Commit ff2ac4df authored by Breno Leitao's avatar Breno Leitao Committed by Jakub Kicinski
Browse files

netdevsim: implement peer queue flow control 



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
  * wake the queues when changing the queue numbers
  * Remove IFF_NO_QUEUE given it will enqueue packets now

The flow control only activates when devices have matching TX/RX queue
counts to ensure proper queue mapping.

Suggested-by: default avatarJakub Kicinski <kuba@kernel.org>
Signed-off-by: default avatarBreno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250711-netdev_flow_control-v3-1-aa1d5a155762@debian.org


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 5777d187

drivers/net/netdevsim/bus.c

+3 −0
Original line number Diff line number Diff line
@@ -384,6 +384,9 @@ static ssize_t unlink_device_store(const struct bus_type *bus, const char *buf, 
	err = 0;

	RCU_INIT_POINTER(nsim->peer, NULL);

	RCU_INIT_POINTER(peer->peer, NULL);

	synchronize_net();

	netif_tx_wake_all_queues(dev);

	netif_tx_wake_all_queues(peer->netdev);



out_put_netns:

	put_net(ns);

drivers/net/netdevsim/ethtool.c

+21 −0
Original line number Diff line number Diff line
@@ -101,6 +101,22 @@ nsim_get_channels(struct net_device *dev, struct ethtool_channels *ch) 
	ch->combined_count = ns->ethtool.channels;

}



static void

nsim_wake_queues(struct net_device *dev)

{

	struct netdevsim *ns = netdev_priv(dev);

	struct netdevsim *peer;



	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();

}



static int

nsim_set_channels(struct net_device *dev, struct ethtool_channels *ch)

{
@@ -113,6 +129,11 @@ nsim_set_channels(struct net_device *dev, struct ethtool_channels *ch) 
		return err;



	ns->ethtool.channels = ch->combined_count;



	/* Only wake up queues if devices are linked */

	if (rcu_access_pointer(ns->peer))

		nsim_wake_queues(dev);



	return 0;

}

drivers/net/netdevsim/netdev.c

+61 −7
Original line number Diff line number Diff line
@@ -37,7 +37,53 @@ MODULE_IMPORT_NS("NETDEV_INTERNAL"); 


#define NSIM_RING_SIZE		256



static int nsim_napi_rx(struct nsim_rq *rq, struct sk_buff *skb)

static void nsim_start_peer_tx_queue(struct net_device *dev, struct nsim_rq *rq)

{

	struct netdevsim *ns = netdev_priv(dev);

	struct net_device *peer_dev;

	struct netdevsim *peer_ns;

	struct netdev_queue *txq;

	u16 idx;



	idx = rq->napi.index;

	rcu_read_lock();

	peer_ns = rcu_dereference(ns->peer);

	if (!peer_ns)

		goto out;



	/* TX device */

	peer_dev = peer_ns->netdev;

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

		goto out;



	txq = netdev_get_tx_queue(peer_dev, idx);

	if (!netif_tx_queue_stopped(txq))

		goto out;



	netif_tx_wake_queue(txq);

out:

	rcu_read_unlock();

}



static void nsim_stop_tx_queue(struct net_device *tx_dev,

			       struct net_device *rx_dev,

			       struct nsim_rq *rq,

			       u16 idx)

{

	/* If different queues size, do not stop, since it is not

	 * easy to find which TX queue is mapped here

	 */

	if (rx_dev->real_num_tx_queues != tx_dev->num_rx_queues)

		return;



	/* rq is the queue on the receive side */

	netif_subqueue_try_stop(tx_dev, idx,

				NSIM_RING_SIZE - skb_queue_len(&rq->skb_queue),

				NSIM_RING_SIZE / 2);

}



static int nsim_napi_rx(struct net_device *tx_dev, struct net_device *rx_dev,

			struct nsim_rq *rq, struct sk_buff *skb)

{

	if (skb_queue_len(&rq->skb_queue) > NSIM_RING_SIZE) {

		dev_kfree_skb_any(skb);
@@ -45,13 +91,22 @@ static int nsim_napi_rx(struct nsim_rq *rq, struct sk_buff *skb) 
	}



	skb_queue_tail(&rq->skb_queue, skb);



	/* Stop the peer TX queue avoiding dropping packets later */

	if (skb_queue_len(&rq->skb_queue) >= NSIM_RING_SIZE)

		nsim_stop_tx_queue(tx_dev, rx_dev, rq,

				   skb_get_queue_mapping(skb));



	return NET_RX_SUCCESS;

}



static int nsim_forward_skb(struct net_device *dev, struct sk_buff *skb,

static int nsim_forward_skb(struct net_device *tx_dev,

			    struct net_device *rx_dev,

			    struct sk_buff *skb,

			    struct nsim_rq *rq)

{

	return __dev_forward_skb(dev, skb) ?: nsim_napi_rx(rq, skb);

	return __dev_forward_skb(rx_dev, skb) ?:

		nsim_napi_rx(tx_dev, rx_dev, rq, skb);

}



static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
@@ -86,7 +141,7 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev) 
		skb_linearize(skb);



	skb_tx_timestamp(skb);

	if (unlikely(nsim_forward_skb(peer_dev, skb, rq) == NET_RX_DROP))

	if (unlikely(nsim_forward_skb(dev, peer_dev, skb, rq) == NET_RX_DROP))

		goto out_drop_cnt;



	if (!hrtimer_active(&rq->napi_timer))
@@ -351,6 +406,7 @@ static int nsim_rcv(struct nsim_rq *rq, int budget) 
			dev_dstats_rx_dropped(dev);

	}



	nsim_start_peer_tx_queue(dev, rq);

	return i;

}
@@ -864,10 +920,8 @@ static void nsim_setup(struct net_device *dev) 
	ether_setup(dev);

	eth_hw_addr_random(dev);



	dev->tx_queue_len = 0;

	dev->flags &= ~IFF_MULTICAST;

	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE |

			   IFF_NO_QUEUE;

	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE;

	dev->features |= NETIF_F_HIGHDMA |

			 NETIF_F_SG |

			 NETIF_F_FRAGLIST |