netdev - Re: [PATCH net-next 2/4] net: ipv4: Add a sysctl to set multipath hash seed

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Date: Mon, 3 Jun 2024 10:25:54 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Petr Machata <petrm@...dia.com>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	Ido Schimmel <idosch@...dia.com>, David Ahern <dsahern@...nel.org>, Jonathan Corbet <corbet@....net>, 
	linux-doc@...r.kernel.org, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next 2/4] net: ipv4: Add a sysctl to set multipath
 hash seed

On Mon, Jun 3, 2024 at 9:30 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Eric Dumazet <edumazet@...gle.com> writes:
>
> > On Wed, May 29, 2024 at 1:21 PM Petr Machata <petrm@...dia.com> wrote:
> >>
> >> When calculating hashes for the purpose of multipath forwarding, both IPv4
> >> and IPv6 code currently fall back on flow_hash_from_keys(). That uses a
> >> randomly-generated seed. That's a fine choice by default, but unfortunately
> >> some deployments may need a tighter control over the seed used.
> >>
> >> In this patch, make the seed configurable by adding a new sysctl key,
> >> net.ipv4.fib_multipath_hash_seed to control the seed. This seed is used
> >> specifically for multipath forwarding and not for the other concerns that
> >> flow_hash_from_keys() is used for, such as queue selection. Expose the knob
> >> as sysctl because other such settings, such as headers to hash, are also
> >> handled that way. Like those, the multipath hash seed is a per-netns
> >> variable.
> >>
> >> Despite being placed in the net.ipv4 namespace, the multipath seed sysctl
> >> is used for both IPv4 and IPv6, similarly to e.g. a number of TCP
> >> variables.
> >>
> > ...
> >
> >> +       rtnl_lock();
> >> +       old = rcu_replace_pointer_rtnl(net->ipv4.sysctl_fib_multipath_hash_seed,
> >> +                                      mphs);
> >> +       rtnl_unlock();
> >> +
> >
> > In case you keep RCU for the next version, please do not use rtnl_lock() here.
> >
> > A simple xchg() will work just fine.
> >
> > old = xchg((__force struct struct sysctl_fib_multipath_hash_seed
> > **)&net->ipv4.sysctl_fib_multipath_hash_seed,
> >                  mphs);
>
> We added a macro to do this kind of thing without triggering any of the
> RCU type linter warnings, in:
>
> 76c8eaafe4f0 ("rcu: Create an unrcu_pointer() to remove __rcu from a pointer")
>
> So as an alternative to open-coding the cast, something like this could
> work - I guess it's mostly a matter of taste:
>
> old = unrcu_pointer(xchg(&net->ipv4.sysctl_fib_multipath_hash_seed, RCU_INITIALIZER(mphs)));

Good to know, thanks.

Not sure why __kernel qualifier has been put there.