric-xor? In case
another device would implement another mode, such as the symmetric
toeplitz of __flow_hash_consistentify, it would be good to be able to
discern the modes.