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: Jakub Kicinski Signed-off-by: Breno Leitao --- Changes in v3: - Wake the queues if we need to in nsim_set_channels() (Jakub) - Remove IFF_NO_QUEUE (Jakub) - Improve function parameters (Jakub) - Link to v2: https://lore.kernel.org/r/20250703-netdev_flow_control-v2-1-ab00341c9cc1@debian.org Changes in v2: - Move the RCU locks inside the function (David) - Use better helpers for waking up all the queues (Jakub) - Link to v1: https://lore.kernel.org/r/20250701-netdev_flow_control-v1-1-240329fc91b1@debian.org --- drivers/net/netdevsim/bus.c | 3 ++ drivers/net/netdevsim/ethtool.c | 21 +++++++++++++ drivers/net/netdevsim/netdev.c | 68 ++++++++++++++++++++++++++++++++++++----- 3 files changed, 85 insertions(+), 7 deletions(-) diff --git a/drivers/net/netdevsim/bus.c b/drivers/net/netdevsim/bus.c index 64c0cdd31bf85..1ba52471f3fbc 100644 --- a/drivers/net/netdevsim/bus.c +++ b/drivers/net/netdevsim/bus.c @@ -366,6 +366,9 @@ 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); diff --git a/drivers/net/netdevsim/ethtool.c b/drivers/net/netdevsim/ethtool.c index 4d191a3293c74..f631d90c428ac 100644 --- a/drivers/net/netdevsim/ethtool.c +++ b/drivers/net/netdevsim/ethtool.c @@ -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; } diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c index e36d3e846c2dc..48da5d4b3a89a 100644 --- a/drivers/net/netdevsim/netdev.c +++ b/drivers/net/netdevsim/netdev.c @@ -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 | --- base-commit: 5fa1fa9184ef1af929ae49c715d4a25667e84df5 change-id: 20250630-netdev_flow_control-2b2d37965377 Best regards, -- Breno Leitao