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

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Message-ID: <CANn89iJeOp4_+YknUxnBheWo7WqGzvoB6trp9RV8Dz9-AogppQ@mail.gmail.com>
Date: Mon, 18 Aug 2025 03:02:38 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: cpaasch@...nai.com
Cc: David Ahern <dsahern@...nel.org>, Nikolay Aleksandrov <razor@...ckwall.org>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Ido Schimmel <idosch@...sch.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 4:13 PM Christoph Paasch via B4 Relay
<devnull+cpaasch.openai.com@...nel.org> 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.
>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>