net: When removing nexthops, don't call synchronize_net if it is not necessary

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[ Upstream commit b0ac6d3b56 ]

When removing a nexthop, commit
90f33bffa3 ("nexthops: don't modify published nexthop groups") added a
call to synchronize_rcu() (later changed to _net()) to make sure
everyone sees the new nexthop-group before the rtnl-lock is released.

When one wants to delete a large number of groups and nexthops, it is
fastest to first flush the groups (ip nexthop flush groups) and then
flush the nexthops themselves (ip -6 nexthop flush). As that way the
groups don't need to be rebalanced.

However, `ip -6 nexthop flush` will still take a long time if there is
a very large number of nexthops because of the call to
synchronize_net(). Now, if there are no more groups, there is no point
in calling synchronize_net(). So, let's skip that entirely by checking
if nh->grp_list is empty.

This gives us a nice speedup:

BEFORE:
=======

$ time sudo ip -6 nexthop flush
Dump was interrupted and may be inconsistent.
Flushed 2097152 nexthops

real	1m45.345s
user	0m0.001s
sys	0m0.005s

$ time sudo ip -6 nexthop flush
Dump was interrupted and may be inconsistent.
Flushed 4194304 nexthops

real	3m10.430s
user	0m0.002s
sys	0m0.004s

AFTER:
======

$ time sudo ip -6 nexthop flush
Dump was interrupted and may be inconsistent.
Flushed 2097152 nexthops

real	0m17.545s
user	0m0.003s
sys	0m0.003s

$ time sudo ip -6 nexthop flush
Dump was interrupted and may be inconsistent.
Flushed 4194304 nexthops

real	0m35.823s
user	0m0.002s
sys	0m0.004s

Signed-off-by: Christoph Paasch <cpaasch@openai.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250816-nexthop_dump-v2-2-491da3462118@openai.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

...

This commit is contained in:

Christoph Paasch 2025-08-16 16:12:49 -07:00 • committed by Greg Kroah-Hartman

parent 6f3dcc8090

commit 2263e086ab

1 changed files with 6 additions and 0 deletions

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net/ipv4/nexthop.c

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