netdev - Re: [PATCH net-next v2] netdevsim: implement peer queue flow control

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Message-ID: <p22efbdqaaypgp7wu4csohhtzowpgrzrtelev7waumidabryty@lq4txzalfbfl>
Date: Fri, 11 Jul 2025 09:29:38 -0700
From: Breno Leitao <leitao@...ian.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel-team@...a.com, dw@...idwei.uk
Subject: Re: [PATCH net-next v2] netdevsim: implement peer queue flow control

On Wed, Jul 09, 2025 at 02:36:27PM -0700, Jakub Kicinski wrote:
> On Wed, 9 Jul 2025 03:34:20 -0700 Breno Leitao wrote:
> > 	+
> > 	+	synchronize_net();
> > 	+       netif_tx_wake_all_queues(dev);
> > 	+       rcu_read_lock();
> > 	+       peer = rcu_dereference(ns->peer);
> > 	+       if (peer)
> > 	+               netif_tx_wake_all_queues(peer->netdev);
> > 	+       rcu_read_unlock();
> 
> That's sufficiently orthogonal to warrant a dedicated function / helper.
> 
> In terms of code I think we can skip the whole dance if peer is NULL?

Sure. We can use rcu_access_pointer() to check if the value is set, and
then get into the slow path.

       if (rcu_access_pointer(ns->peer))
               nsim_wake_queues(dev);

> > Also, with this patch, we will eventually get the following critical
> > message:
> > 
> > 	net_crit_ratelimited("Virtual device %s asks to queue packet!\n", dev->name);
> > 
> > I am wondering if that alert is not valid anymore, and I can simply
> > remove it.
> 
> Ah. In nsim_setup() we should remove IFF_NO_QUEUE and stop setting
> tx_queue_len to 0

That makes sense, thanks!
--breno