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

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Message-ID: <20250708182718.29c4ae45@kernel.org>
Date: Tue, 8 Jul 2025 18:27:18 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Breno Leitao <leitao@...ian.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 Thu, 03 Jul 2025 06:09:31 -0700 Breno Leitao wrote:
> +static int nsim_napi_rx(struct net_device *dev, struct nsim_rq *rq,
> +			struct sk_buff *skb)
>  {
>  	if (skb_queue_len(&rq->skb_queue) > NSIM_RING_SIZE) {
> +		nsim_stop_peer_tx_queue(dev, rq, skb_get_queue_mapping(skb));
>  		dev_kfree_skb_any(skb);
>  		return NET_RX_DROP;
>  	}

we should probably add:

	if (skb_queue_len(&rq->skb_queue) > NSIM_RING_SIZE)
		nsim_stop_tx_queue(dev, rq, skb_get_queue_mapping(skb));

after enqueuing the skb, so that we stop the queue before any drops
happen

> @@ -51,7 +109,7 @@ static int nsim_napi_rx(struct nsim_rq *rq, struct sk_buff *skb)
>  static int nsim_forward_skb(struct net_device *dev, struct sk_buff *skb,
>  			    struct nsim_rq *rq)
>  {
> -	return __dev_forward_skb(dev, skb) ?: nsim_napi_rx(rq, skb);
> +	return __dev_forward_skb(dev, skb) ?: nsim_napi_rx(dev, rq, skb);
>  }
>  
>  static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)

nsim_start_xmit() has both dev and peer_dev, pass them all the way to
nsim_stop_peer_tx_queue() so that you don't have to try to dereference
the peer again.

> +	if (dev->real_num_tx_queues != peer_dev->num_rx_queues)

given that we compare real_num_tx_queues I think we should also kick
the queues in nsim_set_channels(), like we do in unlink_device_store()
-- 
pw-bot: cr