[PATCH net-next v3 0/5] veth: add Byte Queue Limits (BQL) support

[PATCH net-next v3 0/5] veth: add Byte Queue Limits (BQL) support

[hawk]

From: Jesper Dangaard Brouer <hawk@kernel.org>

This series adds BQL (Byte Queue Limits) to the veth driver, reducing
latency by dynamically limiting in-flight packets in the ptr_ring and
moving buffering into the qdisc where AQM algorithms can act on it.

Problem:
  veth's 256-entry ptr_ring acts as a "dark buffer" -- packets queued
  there are invisible to the qdisc's AQM.  Under load, the ring fills
  completely (DRV_XOFF backpressure), adding up to 256 packets of
  unmanaged latency before the qdisc even sees congestion.

Solution:
  BQL (STACK_XOFF) dynamically limits in-flight packets, stopping the
  queue before the ring fills.  This keeps the ring shallow and pushes
  excess packets into the qdisc, where sojourn-based AQM can measure
  and drop them.

  Test setup: veth pair, UDP flood, 13000 iptables rules in consumer
  namespace (slows NAPI-64 cycle to ~6-7ms), ping measures RTT under load.

                   BQL off                    BQL on
  fq_codel:  RTT ~22ms, 4% loss         RTT ~1.3ms, 0% loss
  sfq:       RTT ~24ms, 0% loss         RTT ~1.5ms, 0% loss

  BQL reduces ping RTT by ~17x for both qdiscs.  Consumer throughput
  is unchanged (~10K pps) -- BQL adds no overhead.

CoDel bug discovered during BQL development:
  Our original motivation for BQL was fq_codel ping loss observed under
  load (4-26% depending on NAPI cycle time).  Investigating this led us
  to discover a bug in the CoDel implementation: codel_dequeue() does
  not reset vars->first_above_time when a flow goes empty, contrary to
  the reference algorithm.  This causes stale CoDel state to persist
  across empty periods in fq_codel's per-flow queues, penalizing sparse
  flows like ICMP ping.  A fix for this has been applied to the net tree:
    https://git.kernel.org/netdev/net/c/815980fe6dbb

  BQL remains valuable independently: it reduces RTT by ~17x by moving
  buffering from the dark ptr_ring into the qdisc.  Additionally, BQL
  clears STACK_XOFF per-SKB as each packet completes, rather than
  batch-waking after 64 packets (DRV_XOFF).  This keeps sojourn times
  below fq_codel's target, preventing CoDel from entering dropping
  state on non-congested flows in the first place.

Key design decisions:
  - Charge-under-lock in veth_xdp_rx(): The BQL charge must precede
    the ptr_ring produce, because the NAPI consumer can run on another
    CPU and complete the SKB immediately after it becomes visible.  To
    avoid a pre-charge/undo pattern, the charge is done under the
    ptr_ring producer_lock after confirming the ring is not full.  BQL
    is only charged when produce is guaranteed to succeed, keeping
    num_queued monotonically increasing.  HARD_TX_LOCK already
    serializes dql_queued() (veth requires a qdisc for BQL); the
    ptr_ring lock additionally would allow noqueue to work correctly.

  - Per-SKB BQL tracking via pointer tag: A VETH_BQL_FLAG bit in the
    ptr_ring pointer records whether each SKB was BQL-charged.  This is
    necessary because the qdisc can be replaced live (noqueue->sfq or
    vice versa) while SKBs are in-flight -- the completion side must
    know the charge state that was decided at enqueue time.

  - IFF_NO_QUEUE + BQL coexistence: A new dev->bql flag enables BQL
    sysfs exposure for IFF_NO_QUEUE devices that opt in to DQL
    accounting, without changing IFF_NO_QUEUE semantics.

Background and acknowledgments:
  Mike Freemon reported the veth dark buffer problem internally at
  Cloudflare and showed that recompiling the kernel with a ptr_ring
  size of 30 (down from 256) made fq_codel work dramatically better.
  This was the primary motivation for a proper BQL solution that
  achieves the same effect dynamically without a kernel rebuild.

  Chris Arges wrote a reproducer for the dark buffer latency problem:
    https://github.com/netoptimizer/veth-backpressure-performance-testing
  This is where we first observed ping packets being dropped under
  fq_codel, which became our secondary motivation for BQL.  In
  production we switched to SFQ on veth devices as a workaround.

  Jonas Koeppeler provided extensive testing and code review.
  Together we discovered that the fq_codel ping loss was actually a
  12-year-old CoDel bug (stale first_above_time in empty flows), not
  caused by the dark buffer itself.  A fix has been applied to the net tree:
    https://git.kernel.org/netdev/net/c/815980fe6dbb

Patch overview:
  1. net: add dev->bql flag to allow BQL sysfs for IFF_NO_QUEUE devices
  2. veth: implement Byte Queue Limits (BQL) for latency reduction
  3. veth: add tx_timeout watchdog as BQL safety net
  4. net: sched: add timeout count to NETDEV WATCHDOG message
  5. selftests: net: add veth BQL stress test

Jesper Dangaard Brouer (5):
  net: add dev->bql flag to allow BQL sysfs for IFF_NO_QUEUE devices
  veth: implement Byte Queue Limits (BQL) for latency reduction
  veth: add tx_timeout watchdog as BQL safety net
  net: sched: add timeout count to NETDEV WATCHDOG message
  selftests: net: add veth BQL stress test

 .../networking/net_cachelines/net_device.rst  |   1 +
 drivers/net/veth.c                            |  94 +-
 include/linux/netdevice.h                     |   2 +
 net/core/net-sysfs.c                          |   8 +-
 net/sched/sch_generic.c                       |   8 +-
 tools/testing/selftests/net/Makefile          |   3 +
 tools/testing/selftests/net/config            |   1 +
 tools/testing/selftests/net/napi_poll_hist.bt |  40 +
 tools/testing/selftests/net/veth_bql_test.sh  | 821 ++++++++++++++++++
 .../selftests/net/veth_bql_test_virtme.sh     | 124 +++
 10 files changed, 1086 insertions(+), 16 deletions(-)
 create mode 100644 tools/testing/selftests/net/napi_poll_hist.bt
 create mode 100755 tools/testing/selftests/net/veth_bql_test.sh
 create mode 100755 tools/testing/selftests/net/veth_bql_test_virtme.sh

V2: https://lore.kernel.org/all/20260413094442.1376022-1-hawk@kernel.org/

Changes since V2:
  - Patch 2 (veth BQL): fix syzbot WARNING in veth_napi_del_range():
    clamp BQL reset loop to peer's real_num_tx_queues.  The loop was
    iterating dev->real_num_rx_queues but indexing peer's txq[], which
    goes out of bounds when the peer has fewer TX queues (e.g. veth
    enslaved to a bond with XDP attached).
  - Patch 5 (selftests): fix CONFIG_NET_SCH_SFQ alphabetical ordering
    in net/config (Breno Leitao).

V1: https://lore.kernel.org/all/20260324174719.1224337-1-hawk@kernel.org/

Changes since V1:
  - Patch 1 (dev->bql flag): add kdoc entry for @bql in struct net_device.
  - Patch 2 (veth BQL): charge fixed VETH_BQL_UNIT (1) per packet instead
    of skb->len.  veth has no link speed; the ptr_ring is packet-indexed.
    Byte-based charging lets small packets sneak many entries into the ring.
    Testing: min-size packet flood causes 3.7x ping RTT degradation with
    skb->len vs no change with fixed-unit charging.
  - Patch 3 (tx_timeout watchdog): fix race with peer NAPI: replace
    netdev_tx_reset_queue() with clear_bit(STACK_XOFF) + netif_tx_wake_queue()
    to avoid dql_reset() racing with concurrent dql_completed().
  - Patch 5 (selftests): fix shellcheck warnings and infos:
    - Quote variables passed to kill_process and exit.
    - Declare and assign local variables separately (SC2155).
    - Use read -r to avoid mangling backslashes (SC2162).
    - Add shellcheck disable comments for intentional word splitting
      (set -- $line, tc $qdisc $opts) and indirect invocation (trap).
    - Make iptables-restore failure a hard FAIL instead of continuing.
    - Add veth_bql_test.sh to TEST_PROGS in net/Makefile.
    - Add veth_bql_test_virtme.sh to TEST_FILES (needs kernel build tree).
    - Add napi_poll_hist.bt to TEST_FILES in net/Makefile.
    - Add CONFIG_NET_SCH_SFQ=m to net/config (default qdisc is sfq).
    - Reduce default test duration from 300s to 30s for kselftest CI.
    - Fix virtme wrapper: empty args bug, check vmlinux instead of test path.
  - Cover letter: update CoDel fix reference to merged commit in net tree.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: Chris Arges <carges@cloudflare.com>
Cc: Mike Freemon <mfreemon@cloudflare.com>
Cc: Toke Høiland-Jørgensen <toke@toke.dk>
Cc: Jonas Köppeler <j.koeppeler@tu-berlin.de>
Cc: Breno Leitao <leitao@debian.org>
Cc: kernel-team@cloudflare.com

-- 
2.43.0

[PATCH net-next v3 1/5] net: add dev->bql flag to allow BQL sysfs for IFF_NO_QUEUE devices

[hawk]

From: Jesper Dangaard Brouer <hawk@kernel.org>

Virtual devices with IFF_NO_QUEUE or lltx are excluded from BQL sysfs
by netdev_uses_bql(), since they traditionally lack real hardware
queues. However, some virtual devices like veth implement a real
ptr_ring FIFO with NAPI processing and benefit from BQL to limit
in-flight bytes and reduce latency.

Add a per-device 'bql' bitfield boolean in the priv_flags_slow section
of struct net_device. When set, it overrides the IFF_NO_QUEUE/lltx
exclusion and exposes BQL sysfs entries (/sys/class/net/<dev>/queues/
tx-<n>/byte_queue_limits/). The flag is still gated on CONFIG_BQL.

This allows drivers that use BQL despite being IFF_NO_QUEUE to opt in
to sysfs visibility for monitoring and debugging.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Tested-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
---
 Documentation/networking/net_cachelines/net_device.rst | 1 +
 include/linux/netdevice.h                              | 2 ++
 net/core/net-sysfs.c                                   | 8 +++++++-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/net_cachelines/net_device.rst b/Documentation/networking/net_cachelines/net_device.rst
index 1c19bb7705df..b775d3235a2d 100644
--- a/Documentation/networking/net_cachelines/net_device.rst
+++ b/Documentation/networking/net_cachelines/net_device.rst
@@ -170,6 +170,7 @@ unsigned_long:1                     see_all_hwtstamp_requests
 unsigned_long:1                     change_proto_down
 unsigned_long:1                     netns_immutable
 unsigned_long:1                     fcoe_mtu
+unsigned_long:1                     bql                                                                 netdev_uses_bql(net-sysfs.c)
 struct list_head                    net_notifier_list
 struct macsec_ops*                  macsec_ops
 struct udp_tunnel_nic_info*         udp_tunnel_nic_info
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 47417b2d48a4..7a1a491ecdd5 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2048,6 +2048,7 @@ enum netdev_reg_state {
  *	@change_proto_down: device supports setting carrier via IFLA_PROTO_DOWN
  *	@netns_immutable: interface can't change network namespaces
  *	@fcoe_mtu:	device supports maximum FCoE MTU, 2158 bytes
+ *	@bql:		device uses BQL (DQL sysfs) despite having IFF_NO_QUEUE
  *
  *	@net_notifier_list:	List of per-net netdev notifier block
  *				that follow this device when it is moved
@@ -2462,6 +2463,7 @@ struct net_device {
 	unsigned long		change_proto_down:1;
 	unsigned long		netns_immutable:1;
 	unsigned long		fcoe_mtu:1;
+	unsigned long		bql:1;
 
 	struct list_head	net_notifier_list;
 
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index e430645748a7..4360efc8f241 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1945,10 +1945,16 @@ static const struct kobj_type netdev_queue_ktype = {
 
 static bool netdev_uses_bql(const struct net_device *dev)
 {
+	if (!IS_ENABLED(CONFIG_BQL))
+		return false;
+
+	if (dev->bql)
+		return true;
+
 	if (dev->lltx || (dev->priv_flags & IFF_NO_QUEUE))
 		return false;
 
-	return IS_ENABLED(CONFIG_BQL);
+	return true;
 }
 
 static int netdev_queue_add_kobject(struct net_device *dev, int index)
-- 
2.43.0

[PATCH net-next v3 2/5] veth: implement Byte Queue Limits (BQL) for latency reduction

[hawk]

From: Jesper Dangaard Brouer <hawk@kernel.org>

Commit dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to
reduce TX drops") gave qdiscs control over veth by returning
NETDEV_TX_BUSY when the ptr_ring is full (DRV_XOFF).  That commit noted
a known limitation: the 256-entry ptr_ring sits in front of the qdisc as
a dark buffer, adding base latency because the qdisc has no visibility
into how many bytes are already queued there.

Add BQL support so the qdisc gets feedback and can begin shaping traffic
before the ring fills.  In testing with fq_codel, BQL reduces ping RTT
under UDP load from ~6.61ms to ~0.36ms (18x).

Charge a fixed VETH_BQL_UNIT (1) per packet rather than skb->len, so
the DQL limit tracks packets-in-flight.  Unlike a physical NIC, veth
has no link speed -- the ptr_ring drains at CPU speed and is
packet-indexed, not byte-indexed, so bytes are not the natural unit.
With byte-based charging, small packets sneak many more entries into
the ring before STACK_XOFF fires, deepening the dark buffer under
mixed-size workloads.  Testing with a concurrent min-size packet flood
shows 3.7x ping RTT degradation with skb->len charging versus no
change with fixed-unit charging.

Charge BQL inside veth_xdp_rx() under the ptr_ring producer_lock, after
confirming the ring is not full.  The charge must precede the produce
because the NAPI consumer can run on another CPU and complete the SKB
the instant it becomes visible in the ring.  Doing both under the same
lock avoids a pre-charge/undo pattern -- BQL is only charged when
produce is guaranteed to succeed.

BQL is enabled only when a real qdisc is attached (guarded by
!qdisc_txq_has_no_queue), as HARD_TX_LOCK provides serialization
for TXQ modification like dql_queued(). For lltx devices, like veth,
this HARD_TX_LOCK serialization isn't provided.  The ptr_ring
producer_lock provides additional serialization that would allow
BQL to work correctly even with noqueue, though that combination
is not currently enabled, as the netstack will drop and warn.

Track per-SKB BQL state via a VETH_BQL_FLAG pointer tag in the ptr_ring
entry.  This is necessary because the qdisc can be replaced live while
SKBs are in-flight -- each SKB must carry the charge decision made at
enqueue time rather than re-checking the peer's qdisc at completion.

Complete per-SKB in veth_xdp_rcv() rather than in bulk, so STACK_XOFF
clears promptly when producer and consumer run on different CPUs.

BQL introduces a second independent queue-stop mechanism (STACK_XOFF)
alongside the existing DRV_XOFF (ring full).  Both must be clear for
the queue to transmit.  Reset BQL state in veth_napi_del_range() after
synchronize_net() to avoid racing with in-flight veth_poll() calls.
Clamp the reset loop to the peer's real_num_tx_queues, since the peer
may have fewer TX queues than the local device has RX queues (e.g. when
veth is enslaved to a bond with XDP attached).

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
---
 drivers/net/veth.c | 76 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 65 insertions(+), 11 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index e35df717e65e..3de25ba34a90 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -34,9 +34,13 @@
 #define DRV_VERSION	"1.0"
 
 #define VETH_XDP_FLAG		BIT(0)
+#define VETH_BQL_FLAG		BIT(1)
 #define VETH_RING_SIZE		256
 #define VETH_XDP_HEADROOM	(XDP_PACKET_HEADROOM + NET_IP_ALIGN)
 
+/* Fixed BQL charge: DQL limit tracks packets-in-flight, not bytes */
+#define VETH_BQL_UNIT		1
+
 #define VETH_XDP_TX_BULK_SIZE	16
 #define VETH_XDP_BATCH		16
 
@@ -280,6 +284,21 @@ static bool veth_is_xdp_frame(void *ptr)
 	return (unsigned long)ptr & VETH_XDP_FLAG;
 }
 
+static bool veth_ptr_is_bql(void *ptr)
+{
+	return (unsigned long)ptr & VETH_BQL_FLAG;
+}
+
+static struct sk_buff *veth_ptr_to_skb(void *ptr)
+{
+	return (void *)((unsigned long)ptr & ~VETH_BQL_FLAG);
+}
+
+static void *veth_skb_to_ptr(struct sk_buff *skb, bool bql)
+{
+	return bql ? (void *)((unsigned long)skb | VETH_BQL_FLAG) : skb;
+}
+
 static struct xdp_frame *veth_ptr_to_xdp(void *ptr)
 {
 	return (void *)((unsigned long)ptr & ~VETH_XDP_FLAG);
@@ -295,7 +314,7 @@ static void veth_ptr_free(void *ptr)
 	if (veth_is_xdp_frame(ptr))
 		xdp_return_frame(veth_ptr_to_xdp(ptr));
 	else
-		kfree_skb(ptr);
+		kfree_skb(veth_ptr_to_skb(ptr));
 }
 
 static void __veth_xdp_flush(struct veth_rq *rq)
@@ -309,19 +328,33 @@ static void __veth_xdp_flush(struct veth_rq *rq)
 	}
 }
 
-static int veth_xdp_rx(struct veth_rq *rq, struct sk_buff *skb)
+static int veth_xdp_rx(struct veth_rq *rq, struct sk_buff *skb, bool do_bql,
+		       struct netdev_queue *txq)
 {
-	if (unlikely(ptr_ring_produce(&rq->xdp_ring, skb)))
+	struct ptr_ring *ring = &rq->xdp_ring;
+
+	spin_lock(&ring->producer_lock);
+	if (unlikely(!ring->size) || __ptr_ring_full(ring)) {
+		spin_unlock(&ring->producer_lock);
 		return NETDEV_TX_BUSY; /* signal qdisc layer */
+	}
+
+	/* BQL charge before produce; consumer cannot see entry yet */
+	if (do_bql)
+		netdev_tx_sent_queue(txq, VETH_BQL_UNIT);
+
+	__ptr_ring_produce(ring, veth_skb_to_ptr(skb, do_bql));
+	spin_unlock(&ring->producer_lock);
 
 	return NET_RX_SUCCESS; /* same as NETDEV_TX_OK */
 }
 
 static int veth_forward_skb(struct net_device *dev, struct sk_buff *skb,
-			    struct veth_rq *rq, bool xdp)
+			    struct veth_rq *rq, bool xdp, bool do_bql,
+			    struct netdev_queue *txq)
 {
 	return __dev_forward_skb(dev, skb) ?: xdp ?
-		veth_xdp_rx(rq, skb) :
+		veth_xdp_rx(rq, skb, do_bql, txq) :
 		__netif_rx(skb);
 }
 
@@ -352,6 +385,7 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct net_device *rcv;
 	int length = skb->len;
 	bool use_napi = false;
+	bool do_bql = false;
 	int ret, rxq;
 
 	rcu_read_lock();
@@ -375,8 +409,11 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	skb_tx_timestamp(skb);
+	txq = netdev_get_tx_queue(dev, rxq);
 
-	ret = veth_forward_skb(rcv, skb, rq, use_napi);
+	/* BQL charge happens inside veth_xdp_rx() under producer_lock */
+	do_bql = use_napi && !qdisc_txq_has_no_queue(txq);
+	ret = veth_forward_skb(rcv, skb, rq, use_napi, do_bql, txq);
 	switch (ret) {
 	case NET_RX_SUCCESS: /* same as NETDEV_TX_OK */
 		if (!use_napi)
@@ -388,8 +425,6 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 		/* If a qdisc is attached to our virtual device, returning
 		 * NETDEV_TX_BUSY is allowed.
 		 */
-		txq = netdev_get_tx_queue(dev, rxq);
-
 		if (qdisc_txq_has_no_queue(txq)) {
 			dev_kfree_skb_any(skb);
 			goto drop;
@@ -412,6 +447,7 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 		net_crit_ratelimited("%s(%s): Invalid return code(%d)",
 				     __func__, dev->name, ret);
 	}
+
 	rcu_read_unlock();
 
 	return ret;
@@ -900,7 +936,8 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 
 static int veth_xdp_rcv(struct veth_rq *rq, int budget,
 			struct veth_xdp_tx_bq *bq,
-			struct veth_stats *stats)
+			struct veth_stats *stats,
+			struct netdev_queue *peer_txq)
 {
 	int i, done = 0, n_xdpf = 0;
 	void *xdpf[VETH_XDP_BATCH];
@@ -928,9 +965,13 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
 			}
 		} else {
 			/* ndo_start_xmit */
-			struct sk_buff *skb = ptr;
+			bool bql_charged = veth_ptr_is_bql(ptr);
+			struct sk_buff *skb = veth_ptr_to_skb(ptr);
 
 			stats->xdp_bytes += skb->len;
+			if (peer_txq && bql_charged)
+				netdev_tx_completed_queue(peer_txq, 1, VETH_BQL_UNIT);
+
 			skb = veth_xdp_rcv_skb(rq, skb, bq, stats);
 			if (skb) {
 				if (skb_shared(skb) || skb_unclone(skb, GFP_ATOMIC))
@@ -975,7 +1016,7 @@ static int veth_poll(struct napi_struct *napi, int budget)
 	peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : NULL;
 
 	xdp_set_return_frame_no_direct();
-	done = veth_xdp_rcv(rq, budget, &bq, &stats);
+	done = veth_xdp_rcv(rq, budget, &bq, &stats, peer_txq);
 
 	if (stats.xdp_redirect > 0)
 		xdp_do_flush();
@@ -1073,6 +1114,7 @@ static int __veth_napi_enable(struct net_device *dev)
 static void veth_napi_del_range(struct net_device *dev, int start, int end)
 {
 	struct veth_priv *priv = netdev_priv(dev);
+	struct net_device *peer;
 	int i;
 
 	for (i = start; i < end; i++) {
@@ -1091,6 +1133,17 @@ static void veth_napi_del_range(struct net_device *dev, int start, int end)
 		ptr_ring_cleanup(&rq->xdp_ring, veth_ptr_free);
 	}
 
+	/* Reset BQL on peer's txqs: remaining ring items were freed above
+	 * without BQL completion, so DQL state must be reset.
+	 */
+	peer = rtnl_dereference(priv->peer);
+	if (peer) {
+		int peer_end = min(end, (int)peer->real_num_tx_queues);
+
+		for (i = start; i < peer_end; i++)
+			netdev_tx_reset_queue(netdev_get_tx_queue(peer, i));
+	}
+
 	for (i = start; i < end; i++) {
 		page_pool_destroy(priv->rq[i].page_pool);
 		priv->rq[i].page_pool = NULL;
@@ -1740,6 +1793,7 @@ static void veth_setup(struct net_device *dev)
 	dev->priv_flags |= IFF_PHONY_HEADROOM;
 	dev->priv_flags |= IFF_DISABLE_NETPOLL;
 	dev->lltx = true;
+	dev->bql = true;
 
 	dev->netdev_ops = &veth_netdev_ops;
 	dev->xdp_metadata_ops = &veth_xdp_metadata_ops;
-- 
2.43.0

[PATCH net-next v3 3/5] veth: add tx_timeout watchdog as BQL safety net

[hawk]

From: Jesper Dangaard Brouer <hawk@kernel.org>

With the introduction of BQL (Byte Queue Limits) for veth, there are
now two independent mechanisms that can stop a transmit queue:

 - DRV_XOFF: set by netif_tx_stop_queue() when the ptr_ring is full
 - STACK_XOFF: set by BQL when the byte-in-flight limit is reached

If either mechanism stalls without a corresponding wake/completion,
the queue stops permanently. Enable the net device watchdog timer and
implement ndo_tx_timeout as a failsafe recovery.

The timeout handler resets BQL state (clearing STACK_XOFF) and wakes
the queue (clearing DRV_XOFF), covering both stop mechanisms. The
watchdog fires after 16 seconds, which accommodates worst-case NAPI
processing (budget=64 packets x 250ms per-packet consumer delay)
without false positives under normal backpressure.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Tested-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
---
 drivers/net/veth.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 3de25ba34a90..9d7b085c9548 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -1431,6 +1431,22 @@ static int veth_set_channels(struct net_device *dev,
 	goto out;
 }
 
+static void veth_tx_timeout(struct net_device *dev, unsigned int txqueue)
+{
+	struct netdev_queue *txq = netdev_get_tx_queue(dev, txqueue);
+
+	netdev_err(dev,
+		   "veth backpressure(0x%lX) stalled(n:%ld) TXQ(%u) re-enable\n",
+		   txq->state, atomic_long_read(&txq->trans_timeout), txqueue);
+
+	/* Cannot call netdev_tx_reset_queue(): dql_reset() races with
+	 * peer NAPI calling dql_completed() concurrently.
+	 * Just clear the stop bits; the qdisc will re-stop if still stuck.
+	 */
+	clear_bit(__QUEUE_STATE_STACK_XOFF, &txq->state);
+	netif_tx_wake_queue(txq);
+}
+
 static int veth_open(struct net_device *dev)
 {
 	struct veth_priv *priv = netdev_priv(dev);
@@ -1769,6 +1785,7 @@ static const struct net_device_ops veth_netdev_ops = {
 	.ndo_bpf		= veth_xdp,
 	.ndo_xdp_xmit		= veth_ndo_xdp_xmit,
 	.ndo_get_peer_dev	= veth_peer_dev,
+	.ndo_tx_timeout		= veth_tx_timeout,
 };
 
 static const struct xdp_metadata_ops veth_xdp_metadata_ops = {
@@ -1808,6 +1825,7 @@ static void veth_setup(struct net_device *dev)
 	dev->priv_destructor = veth_dev_free;
 	dev->pcpu_stat_type = NETDEV_PCPU_STAT_TSTATS;
 	dev->max_mtu = ETH_MAX_MTU;
+	dev->watchdog_timeo = msecs_to_jiffies(16000);
 
 	dev->hw_features = VETH_FEATURES;
 	dev->hw_enc_features = VETH_FEATURES;
-- 
2.43.0

[PATCH net-next v3 4/5] net: sched: add timeout count to NETDEV WATCHDOG message

[hawk]

From: Jesper Dangaard Brouer <hawk@kernel.org>

Add the per-queue timeout counter (trans_timeout) to the core NETDEV
WATCHDOG log message.  This makes it easy to determine how frequently
a particular queue is stalling from a single log line, without having
to search through and correlate spaced-out log entries.

Useful for production monitoring where timeouts are spaced by the
watchdog interval, making frequency hard to judge.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/all/20251107175445.58eba452@kernel.org/
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Tested-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
---
 net/sched/sch_generic.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index a93321db8fd7..3e2e2e887a86 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -533,13 +533,12 @@ static void dev_watchdog(struct timer_list *t)
 		    netif_running(dev) &&
 		    netif_carrier_ok(dev)) {
 			unsigned int timedout_ms = 0;
+			struct netdev_queue *txq;
 			unsigned int i;
 			unsigned long trans_start;
 			unsigned long oldest_start = jiffies;
 
 			for (i = 0; i < dev->num_tx_queues; i++) {
-				struct netdev_queue *txq;
-
 				txq = netdev_get_tx_queue(dev, i);
 				if (!netif_xmit_stopped(txq))
 					continue;
@@ -561,9 +560,10 @@ static void dev_watchdog(struct timer_list *t)
 
 			if (unlikely(timedout_ms)) {
 				trace_net_dev_xmit_timeout(dev, i);
-				netdev_crit(dev, "NETDEV WATCHDOG: CPU: %d: transmit queue %u timed out %u ms\n",
+				netdev_crit(dev, "NETDEV WATCHDOG: CPU: %d: transmit queue %u timed out %u ms (n:%ld)\n",
 					    raw_smp_processor_id(),
-					    i, timedout_ms);
+					    i, timedout_ms,
+					    atomic_long_read(&txq->trans_timeout));
 				netif_freeze_queues(dev);
 				dev->netdev_ops->ndo_tx_timeout(dev, i);
 				netif_unfreeze_queues(dev);
-- 
2.43.0

[PATCH net-next v3 5/5] selftests: net: add veth BQL stress test

[hawk]

From: Jesper Dangaard Brouer <hawk@kernel.org>

Add a selftest that exercises veth's BQL (Byte Queue Limits) code path
under sustained UDP load. The test creates a veth pair with GRO enabled
(activating the NAPI path and BQL), attaches a qdisc, optionally loads
iptables rules in the consumer namespace to slow NAPI processing, and
floods UDP packets for a configurable duration.

The test serves two purposes: benchmarking BQL's latency impact under
configurable load (iptables rules, qdisc type and parameters), and
detecting kernel BUG/Oops from DQL accounting mismatches. It monitors
dmesg throughout the run and reports PASS/FAIL via kselftest (lib.sh).

Diagnostic output is printed every 5 seconds:
  - BQL sysfs inflight/limit and watchdog tx_timeout counter
  - qdisc stats: packets, drops, requeues, backlog, qlen, overlimits
  - consumer PPS and NAPI-64 cycle time (shows fq_codel target impact)
  - sink PPS (per-period delta), latency min/avg/max (stddev at exit)
  - ping RTT to measure latency under load

Generating enough traffic to fill the 256-entry ptr_ring requires care:
the UDP sendto() path charges each SKB to sk_wmem_alloc, and the SKB
stays charged (via sock_wfree destructor) until the consumer NAPI thread
finishes processing it -- including any iptables rules in the receive
path. With the default sk_sndbuf (~208KB from wmem_default), only ~93
packets can be in-flight before sendto(MSG_DONTWAIT) returns EAGAIN.
Since 93 < 256 ring entries, the ring never fills and no backpressure
occurs. The test raises wmem_max via sysctl and sets SO_SNDBUF=1MB on
the flood socket to remove this bottleneck. An earlier multi-namespace
routing approach avoided this limit because ip_forward creates new SKBs
detached from the sender's socket.

The --bql-disable option (sets limit_min=1GB) enables A/B comparison.
Typical results with --nrules 6000 --qdisc-opts 'target 2ms interval 20ms':

  fq_codel + BQL disabled:  ping RTT ~10.8ms, 15% loss, 400KB in ptr_ring
  fq_codel + BQL enabled:   ping RTT ~0.6ms,   0% loss, 4KB in ptr_ring

Both cases show identical consumer speed (~20Kpps) and fq_codel drops
(~255K), proving the improvement comes purely from where packets buffer.

BQL moves buffering from the ptr_ring into the qdisc, where AQM
(fq_codel/CAKE) can act on it -- eliminating the "dark buffer" that
hides congestion from the scheduler.

The --qdisc-replace mode cycles through sfq/pfifo/fq_codel/noqueue
under active traffic to verify that stale BQL state (STACK_XOFF) is
properly handled during live qdisc transitions.

A companion wrapper (veth_bql_test_virtme.sh) launches the test inside
a virtme-ng VM, with .config validation to prevent silent stalls.

Usage:
  sudo ./veth_bql_test.sh [--duration 300] [--nrules 100]
                          [--qdisc sfq] [--qdisc-opts '...']
                          [--bql-disable] [--normal-napi]
                          [--qdisc-replace]

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Tested-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
Tested-by: Breno Leitao <leitao@debian.org>
---
 tools/testing/selftests/net/Makefile          |   3 +
 tools/testing/selftests/net/config            |   1 +
 tools/testing/selftests/net/napi_poll_hist.bt |  40 +
 tools/testing/selftests/net/veth_bql_test.sh  | 821 ++++++++++++++++++
 .../selftests/net/veth_bql_test_virtme.sh     | 124 +++
 5 files changed, 989 insertions(+)
 create mode 100644 tools/testing/selftests/net/napi_poll_hist.bt
 create mode 100755 tools/testing/selftests/net/veth_bql_test.sh
 create mode 100755 tools/testing/selftests/net/veth_bql_test_virtme.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 231245a95879..7f6524169b93 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -119,6 +119,7 @@ TEST_PROGS := \
 	udpgso_bench.sh \
 	unicast_extensions.sh \
 	veth.sh \
+	veth_bql_test.sh \
 	vlan_bridge_binding.sh \
 	vlan_hw_filter.sh \
 	vrf-xfrm-tests.sh \
@@ -196,7 +197,9 @@ TEST_FILES := \
 	fcnal-test.sh \
 	in_netns.sh \
 	lib.sh \
+	napi_poll_hist.bt \
 	settings \
+	veth_bql_test_virtme.sh \
 # end of TEST_FILES
 
 # YNL files, must be before "include ..lib.mk"
diff --git a/tools/testing/selftests/net/config b/tools/testing/selftests/net/config
index 2a390cae41bf..b53cf11232ea 100644
--- a/tools/testing/selftests/net/config
+++ b/tools/testing/selftests/net/config
@@ -101,6 +101,7 @@ CONFIG_NET_SCH_HTB=m
 CONFIG_NET_SCH_INGRESS=m
 CONFIG_NET_SCH_NETEM=y
 CONFIG_NET_SCH_PRIO=m
+CONFIG_NET_SCH_SFQ=m
 CONFIG_NET_VRF=y
 CONFIG_NF_CONNTRACK=m
 CONFIG_NF_CONNTRACK_OVS=y
diff --git a/tools/testing/selftests/net/napi_poll_hist.bt b/tools/testing/selftests/net/napi_poll_hist.bt
new file mode 100644
index 000000000000..34d1a43906bf
--- /dev/null
+++ b/tools/testing/selftests/net/napi_poll_hist.bt
@@ -0,0 +1,40 @@
+#!/usr/bin/env bpftrace
+// SPDX-License-Identifier: GPL-2.0
+// napi_poll work histogram for veth BQL testing.
+// Shows how many packets each NAPI poll processes (0..64).
+// Full-budget (64) polls mean more work is pending; partial (<64) means
+// the ring drained before the budget was exhausted.
+//
+// Usage: bpftrace napi_poll_hist.bt
+// Interval output is a single compact line for easy script parsing.
+
+tracepoint:napi:napi_poll
+/str(args->dev_name, 8) == "veth_bql"/
+{
+	@work = lhist(args->work, 0, 65, 1);
+	@total++;
+	@sum += args->work;
+	if (args->work == args->budget) {
+		@full++;
+	}
+}
+
+interval:s:5
+{
+	$avg = @total > 0 ? @sum / @total : 0;
+	printf("napi_poll: polls=%llu full_budget=%llu partial=%llu avg_work=%llu\n",
+	       @total, @full, @total - @full, $avg);
+	clear(@total);
+	clear(@full);
+	clear(@sum);
+}
+
+END
+{
+	printf("\n--- napi_poll work histogram (lifetime) ---\n");
+	print(@work);
+	clear(@work);
+	clear(@total);
+	clear(@full);
+	clear(@sum);
+}
diff --git a/tools/testing/selftests/net/veth_bql_test.sh b/tools/testing/selftests/net/veth_bql_test.sh
new file mode 100755
index 000000000000..bfbbb3432a8f
--- /dev/null
+++ b/tools/testing/selftests/net/veth_bql_test.sh
@@ -0,0 +1,821 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Veth BQL (Byte Queue Limits) stress test and A/B benchmarking tool.
+#
+# Creates a veth pair with GRO on and TSO off (ensures all packets use
+# the NAPI/ptr_ring path where BQL operates), attaches a configurable
+# qdisc, optionally loads iptables rules to slow the consumer NAPI
+# processing, and floods UDP packets at maximum rate.
+#
+# Primary uses:
+#   1) A/B comparison of latency with/without BQL (--bql-disable flag)
+#   2) Testing different qdiscs and their parameters (--qdisc, --qdisc-opts)
+#   3) Detecting kernel BUG/Oops from DQL accounting mismatches
+#
+# Key design detail -- SO_SNDBUF and wmem_max:
+#   The UDP sendto() path charges each SKB to the socket's sk_wmem_alloc
+#   counter.  The SKB carries a destructor (sock_wfree) that releases the
+#   charge only after the consumer NAPI thread on the peer veth finishes
+#   processing it -- including any iptables rules in the receive path.
+#   With the default sk_sndbuf (~208KB from wmem_default), only ~93
+#   packets (1442B each) can be in-flight before sendto() returns EAGAIN.
+#   Since 93 < 256 ptr_ring entries, the ring never fills and no qdisc
+#   backpressure occurs.  The test temporarily raises the global wmem_max
+#   sysctl and sets SO_SNDBUF=1MB to allow enough in-flight SKBs to
+#   saturate the ptr_ring.  The original wmem_max is restored on exit.
+#
+# Two TX-stop mechanisms and the dark-buffer problem:
+#   DRV_XOFF backpressure (commit dc82a33297fc) stops the TX queue when
+#   the 256-entry ptr_ring is full.  The queue is released at the end of
+#   veth_poll() (commit 5442a9da6978) after processing up to 64 packets
+#   (NAPI budget).  Without BQL, the entire ring is a FIFO "dark buffer"
+#   in front of the qdisc -- packets there are invisible to AQM.
+#
+#   BQL adds STACK_XOFF, which dynamically limits in-flight bytes and
+#   stops the queue *before* the ring fills.  This keeps the ring
+#   shallow and moves buffering into the qdisc where sojourn-based AQM
+#   (codel, fq_codel, CAKE/COBALT) can measure and drop packets.
+#
+# Sojourn time and NAPI budget interaction:
+#   DRV_XOFF releases backpressure once per NAPI poll (up to 64 pkts).
+#   During that cycle, packets queued in the qdisc accumulate sojourn
+#   time.  With fq_codel's default target of 5ms, the threshold is:
+#     5000us / 64 pkts = 78us/pkt --> ~12,800 pps consumer speed.
+#   Below that rate the NAPI-64 cycle exceeds the target and fq_codel
+#   starts dropping.  Use --nrules and --qdisc-opts to experiment.
+#
+cd "$(dirname -- "$0")" || exit 1
+source lib.sh
+
+# Defaults
+DURATION=30       # seconds; use longer --duration to reach DQL counter wrap
+NRULES=3500       # iptables rules in consumer NS (0 to disable)
+QDISC=sfq         # qdisc to use (sfq, pfifo, fq_codel, etc.)
+QDISC_OPTS=""     # extra qdisc parameters (e.g. "target 1ms interval 10ms")
+BQL_DISABLE=0     # 1 to disable BQL (sets limit_min high)
+NORMAL_NAPI=0     # 1 to use normal softirq NAPI (skip threaded NAPI)
+QDISC_REPLACE=0   # 1 to test qdisc replacement under active traffic
+TINY_FLOOD=0      # 1 to add 2nd UDP thread with min-size packets
+VETH_A="veth_bql0"
+VETH_B="veth_bql1"
+IP_A="10.99.0.1"
+IP_B="10.99.0.2"
+PORT=9999
+PKT_SIZE=1400     # large packets: slower producer, bigger BQL charges
+
+usage() {
+    echo "Usage: $0 [OPTIONS]"
+    echo "  --duration SEC   test duration (default: $DURATION)"
+    echo "  --nrules N       iptables rules to slow consumer (default: $NRULES, 0=disable)"
+    echo "  --qdisc NAME     qdisc to install (default: $QDISC)"
+    echo "  --qdisc-opts STR extra qdisc params (e.g. 'target 1ms interval 10ms')"
+    echo "  --bql-disable    disable BQL for A/B comparison"
+    echo "  --normal-napi    use softirq NAPI instead of threaded NAPI"
+    echo "  --qdisc-replace  test qdisc replacement under active traffic"
+    echo "  --tiny-flood     add 2nd UDP thread with min-size packets (stress BQL bytes)"
+    exit 1
+}
+
+while [ $# -gt 0 ]; do
+    case "$1" in
+    --duration)   DURATION="$2"; shift 2 ;;
+    --nrules)     NRULES="$2"; shift 2 ;;
+    --qdisc)      QDISC="$2"; shift 2 ;;
+    --qdisc-opts) QDISC_OPTS="$2"; shift 2 ;;
+    --bql-disable) BQL_DISABLE=1; shift ;;
+    --normal-napi) NORMAL_NAPI=1; shift ;;
+    --qdisc-replace) QDISC_REPLACE=1; shift ;;
+    --tiny-flood) TINY_FLOOD=1; shift ;;
+    --help|-h)    usage ;;
+    *)            echo "Unknown option: $1" >&2; usage ;;
+    esac
+done
+
+TMPDIR=$(mktemp -d)
+
+FLOOD_PID=""
+FLOOD2_PID=""
+SINK_PID=""
+PING_PID=""
+BPFTRACE_PID=""
+
+# shellcheck disable=SC2329  # cleanup is invoked indirectly via trap
+cleanup() {
+    [ -n "$BPFTRACE_PID" ] && kill_process "$BPFTRACE_PID"
+    [ -n "$FLOOD_PID" ] && kill_process "$FLOOD_PID"
+    [ -n "$FLOOD2_PID" ] && kill_process "$FLOOD2_PID"
+    [ -n "$SINK_PID" ] && kill_process "$SINK_PID"
+    [ -n "$PING_PID" ] && kill_process "$PING_PID"
+    cleanup_all_ns
+    ip link del "$VETH_A" 2>/dev/null || true
+    [ -n "$ORIG_WMEM_MAX" ] && sysctl -qw net.core.wmem_max="$ORIG_WMEM_MAX"
+    rm -rf "$TMPDIR"
+}
+trap cleanup EXIT
+
+require_command gcc
+require_command ethtool
+require_command tc
+
+# --- Function definitions ---
+
+compile_tools() {
+    echo "--- Compiling UDP flood tool ---"
+cat > "$TMPDIR"/udp_flood.c << 'CEOF'
+#include <arpa/inet.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <time.h>
+#include <unistd.h>
+
+static volatile int running = 1;
+
+static void stop(int sig) { running = 0; }
+
+struct pkt_hdr {
+	struct timespec ts;
+	unsigned long seq;
+};
+
+int main(int argc, char **argv)
+{
+	struct sockaddr_in dst;
+	struct pkt_hdr hdr;
+	unsigned long count = 0;
+	char buf[1500];
+	int sndbuf = 1048576;
+	int pkt_size, max_pkt_size;
+	int cur_size;
+	int duration;
+	int fd;
+
+	if (argc < 5) {
+		fprintf(stderr, "Usage: %s <ip> <pkt_size> <port> <duration> [max_pkt_size]\n",
+			argv[0]);
+		return 1;
+	}
+
+	pkt_size = atoi(argv[2]);
+	if (pkt_size < (int)sizeof(struct pkt_hdr))
+		pkt_size = sizeof(struct pkt_hdr);
+	if (pkt_size > (int)sizeof(buf))
+		pkt_size = sizeof(buf);
+	max_pkt_size = (argc > 5) ? atoi(argv[5]) : pkt_size;
+	if (max_pkt_size < pkt_size)
+		max_pkt_size = pkt_size;
+	if (max_pkt_size > (int)sizeof(buf))
+		max_pkt_size = sizeof(buf);
+	duration = atoi(argv[4]);
+
+	memset(&dst, 0, sizeof(dst));
+	dst.sin_family = AF_INET;
+	dst.sin_port = htons(atoi(argv[3]));
+	inet_pton(AF_INET, argv[1], &dst.sin_addr);
+
+	fd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (fd < 0) {
+		perror("socket");
+		return 1;
+	}
+
+	/* Raise send buffer so sk_wmem_alloc limit doesn't cap
+	 * in-flight packets before the ptr_ring (256 entries) fills.
+	 * Default wmem_default ~208K only allows ~93 packets.
+	 */
+	setsockopt(fd, SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf));
+
+	memset(buf, 0xAA, sizeof(buf));
+	signal(SIGINT, stop);
+	signal(SIGTERM, stop);
+	signal(SIGALRM, stop);
+	alarm(duration);
+
+	while (running) {
+		if (max_pkt_size > pkt_size)
+			cur_size = pkt_size + (rand() % (max_pkt_size - pkt_size + 1));
+		else
+			cur_size = pkt_size;
+		clock_gettime(CLOCK_MONOTONIC, &hdr.ts);
+		hdr.seq = count;
+		memcpy(buf, &hdr, sizeof(hdr));
+		sendto(fd, buf, cur_size, MSG_DONTWAIT,
+		       (struct sockaddr *)&dst, sizeof(dst));
+		count++;
+		if (!(count % 10000000))
+			fprintf(stderr, "  sent: %lu M packets\n",
+				count / 1000000);
+	}
+
+	fprintf(stderr, "Total sent: %lu packets (%.1f M)\n",
+		count, (double)count / 1e6);
+	close(fd);
+	return 0;
+}
+CEOF
+gcc -O2 -Wall -o "$TMPDIR"/udp_flood "$TMPDIR"/udp_flood.c || exit $ksft_fail
+
+# UDP sink with latency measurement
+cat > "$TMPDIR"/udp_sink.c << 'CEOF'
+#include <arpa/inet.h>
+#include <math.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <time.h>
+#include <unistd.h>
+
+static volatile int running = 1;
+
+static void stop(int sig) { running = 0; }
+
+struct pkt_hdr {
+	struct timespec ts;
+	unsigned long seq;
+};
+
+static void print_periodic(unsigned long count, unsigned long delta_count,
+			   double delta_sec, unsigned long drops,
+			   unsigned long reorders,
+			   double lat_min, double lat_sum,
+			   double lat_max)
+{
+	unsigned long pps;
+
+	if (!count)
+		return;
+	pps = delta_sec > 0 ? (unsigned long)(delta_count / delta_sec) : 0;
+	fprintf(stderr, "  sink: %lu pkts (%lu pps)  drops=%lu  reorders=%lu"
+		"  latency min/avg/max = %.3f/%.3f/%.3f ms\n",
+		count, pps, drops, reorders,
+		lat_min * 1e3, (lat_sum / count) * 1e3,
+		lat_max * 1e3);
+}
+
+static void print_final(unsigned long count, double elapsed_sec,
+			unsigned long drops, unsigned long reorders,
+			double lat_min, double lat_sum,
+			double lat_sum_sq, double lat_max)
+{
+	unsigned long pps;
+	double avg, stddev;
+
+	if (!count)
+		return;
+	pps = elapsed_sec > 0 ? (unsigned long)(count / elapsed_sec) : 0;
+	avg = lat_sum / count;
+	stddev = sqrt(lat_sum_sq / count - avg * avg);
+	fprintf(stderr, "  sink: %lu pkts (%lu avg pps)  drops=%lu  reorders=%lu"
+		"  latency min/avg/max/stddev = %.3f/%.3f/%.3f/%.3f ms\n",
+		count, pps, drops, reorders,
+		lat_min * 1e3, avg * 1e3,
+		lat_max * 1e3, stddev * 1e3);
+}
+
+int main(int argc, char **argv)
+{
+	unsigned long next_seq = 0, drops = 0, reorders = 0;
+	double lat_min = 1e9, lat_max = 0, lat_sum = 0, lat_sum_sq = 0;
+	unsigned long count = 0, last_count = 0;
+	struct sockaddr_in addr;
+	char buf[2048];
+	int fd, one = 1;
+
+	if (argc < 2) {
+		fprintf(stderr, "Usage: %s <port>\n", argv[0]);
+		return 1;
+	}
+
+	fd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (fd < 0) {
+		perror("socket");
+		return 1;
+	}
+	setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
+
+	/* Timeout so recv() unblocks periodically to check 'running' flag.
+	 * Needed because glibc signal() sets SA_RESTART, so SIGTERM
+	 * does not interrupt recv().
+	 */
+	struct timeval tv = { .tv_sec = 1 };
+	setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
+
+	memset(&addr, 0, sizeof(addr));
+	addr.sin_family = AF_INET;
+	addr.sin_port = htons(atoi(argv[1]));
+	addr.sin_addr.s_addr = INADDR_ANY;
+	if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
+		perror("bind");
+		return 1;
+	}
+
+	signal(SIGINT, stop);
+	signal(SIGTERM, stop);
+
+	struct timespec t_start, t_last_print;
+
+	clock_gettime(CLOCK_MONOTONIC, &t_start);
+	t_last_print = t_start;
+
+	while (running) {
+		struct pkt_hdr hdr;
+		struct timespec now;
+		ssize_t n;
+		double lat;
+
+		n = recv(fd, buf, sizeof(buf), 0);
+		if (n < (ssize_t)sizeof(struct pkt_hdr))
+			continue;
+
+		clock_gettime(CLOCK_MONOTONIC, &now);
+		memcpy(&hdr, buf, sizeof(hdr));
+
+		/* Track drops (gaps) and reorders (late arrivals) */
+		if (hdr.seq > next_seq)
+			drops += hdr.seq - next_seq;
+		if (hdr.seq < next_seq)
+			reorders++;
+		if (hdr.seq >= next_seq)
+			next_seq = hdr.seq + 1;
+
+		lat = (now.tv_sec - hdr.ts.tv_sec) +
+		      (now.tv_nsec - hdr.ts.tv_nsec) * 1e-9;
+
+		if (lat < lat_min)
+			lat_min = lat;
+		if (lat > lat_max)
+			lat_max = lat;
+		lat_sum += lat;
+		lat_sum_sq += lat * lat;
+		count++;
+
+		{
+			double since_print;
+
+			since_print = (now.tv_sec - t_last_print.tv_sec) +
+				      (now.tv_nsec - t_last_print.tv_nsec) * 1e-9;
+			if (since_print >= 5.0) {
+				print_periodic(count, count - last_count,
+					       since_print, drops,
+					       reorders, lat_min,
+					       lat_sum, lat_max);
+				last_count = count;
+				t_last_print = now;
+			}
+		}
+	}
+
+	{
+		struct timespec t_now;
+		double elapsed;
+
+		clock_gettime(CLOCK_MONOTONIC, &t_now);
+		elapsed = (t_now.tv_sec - t_start.tv_sec) +
+			  (t_now.tv_nsec - t_start.tv_nsec) * 1e-9;
+		print_final(count, elapsed, drops, reorders,
+			    lat_min, lat_sum, lat_sum_sq, lat_max);
+	}
+	close(fd);
+	return 0;
+}
+CEOF
+gcc -O2 -Wall -o "$TMPDIR"/udp_sink "$TMPDIR"/udp_sink.c -lm || exit $ksft_fail
+}
+
+setup_veth() {
+    log_info "Setting up veth pair with GRO"
+    setup_ns NS || exit $ksft_skip
+    ip link add "$VETH_A" type veth peer name "$VETH_B" || \
+        { echo "Failed to create veth pair (need root?)"; exit $ksft_skip; }
+    ip link set "$VETH_B" netns "$NS" || \
+        { echo "Failed to move veth to namespace"; exit $ksft_skip; }
+
+    # Configure IPs
+    ip addr add "${IP_A}/24" dev "$VETH_A"
+    ip link set "$VETH_A" up
+
+    ip -netns "$NS" addr add "${IP_B}/24" dev "$VETH_B"
+    ip -netns "$NS" link set "$VETH_B" up
+
+    # Raise wmem_max so the flood tool's SO_SNDBUF takes effect.
+    # Default 212992 caps in-flight to ~93 packets (sk_wmem_alloc limit),
+    # which is less than the 256-entry ptr_ring and prevents backpressure.
+    ORIG_WMEM_MAX=$(sysctl -n net.core.wmem_max)
+    sysctl -qw net.core.wmem_max=1048576
+
+    # Enable GRO on both ends -- activates NAPI -- BQL code path
+    ethtool -K "$VETH_A" gro on 2>/dev/null || true
+    ip netns exec "$NS" ethtool -K "$VETH_B" gro on 2>/dev/null || true
+
+    # Disable TSO so veth_skb_is_eligible_for_gro() returns true for all
+    # packets, ensuring every SKB takes the NAPI/ptr_ring path.  With TSO
+    # enabled, only packets matching sock_wfree + GRO features are eligible;
+    # disabling TSO removes that filter unconditionally.
+    ethtool -K "$VETH_A" tso off gso off 2>/dev/null || true
+    ip netns exec "$NS" ethtool -K "$VETH_B" tso off gso off 2>/dev/null || true
+
+    # Enable threaded NAPI -- this is critical: BQL backpressure (STACK_XOFF)
+    # only engages when producer and consumer run on separate CPUs.
+    # Without threaded NAPI, softirq completions happen too fast for BQL
+    # to build up enough in-flight bytes to trigger the limit.
+    if [ "$NORMAL_NAPI" -eq 0 ]; then
+        echo 1 > /sys/class/net/"$VETH_A"/threaded 2>/dev/null || true
+        ip netns exec "$NS" sh -c "echo 1 > /sys/class/net/$VETH_B/threaded" 2>/dev/null || true
+        log_info "Threaded NAPI enabled"
+    else
+        log_info "Using normal softirq NAPI (threaded NAPI disabled)"
+    fi
+}
+
+install_qdisc() {
+    local qdisc="${1:-$QDISC}"
+    local opts="${2:-}"
+    # Add a qdisc -- veth defaults to noqueue, but BQL needs a qdisc
+    # because STACK_XOFF is checked by the qdisc layer.
+    # Note: qdisc_create() auto-fixes txqueuelen=0 on IFF_NO_QUEUE devices
+    # to DEFAULT_TX_QUEUE_LEN (commit 84c46dd86538).
+    log_info "Installing qdisc: $qdisc $opts"
+    # shellcheck disable=SC2086  # $opts must word-split for tc arguments
+    tc qdisc replace dev "$VETH_A" root $qdisc $opts
+    # shellcheck disable=SC2086
+    ip netns exec "$NS" tc qdisc replace dev "$VETH_B" root $qdisc $opts
+}
+
+remove_qdisc() {
+    log_info "Removing qdisc (reverting to noqueue)"
+    tc qdisc del dev "$VETH_A" root 2>/dev/null || true
+    ip netns exec "$NS" tc qdisc del dev "$VETH_B" root 2>/dev/null || true
+}
+
+setup_iptables() {
+    # Bulk-load iptables rules in consumer namespace to slow NAPI processing.
+    # Many rules force per-packet linear rule traversal, increasing consumer
+    # overhead and BQL inflight bytes -- simulates realistic k8s-like workload.
+    if [ "$NRULES" -gt 0 ]; then
+        # shellcheck disable=SC2016  # single quotes intentional
+        ip netns exec "$NS" bash -c '
+        iptables-restore < <(
+        echo "*filter"
+        for n in $(seq 1 '"$NRULES"'); do
+          echo "-I INPUT -d '"$IP_B"'"
+        done
+        echo "COMMIT"
+        )
+        ' 2>/dev/null || { RET=$ksft_fail retmsg="iptables not available" \
+            log_test "iptables"; exit "$EXIT_STATUS"; }
+        log_info "Loaded $NRULES iptables rules in consumer NS"
+    fi
+}
+
+check_bql_sysfs() {
+    BQL_DIR="/sys/class/net/${VETH_A}/queues/tx-0/byte_queue_limits"
+    if [ -d "$BQL_DIR" ]; then
+        log_info "BQL sysfs found: $BQL_DIR"
+        if [ "$BQL_DISABLE" -eq 1 ]; then
+            echo 1073741824 > "$BQL_DIR/limit_min"
+            log_info "BQL effectively disabled (limit_min=1G)"
+        fi
+    else
+        log_info "BQL sysfs absent (veth IFF_NO_QUEUE+lltx, DQL accounting still active)"
+        BQL_DIR=""
+    fi
+}
+
+start_traffic() {
+    # Snapshot dmesg before test
+    DMESG_BEFORE=$(dmesg | wc -l)
+
+    log_info "Starting UDP sink in namespace"
+    ip netns exec "$NS" "$TMPDIR"/udp_sink "$PORT" &
+    SINK_PID=$!
+    sleep 0.2
+
+    log_info "Starting ping to $IP_B (5/s) to measure latency under load"
+    ping -i 0.2 -w "$DURATION" "$IP_B" > "$TMPDIR"/ping.log 2>&1 &
+    PING_PID=$!
+
+    log_info "Flooding ${PKT_SIZE}-byte UDP packets for ${DURATION}s"
+    "$TMPDIR"/udp_flood "$IP_B" "$PKT_SIZE" "$PORT" "$DURATION" &
+    FLOOD_PID=$!
+
+    # Optional: 2nd UDP thread with tiny packets to stress byte-based BQL.
+    # Small packets charge few BQL bytes, letting many more into the
+    # ptr_ring before STACK_XOFF fires -- exposing the dark buffer.
+    if [ "$TINY_FLOOD" -eq 1 ]; then
+        local port2=$((PORT + 1))
+        ip netns exec "$NS" "$TMPDIR"/udp_sink "$port2" &
+        log_info "Starting 2nd UDP flood (min-size pkts) on port $port2"
+        "$TMPDIR"/udp_flood "$IP_B" 24 "$port2" "$DURATION" &
+        FLOOD2_PID=$!
+    fi
+
+    # Optional: start bpftrace napi_poll histogram (best-effort)
+    local bt_script
+    bt_script="$(dirname -- "$0")/napi_poll_hist.bt"
+    if command -v bpftrace >/dev/null 2>&1 && [ -f "$bt_script" ]; then
+        bpftrace "$bt_script" > "$TMPDIR"/napi_poll.log 2>&1 &
+        BPFTRACE_PID=$!
+        log_info "bpftrace napi_poll histogram started (pid=$BPFTRACE_PID)"
+    fi
+}
+
+stop_traffic() {
+    [ -n "$FLOOD_PID" ] && kill_process "$FLOOD_PID"
+    FLOOD_PID=""
+    [ -n "$FLOOD2_PID" ] && kill_process "$FLOOD2_PID"
+    FLOOD2_PID=""
+    [ -n "$SINK_PID" ] && kill_process "$SINK_PID"
+    SINK_PID=""
+    [ -n "$PING_PID" ] && kill_process "$PING_PID"
+    PING_PID=""
+    [ -n "$BPFTRACE_PID" ] && kill_process "$BPFTRACE_PID"
+    BPFTRACE_PID=""
+}
+
+check_dmesg_bug() {
+    local bug_pattern='kernel BUG|BUG:|Oops:|dql_completed'
+    local warn_pattern='WARNING:|asks to queue packet|NETDEV WATCHDOG'
+    if dmesg | tail -n +$((DMESG_BEFORE + 1)) | \
+       grep -qE "$bug_pattern"; then
+        dmesg | tail -n +$((DMESG_BEFORE + 1)) | \
+            grep -B2 -A20 -E "$bug_pattern|$warn_pattern"
+        return 1
+    fi
+    # Log new warnings since last check (don't repeat old ones)
+    local cur_lines
+    cur_lines=$(dmesg | wc -l)
+    if [ "$cur_lines" -gt "${DMESG_WARN_SEEN:-$DMESG_BEFORE}" ]; then
+        local new_warns
+        new_warns=$(dmesg | tail -n +$(("${DMESG_WARN_SEEN:-$DMESG_BEFORE}" + 1)) | \
+            grep -E "$warn_pattern") || true
+        if [ -n "$new_warns" ]; then
+            local cnt
+            cnt=$(echo "$new_warns" | wc -l)
+            echo "  WARN: $cnt new kernel warning(s):"
+            echo "$new_warns" | tail -5
+        fi
+    fi
+    DMESG_WARN_SEEN=$cur_lines
+    return 0
+}
+
+print_periodic_stats() {
+    local elapsed="$1"
+
+    # BQL stats and watchdog counter
+    WD_CNT=$(cat /sys/class/net/${VETH_A}/queues/tx-0/tx_timeout \
+        2>/dev/null) || WD_CNT="?"
+    if [ -n "$BQL_DIR" ] && [ -d "$BQL_DIR" ]; then
+        INFLIGHT=$(cat "$BQL_DIR/inflight" 2>/dev/null || echo "?")
+        LIMIT=$(cat "$BQL_DIR/limit" 2>/dev/null || echo "?")
+        echo "  [${elapsed}s] BQL inflight=${INFLIGHT} limit=${LIMIT}" \
+            "watchdog=${WD_CNT}"
+    else
+        echo "  [${elapsed}s] watchdog=${WD_CNT} (no BQL sysfs)"
+    fi
+
+    # Qdisc stats
+    JQ_FMT='"qdisc \(.kind) pkts=\(.packets) drops=\(.drops)'
+    JQ_FMT+=' requeues=\(.requeues) backlog=\(.backlog)'
+    JQ_FMT+=' qlen=\(.qlen) overlimits=\(.overlimits)"'
+    CUR_QPKTS=$(tc -j -s qdisc show dev "$VETH_A" root 2>/dev/null |
+        jq -r '.[0].packets // 0' 2>/dev/null) || CUR_QPKTS=0
+    QSTATS=$(tc -j -s qdisc show dev "$VETH_A" root 2>/dev/null |
+        jq -r ".[0] | $JQ_FMT" 2>/dev/null) &&
+        echo "  [${elapsed}s] $QSTATS" || true
+
+    # Consumer PPS and per-packet processing time
+    if [ "$PREV_QPKTS" -gt 0 ] 2>/dev/null; then
+        DELTA=$((CUR_QPKTS - PREV_QPKTS))
+        PPS=$((DELTA / INTERVAL))
+        if [ "$PPS" -gt 0 ]; then
+            PKT_MS=$(awk "BEGIN {printf \"%.3f\", 1000.0/$PPS}")
+            NAPI_MS=$(awk "BEGIN {printf \"%.1f\", 64000.0/$PPS}")
+            echo "  [${elapsed}s] consumer: ${PPS} pps" \
+                "(~${PKT_MS}ms/pkt, NAPI-64 cycle ~${NAPI_MS}ms)"
+        fi
+    fi
+    PREV_QPKTS=$CUR_QPKTS
+
+    # softnet_stat: per-CPU tracking to detect same-CPU vs multi-CPU NAPI
+    # /proc/net/softnet_stat columns: processed, dropped, time_squeeze (hex, per-CPU)
+    local cpu=0 total_proc=0 total_sq=0 active_cpus=""
+    while read -r line; do
+        # shellcheck disable=SC2086  # word splitting on $line is intentional
+        set -- $line
+        local cur_p=$((0x${1})) cur_sq=$((0x${3}))
+        if [ -f "$TMPDIR/softnet_cpu${cpu}" ]; then
+            read -r prev_p prev_sq < "$TMPDIR/softnet_cpu${cpu}"
+            local dp=$((cur_p - prev_p)) dsq=$((cur_sq - prev_sq))
+            total_proc=$((total_proc + dp))
+            total_sq=$((total_sq + dsq))
+            [ "$dp" -gt 0 ] && active_cpus="${active_cpus} cpu${cpu}(+${dp})"
+        fi
+        echo "$cur_p $cur_sq" > "$TMPDIR/softnet_cpu${cpu}"
+        cpu=$((cpu + 1))
+    done < /proc/net/softnet_stat
+    local n_active
+    n_active=$(echo "$active_cpus" | wc -w)
+    local cpu_mode="single-CPU"
+    [ "$n_active" -gt 1 ] && cpu_mode="multi-CPU(${n_active})"
+    if [ "$total_sq" -gt 0 ] && [ "$INTERVAL" -gt 0 ]; then
+        echo "  [${elapsed}s] softnet: processed=${total_proc}" \
+            "time_squeeze=${total_sq} (${total_sq}/${INTERVAL}s)" \
+            "${cpu_mode}:${active_cpus}"
+    else
+        echo "  [${elapsed}s] softnet: processed=${total_proc}" \
+            "time_squeeze=${total_sq}" \
+            "${cpu_mode}:${active_cpus}"
+    fi
+
+    # napi_poll histogram (from bpftrace, if running)
+    if [ -n "$BPFTRACE_PID" ] && [ -f "$TMPDIR"/napi_poll.log ]; then
+        local napi_line
+        napi_line=$(grep '^napi_poll:' "$TMPDIR"/napi_poll.log | tail -1)
+        [ -n "$napi_line" ] && echo "  [${elapsed}s] $napi_line"
+    fi
+
+    # Ping RTT
+    PING_RTT=$(tail -1 "$TMPDIR"/ping.log 2>/dev/null | grep -oP 'time=\K[0-9.]+') &&
+        echo "  [${elapsed}s] ping RTT=${PING_RTT}ms" || true
+}
+
+monitor_loop() {
+    ELAPSED=0
+    INTERVAL=5
+    PREV_QPKTS=0
+    # Seed per-CPU softnet baselines
+    local cpu=0
+    while read -r line; do
+        # shellcheck disable=SC2086  # word splitting on $line is intentional
+        set -- $line
+        echo "$((0x${1})) $((0x${3}))" > "$TMPDIR/softnet_cpu${cpu}"
+        cpu=$((cpu + 1))
+    done < /proc/net/softnet_stat
+    while kill -0 "$FLOOD_PID" 2>/dev/null; do
+        sleep "$INTERVAL"
+        ELAPSED=$((ELAPSED + INTERVAL))
+
+        if ! check_dmesg_bug; then
+            RET=$ksft_fail
+            retmsg="BUG_ON triggered in dql_completed at ${ELAPSED}s"
+            log_test "veth_bql"
+            exit "$EXIT_STATUS"
+        fi
+
+        print_periodic_stats "$ELAPSED"
+    done
+    wait "$FLOOD_PID" || true
+    FLOOD_PID=""
+}
+
+# Verify traffic is flowing by checking device tx_packets counter.
+# Works for both qdisc and noqueue modes.
+verify_traffic_flowing() {
+    local label="$1"
+    local prev_tx cur_tx
+
+    # Skip check if flood producer already exited (not a stall)
+    if [ -n "$FLOOD_PID" ] && ! kill -0 "$FLOOD_PID" 2>/dev/null; then
+        log_info "$label flood producer exited (duration reached)"
+        return 0
+    fi
+
+    prev_tx=$(cat /sys/class/net/${VETH_A}/statistics/tx_packets \
+        2>/dev/null) || prev_tx=0
+    sleep 0.5
+    cur_tx=$(cat /sys/class/net/${VETH_A}/statistics/tx_packets \
+        2>/dev/null) || cur_tx=0
+    if [ "$cur_tx" -gt "$prev_tx" ]; then
+        log_info "$label traffic flowing (tx: $prev_tx -> $cur_tx)"
+        return 0
+    fi
+    log_info "$label traffic STALLED (tx: $prev_tx -> $cur_tx)"
+    return 1
+}
+
+collect_results() {
+    local test_name="${1:-veth_bql}"
+
+    # Ping summary
+    wait "$PING_PID" 2>/dev/null || true
+    PING_PID=""
+    if [ -f "$TMPDIR"/ping.log ]; then
+        PING_LOSS=$(grep -o '[0-9.]*% packet loss' "$TMPDIR"/ping.log) &&
+            log_info "Ping loss: $PING_LOSS"
+        PING_SUMMARY=$(tail -1 "$TMPDIR"/ping.log)
+        log_info "Ping summary: $PING_SUMMARY"
+    fi
+
+    # Watchdog summary
+    WD_FINAL=$(cat /sys/class/net/${VETH_A}/queues/tx-0/tx_timeout \
+        2>/dev/null) || WD_FINAL=0
+    if [ "$WD_FINAL" -gt 0 ] 2>/dev/null; then
+        log_info "Watchdog fired ${WD_FINAL} time(s)"
+        dmesg | tail -n +$((DMESG_BEFORE + 1)) | \
+            grep -E 'NETDEV WATCHDOG|veth backpressure' || true
+    fi
+
+    # Final dmesg check -- only upgrade to fail, never override existing fail
+    if ! check_dmesg_bug; then
+        RET=$ksft_fail
+        retmsg="BUG_ON triggered in dql_completed"
+    fi
+    log_test "$test_name"
+    exit "$EXIT_STATUS"
+}
+
+# --- Test modes ---
+
+test_bql_stress() {
+    RET=$ksft_pass
+    compile_tools
+    setup_veth
+    install_qdisc "$QDISC" "$QDISC_OPTS"
+    setup_iptables
+    log_info "kernel: $(uname -r)"
+    check_bql_sysfs
+    start_traffic
+    monitor_loop
+    collect_results "veth_bql"
+}
+
+# Test qdisc replacement under active traffic.  Cycles through several
+# qdiscs including a transition to noqueue (tc qdisc del) to verify
+# that stale BQL state (STACK_XOFF) is properly reset during qdisc
+# transitions.
+test_qdisc_replace() {
+    local qdiscs=("sfq" "pfifo" "fq_codel")
+    local step=2
+    local elapsed=0
+    local idx
+
+    RET=$ksft_pass
+    compile_tools
+    setup_veth
+    install_qdisc "$QDISC" "$QDISC_OPTS"
+    setup_iptables
+    log_info "kernel: $(uname -r)"
+    check_bql_sysfs
+    start_traffic
+
+    while [ "$elapsed" -lt "$DURATION" ] && kill -0 "$FLOOD_PID" 2>/dev/null; do
+        sleep "$step"
+        elapsed=$((elapsed + step))
+
+        if ! check_dmesg_bug; then
+            RET=$ksft_fail
+            retmsg="BUG_ON during qdisc replacement at ${elapsed}s"
+            break
+        fi
+
+        # Cycle: sfq -> pfifo -> fq_codel -> noqueue -> sfq -> ...
+        idx=$(( (elapsed / step - 1) % (${#qdiscs[@]} + 1) ))
+        if [ "$idx" -eq "${#qdiscs[@]}" ]; then
+            remove_qdisc
+        else
+            install_qdisc "${qdiscs[$idx]}"
+        fi
+
+        # Print BQL and qdisc stats after each replacement
+        if [ -n "$BQL_DIR" ] && [ -d "$BQL_DIR" ]; then
+            local inflight limit limit_min limit_max holding
+            inflight=$(cat "$BQL_DIR/inflight" 2>/dev/null || echo "?")
+            limit=$(cat "$BQL_DIR/limit" 2>/dev/null || echo "?")
+            limit_min=$(cat "$BQL_DIR/limit_min" 2>/dev/null || echo "?")
+            limit_max=$(cat "$BQL_DIR/limit_max" 2>/dev/null || echo "?")
+            holding=$(cat "$BQL_DIR/holding_time" 2>/dev/null || echo "?")
+            echo "  [${elapsed}s] BQL inflight=${inflight} limit=${limit}" \
+                "limit_min=${limit_min} limit_max=${limit_max}" \
+                "holding=${holding}"
+        fi
+        local cur_qdisc
+        cur_qdisc=$(tc qdisc show dev "$VETH_A" root 2>/dev/null | \
+            awk '{print $2}') || cur_qdisc="none"
+        local txq_state
+        txq_state=$(cat /sys/class/net/${VETH_A}/queues/tx-0/tx_timeout \
+            2>/dev/null) || txq_state="?"
+        echo "  [${elapsed}s] qdisc=${cur_qdisc} watchdog=${txq_state}"
+
+        if ! verify_traffic_flowing "[${elapsed}s]"; then
+            RET=$ksft_fail
+            retmsg="Traffic stalled after qdisc replacement at ${elapsed}s"
+            break
+        fi
+    done
+
+    stop_traffic
+    collect_results "veth_bql_qdisc_replace"
+}
+
+# --- Main ---
+if [ "$QDISC_REPLACE" -eq 1 ]; then
+    test_qdisc_replace
+else
+    test_bql_stress
+fi
diff --git a/tools/testing/selftests/net/veth_bql_test_virtme.sh b/tools/testing/selftests/net/veth_bql_test_virtme.sh
new file mode 100755
index 000000000000..bb8dde0f6c00
--- /dev/null
+++ b/tools/testing/selftests/net/veth_bql_test_virtme.sh
@@ -0,0 +1,124 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Launch veth BQL test inside virtme-ng
+#
+# Must be run from the kernel build tree root.
+#
+# Options:
+#   --verbose       Show kernel console (vng boot messages) in real time.
+#                   Useful for debugging kernel panics / BUG_ON crashes.
+#   All other options are forwarded to veth_bql_test.sh (see --help there).
+#
+# Examples (run from kernel tree root):
+#   ./tools/testing/selftests/net/veth_bql_test_virtme.sh [OPTIONS]
+#     --duration 20 --nrules 1000
+#     --qdisc fq_codel --bql-disable
+#     --verbose --qdisc-replace --duration 60
+
+set -eu
+
+# Parse --verbose (consumed here, not forwarded to the inner test).
+VERBOSE=""
+INNER_ARGS=()
+for arg in "$@"; do
+    if [ "$arg" = "--verbose" ]; then
+        VERBOSE="--verbose"
+    else
+        INNER_ARGS+=("$arg")
+    fi
+done
+TEST_ARGS=""
+[ ${#INNER_ARGS[@]} -gt 0 ] && TEST_ARGS=$(printf '%q ' "${INNER_ARGS[@]}")
+
+if [ ! -f "vmlinux" ]; then
+    echo "ERROR: virtme-ng needs vmlinux; run from a compiled kernel tree:" >&2
+    echo "  cd /path/to/kernel && $0" >&2
+    exit 1
+fi
+
+# Verify .config has the options needed for virtme-ng and this test.
+# Without these the VM silently stalls with no output.
+KCONFIG=".config"
+if [ ! -f "$KCONFIG" ]; then
+    echo "ERROR: No .config found -- build the kernel first" >&2
+    exit 1
+fi
+
+MISSING=""
+for opt in CONFIG_VIRTIO CONFIG_VIRTIO_PCI CONFIG_VIRTIO_NET \
+           CONFIG_VIRTIO_CONSOLE CONFIG_NET_9P CONFIG_NET_9P_VIRTIO \
+           CONFIG_9P_FS CONFIG_VETH CONFIG_BQL; do
+    if ! grep -q "^${opt}=[ym]" "$KCONFIG"; then
+        MISSING+="  $opt\n"
+    fi
+done
+if [ -n "$MISSING" ]; then
+    echo "ERROR: .config is missing options required by virtme-ng:" >&2
+    echo -e "$MISSING" >&2
+    echo "Consider: vng --kconfig (or make defconfig + enable above)" >&2
+    exit 1
+fi
+
+TESTDIR="tools/testing/selftests/net"
+TESTNAME="veth_bql_test.sh"
+LOGFILE="veth_bql_test.log"
+LOGPATH="$TESTDIR/$LOGFILE"
+CONSOLELOG="veth_bql_console.log"
+rm -f "$LOGPATH" "$CONSOLELOG"
+
+echo "Starting VM... test output in $LOGPATH, kernel console in $CONSOLELOG"
+echo "(VM is booting, please wait ~30s)"
+
+# Always capture kernel console to a file via a second QEMU serial port.
+# vng claims ttyS0 (mapped to /dev/null); --qemu-opts adds ttyS1 on COM2.
+# earlycon registers COM2's I/O port (0x2f8) as a persistent console.
+# (plain console=ttyS1 does NOT work: the 8250 driver registers once,
+# ttyS0 wins, and ttyS1 is never picked up.)
+# --verbose additionally shows kernel console in real time on the terminal.
+SERIAL_CONSOLE="earlycon=uart8250,io,0x2f8,115200"
+SERIAL_CONSOLE+=" console=uart8250,io,0x2f8,115200"
+set +e
+vng $VERBOSE --cpus 4 --memory 2G \
+    --rwdir "$TESTDIR" \
+    --append "panic=5 loglevel=4 $SERIAL_CONSOLE" \
+    --qemu-opts="-serial file:$CONSOLELOG" \
+    --exec "cd $TESTDIR && \
+        ./$TESTNAME $TEST_ARGS 2>&1 | \
+        tee $LOGFILE; echo EXIT_CODE=\$? >> $LOGFILE"
+VNG_RC=$?
+set -e
+
+echo ""
+if [ "$VNG_RC" -ne 0 ]; then
+    echo "***********************************************************"
+    echo "* VM CRASHED -- kernel panic or BUG_ON (vng rc=$VNG_RC)"
+    echo "***********************************************************"
+    if [ -s "$CONSOLELOG" ] && \
+       grep -qiE 'kernel BUG|BUG:|Oops:|panic|dql_completed' "$CONSOLELOG"; then
+        echo ""
+        echo "--- kernel backtrace ($CONSOLELOG) ---"
+        grep -iE -A30 'kernel BUG|BUG:|Oops:|panic|dql_completed' \
+            "$CONSOLELOG" | head -50
+    else
+        echo ""
+        echo "Re-run with --verbose to see the kernel backtrace:"
+        echo "  $0 --verbose ${INNER_ARGS[*]}"
+    fi
+    exit 1
+elif [ ! -f "$LOGPATH" ]; then
+    echo "No log file found -- VM may have crashed before writing output"
+    exit 2
+else
+    echo "=== VM finished ==="
+fi
+
+# Scan console log for unexpected kernel warnings (even on clean exit)
+if [ -s "$CONSOLELOG" ]; then
+    WARN_PATTERN='kernel BUG|BUG:|Oops:|dql_completed|WARNING:|asks to queue packet|NETDEV WATCHDOG'
+    WARN_LINES=$(grep -cE "$WARN_PATTERN" "$CONSOLELOG" 2>/dev/null) || WARN_LINES=0
+    if [ "$WARN_LINES" -gt 0 ]; then
+        echo ""
+        echo "*** kernel warnings in $CONSOLELOG ($WARN_LINES lines) ***"
+        grep -E "$WARN_PATTERN" "$CONSOLELOG" | head -20
+    fi
+fi
-- 
2.43.0

Re: [PATCH net-next v3 5/5] selftests: net: add veth BQL stress test

[Jakub Kicinski]

On Wed, 29 Apr 2026 19:20:32 +0200 hawk@kernel.org wrote:
> A companion wrapper (veth_bql_test_virtme.sh) launches the test inside
> a virtme-ng VM, with .config validation to prevent silent stalls.
> 
> Usage:
>   sudo ./veth_bql_test.sh [--duration 300] [--nrules 100]
>                           [--qdisc sfq] [--qdisc-opts '...']
>                           [--bql-disable] [--normal-napi]
>                           [--qdisc-replace]

Not convinced we need to carry this in the tree.

Honestly:

> +	close(fd);
> +	return 0;
> +}
> +CEOF
> +gcc -O2 -Wall -o "$TMPDIR"/udp_sink "$TMPDIR"/udp_sink.c -lm || exit $ksft_fail
> +}
> +
> +setup_veth() {
> +    log_info "Setting up

This whole test looks like outrageous slop.
Did you even read this? :|

Re: [PATCH net-next v3 5/5] selftests: net: add veth BQL stress test

[Jesper Dangaard Brouer]

On 01/05/2026 04.00, Jakub Kicinski wrote:
> On Wed, 29 Apr 2026 19:20:32 +0200 hawk@kernel.org wrote:
>> A companion wrapper (veth_bql_test_virtme.sh) launches the test inside
>> a virtme-ng VM, with .config validation to prevent silent stalls.
>>
>> Usage:
>>    sudo ./veth_bql_test.sh [--duration 300] [--nrules 100]
>>                            [--qdisc sfq] [--qdisc-opts '...']
>>                            [--bql-disable] [--normal-napi]
>>                            [--qdisc-replace]
> 
> Not convinced we need to carry this in the tree.

I agree, I will drop the selftest in V4.

The selftest have been very useful during *development* phase.
The number of options also reveal that we (Jonas and I) have been using
this beyond a selftest. The veth_bql_test_virtme.sh wrapper have been a
huge help to accelerate test-based development cycles,  but it's not a
selftest component per say. This have caught actual bugs in our BQL/DQL
API usage, but all those BUGs are fixed in code.

It makes a lot of sense to avoid having to maintain this code in tree.
I can place it somewhere on GitHub, else it can be pulled from lore.

--Jesper

Re: [PATCH net-next v3 5/5] selftests: net: add veth BQL stress test

[Breno Leitao]

On Wed, Apr 29, 2026 at 07:20:32PM +0200, hawk@kernel.org wrote:
> +compile_tools() {
> +    echo "--- Compiling UDP flood tool ---"
> +cat > "$TMPDIR"/udp_flood.c << 'CEOF'

The biggest structural issue: the test ships ~250 lines of C source as
heredocs and shells out to gcc at runtime to build them. This is not
how selftests normally work.

Re: [PATCH net-next v3 0/5] veth: add Byte Queue Limits (BQL) support

[Jakub Kicinski]

On Wed, 29 Apr 2026 19:20:27 +0200 hawk@kernel.org wrote:
> This series adds BQL (Byte Queue Limits) to the veth driver, reducing
> latency by dynamically limiting in-flight packets in the ptr_ring and
> moving buffering into the qdisc where AQM algorithms can act on it.

Does not apply (am I remembering right that it's not the first time
this series doesn't apply?)
-- 
pw-bot: cr

Re: [PATCH net-next v3 0/5] veth: add Byte Queue Limits (BQL) support

[Jesper Dangaard Brouer]

On 01/05/2026 03.58, Jakub Kicinski wrote:
> On Wed, 29 Apr 2026 19:20:27 +0200 hawk@kernel.org wrote:
>> This series adds BQL (Byte Queue Limits) to the veth driver, reducing
>> latency by dynamically limiting in-flight packets in the ptr_ring and
>> moving buffering into the qdisc where AQM algorithms can act on it.
> 
> Does not apply (am I remembering right that it's not the first time
> this series doesn't apply?)

Rebased posted
  V4: https://lore.kernel.org/all/20260501071633.644353-1-hawk@kernel.org/

The conflict was on tools/testing/selftests/net/config for a change 
applied 16 Apr, and I send patchset 29 Apr.  So, yes it seems I forgot 
to rebase for that period, sorry.

--Jesper