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

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Message-ID: <e87a5ba3-956e-401e-9a1e-fc40dadf3d87@davidwei.uk>
Date: Tue, 1 Jul 2025 15:26:07 -0700
From: David Wei <dw@...idwei.uk>
To: Breno Leitao <leitao@...ian.org>, Jakub Kicinski <kuba@...nel.org>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH net-next] netdevsim: implement peer queue flow control

On 2025-07-01 11:10, Breno Leitao wrote:
> Add flow control mechanism between paired netdevsim devices to stop the
> TX queue during high traffic scenarios. When a receive queue becomes
> congested (approaching NSIM_RING_SIZE limit), the corresponding transmit
> queue on the peer device is stopped using netif_subqueue_try_stop().
> 
> Once the receive queue has sufficient capacity again, the peer's
> transmit queue is resumed with netif_tx_wake_queue().
> 
> Key changes:
>    * Add nsim_stop_peer_tx_queue() to pause peer TX when RX queue is full
>    * Add nsim_start_peer_tx_queue() to resume peer TX when RX queue drains
>    * Implement queue mapping validation to ensure TX/RX queue count match
>    * Wake all queues during device unlinking to prevent stuck queues
>    * Use RCU protection when accessing peer device references
> 
> The flow control only activates when devices have matching TX/RX queue
> counts to ensure proper queue mapping.
> 
> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> ---
>   drivers/net/netdevsim/bus.c    | 16 +++++++++++
>   drivers/net/netdevsim/netdev.c | 62 ++++++++++++++++++++++++++++++++++++++++--
>   2 files changed, 76 insertions(+), 2 deletions(-)
> 
[...]> diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
> index e36d3e846c2dc..43f31bc134b0a 100644
> --- a/drivers/net/netdevsim/netdev.c
> +++ b/drivers/net/netdevsim/netdev.c
> @@ -351,6 +406,9 @@ static int nsim_rcv(struct nsim_rq *rq, int budget)
>   			dev_dstats_rx_dropped(dev);
>   	}
>   
> +	rcu_read_lock();
> +	nsim_start_peer_tx_queue(dev, rq);
> +	rcu_read_unlock();

Could the rcu_read_{un}lock() be moved into the
nsim_start/stop_peer_tx_queue() functions to keep it together with
rcu_dereference()?

>   	return i;
>   }
>   
> 
> ---
> base-commit: f6e98f17ad6829c48573952ede3f52ed00c1377f
> change-id: 20250630-netdev_flow_control-2b2d37965377
> 
> Best regards,
> --
> Breno Leitao <leitao@...ian.org>
>