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Message-Id: <20250711-netdev_flow_control-v3-1-aa1d5a155762@debian.org>
Date: Fri, 11 Jul 2025 10:06:59 -0700
From: Breno Leitao <leitao@...ian.org>
To: 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, dw@...idwei.uk,
kernel-team@...a.com, Breno Leitao <leitao@...ian.org>
Subject: [PATCH net-next v3] 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: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Breno Leitao <leitao@...ian.org>
---
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 <leitao@...ian.org>
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