netdev - Re: [PATCH net-next 0/4] net: reduce RTNL pressure in unregister

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Message-ID: <20250107121148.7054518d@kernel.org>
Date: Tue, 7 Jan 2025 12:11:48 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Paolo Abeni
 <pabeni@...hat.com>, netdev@...r.kernel.org, Simon Horman
 <horms@...nel.org>, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 0/4] net: reduce RTNL pressure in
 unregister_netdevice()

On Tue,  7 Jan 2025 17:38:34 +0000 Eric Dumazet wrote:
> One major source of RTNL contention resides in unregister_netdevice()
> 
> Due to RCU protection of various network structures, and
> unregister_netdevice() being a synchronous function,
> it is calling potentially slow functions while holding RTNL.
> 
> I think we can release RTNL in two points, so that three
> slow functions are called while RTNL can be used
> by other threads.

I think we'll need:

diff --git a/net/devlink/port.c b/net/devlink/port.c
index 939081a0e615..cdfa22453a55 100644
--- a/net/devlink/port.c
+++ b/net/devlink/port.c
@@ -1311,6 +1311,7 @@ int devlink_port_netdevice_event(struct notifier_block *nb,
 		__devlink_port_type_set(devlink_port, devlink_port->type,
 					netdev);
 		break;
+	case NETDEV_UNREGISTERING:
 	case NETDEV_UNREGISTER:
 		if (devlink_net(devlink) != dev_net(netdev))
 			return NOTIFY_OK;

There is no other way to speed things up? Use RT prio for the work?
Maybe WRITE_ONCE() a special handler into backlog.poll, and schedule it?

I'm not gonna stand in your way but in general re-taking caller locks
in a callee is a bit ugly :(