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

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Date: Sat, 1 Jun 2024 10:46:16 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Petr Machata <petrm@...dia.com>
Cc: "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 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);