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

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Message-ID: <CANn89iJkxX1d-SKN6WVJST=5X7KqXdJ+OKcCVDEFCedJ7ArSig@mail.gmail.com>
Date: Tue, 7 Jan 2025 21:22:15 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
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, Jan 7, 2025 at 9:11 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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:

Not sure I follow ?

>         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 :(

We might restrict this stuff to cleanup_net() caller only, we know the
netns are disappearing
and that no other thread can mess with them.