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>
b0ac6d3b56