netdev - Re: [PATCH net-next v2 2/2] net: When removing nexthops, don't call synchronize

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Message-ID: <aKGWg7tmkEAC9Wu0@shredder>
Date: Sun, 17 Aug 2025 11:44:51 +0300
From: Ido Schimmel <idosch@...sch.org>
To: cpaasch@...nai.com
Cc: David Ahern <dsahern@...nel.org>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/2] net: When removing nexthops, don't call
 synchronize_net if it is not necessary

On Sat, Aug 16, 2025 at 04:12:49PM -0700, Christoph Paasch via B4 Relay wrote:
> From: Christoph Paasch <cpaasch@...nai.com>
> 
> When removing a nexthop, commit
> 90f33bffa382 ("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.

[...]

> Signed-off-by: Christoph Paasch <cpaasch@...nai.com>

Reviewed-by: Ido Schimmel <idosch@...dia.com>