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Message-Id: <20250701-netdev_flow_control-v1-1-240329fc91b1@debian.org>
Date: Tue, 01 Jul 2025 11:10:56 -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] 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
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/bus.c b/drivers/net/netdevsim/bus.c
index 64c0cdd31bf85..44f3e2f673c16 100644
--- a/drivers/net/netdevsim/bus.c
+++ b/drivers/net/netdevsim/bus.c
@@ -323,6 +323,19 @@ static ssize_t link_device_store(const struct bus_type *bus, const char *buf, si
}
static BUS_ATTR_WO(link_device);
+static void nsim_wake_queue(struct net_device *net, struct netdev_queue *txq, void *unused)
+{
+ if (!(netif_tx_queue_stopped(txq)))
+ return;
+ netif_tx_wake_queue(txq);
+}
+
+static void nsim_wake_all_queues(struct net_device *dev, struct net_device *peer_dev)
+{
+ netdev_for_each_tx_queue(peer_dev, nsim_wake_queue, NULL);
+ netdev_for_each_tx_queue(dev, nsim_wake_queue, NULL);
+}
+
static ssize_t unlink_device_store(const struct bus_type *bus, const char *buf, size_t count)
{
struct netdevsim *nsim, *peer;
@@ -367,6 +380,9 @@ static ssize_t unlink_device_store(const struct bus_type *bus, const char *buf,
RCU_INIT_POINTER(nsim->peer, NULL);
RCU_INIT_POINTER(peer->peer, NULL);
+ synchronize_net();
+ nsim_wake_all_queues(dev, peer->netdev);
+
out_put_netns:
put_net(ns);
rtnl_unlock();
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
@@ -37,9 +37,64 @@ 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;
+ peer_ns = rcu_dereference(ns->peer);
+ if (!peer_ns)
+ return;
+
+ /* TX device */
+ peer_dev = peer_ns->netdev;
+
+ if (dev->real_num_tx_queues != peer_dev->num_rx_queues)
+ return;
+
+ txq = netdev_get_tx_queue(peer_dev, idx);
+ if (!(netif_tx_queue_stopped(txq)))
+ return;
+
+ netif_tx_wake_queue(txq);
+}
+
+static void nsim_stop_peer_tx_queue(struct net_device *dev, struct nsim_rq *rq,
+ u16 idx)
+{
+ struct netdevsim *ns = netdev_priv(dev);
+ struct net_device *peer_dev;
+ struct netdevsim *peer_ns;
+
+ peer_ns = rcu_dereference(ns->peer);
+ if (!peer_ns)
+ return;
+
+ /* TX device */
+ peer_dev = peer_ns->netdev;
+
+ /* If different queues size, do not stop, since it is not
+ * easy to find which TX queue is mapped here
+ */
+ if (dev->real_num_tx_queues != peer_dev->num_rx_queues)
+ return;
+
+ netif_subqueue_try_stop(peer_dev, idx,
+ NSIM_RING_SIZE - skb_queue_len(&rq->skb_queue),
+ NSIM_RING_SIZE / 2);
+}
+
+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) {
+ rcu_read_lock();
+ nsim_stop_peer_tx_queue(dev, rq, skb_get_queue_mapping(skb));
+ rcu_read_unlock();
dev_kfree_skb_any(skb);
return NET_RX_DROP;
}
@@ -51,7 +106,7 @@ 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)
@@ -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();
return i;
}
---
base-commit: f6e98f17ad6829c48573952ede3f52ed00c1377f
change-id: 20250630-netdev_flow_control-2b2d37965377
Best regards,
--
Breno Leitao <leitao@...ian.org>
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