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net: ipv4: Add a sysctl to set multipath hash seed

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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.

The seed used by flow_hash_from_keys() is a 128-bit quantity. However it
seems that usually the seed is a much more modest value. 32 bits seem
typical (Cisco, Cumulus), some systems go even lower. For that reason, and
to decouple the user interface from implementation details, go with a
32-bit quantity, which is then quadruplicated to form the siphash key.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240607151357.421181-3-petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Petr Machata
2024-06-07 17:13:54 +02:00
committed by Jakub Kicinski
parent 3e453ca122
commit 4ee2a8cace
6 changed files with 119 additions and 1 deletions
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14
Documentation/networking/ip-sysctl.rst
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@@ -131,6 +131,20 @@ fib_multipath_hash_fields - UNSIGNED INTEGER
Default: 0x0007 (source IP, destination IP and IP protocol)
fib_multipath_hash_seed - UNSIGNED INTEGER
The seed value used when calculating hash for multipath routes. Applies
to both IPv4 and IPv6 datapath. Only present for kernels built with
CONFIG_IP_ROUTE_MULTIPATH enabled.
When set to 0, the seed value used for multipath routing defaults to an
internal random-generated one.
The actual hashing algorithm is not specified -- there is no guarantee
that a next hop distribution effected by a given seed will keep stable
across kernel versions.
Default: 0 (random)
fib_sync_mem - UNSIGNED INTEGER
Amount of dirty memory from fib entries that can be backlogged before
synchronize_rcu is forced.