[PATCH net v2] net: dsa: clear the conduit RX queue on receive

[John Macias]

A multi-queue conduit records its RX queue in skb->queue_mapping with
skb_record_rx_queue(), as stmmac, mtk_eth_soc, fec and dpaa2 do.
dsa_switch_rcv() passes the skb on with that index unchanged, although
it counts the conduit's queues, not those of skb->dev. A user port has
a single RX queue. For a packet from conduit queue 1 or above,
get_rps_cpu() finds the index past the port's real_num_rx_queues and
returns without steering, and its WARN_ONCE() stays silent because the
port has only one queue. So rps_cpus on a user port only takes effect
for packets from conduit queue 0.

Trunk-tagged frames go straight to a bond or team (tag_dsa), through
the netif_rx() branch for devices that are not user ports. That device
has its own RX queue count, so the conduit's index there selects the
RPS map of an unrelated rx-N queue, skips RPS, or, when the device has
more than one queue and an index at or above that count, triggers
the WARN_ONCE().

Clear the queue mapping once the tagger has chosen skb->dev, before
that branch, so both paths are covered.

This leaves no RX queue recorded, rather than recording queue 0. Both
make get_rps_cpu() use the first RX queue's map. They differ for code
that reads the recorded queue later:

- skb_tx_hash() picks the TX queue from a recorded RX queue when no
  socket mapping or XPS map decides. A flow forwarded from a DSA port
  to a multi-queue device now gets its TX queue from the flow hash
  instead of the conduit's RX queue number. Recording queue 0 would
  instead put every such flow on the first TX queue of its traffic
  class. The selected TX queue is also an input to several taggers:
  tag_brcm encodes it directly, while tag_ksz, tag_sja1105,
  tag_ocelot_8021q, tag_netc and tag_vsc73xx_8021q derive a traffic
  class or PCP from it. A different queue within the same traffic
  class need not change that encoded class.
- With CONFIG_SOCK_RX_QUEUE_MAPPING, packets received through DSA ports
  no longer update sk_rx_queue_mapping, so XPS by RX queue (xps_rxqs)
  stops following the conduit's queue number for those sockets. A
  mapping the socket already holds is left as it is.

Found on an IPQ8074 board (OpenWrt, Linux 6.18, conduit with four RX
rings): with rps_cpus set on the LAN and WAN user ports,
/proc/net/softnet_stat showed RPS only for traffic hashed to conduit
ring 0.

Claude and Codex assisted with diagnosing the queue-metadata issue,
preparing the change and its explanation, and checking source and test
results.

Fixes: 91da11f870f0 ("net: Distributed Switch Architecture protocol support")
Signed-off-by: John Macias <dencity7@xxxxxxxxx>
---
Changes in v2:
- Clear the queue mapping before the branch for devices that are not
  user ports, so trunk-tagged frames delivered to a bond or team are
  covered too (Sashiko review).
- Replace the claim that TX queue selection is unaffected. The changelog
  now describes the effect on skb_tx_hash(), taggers that consume the
  selected TX queue, and sk_rx_queue_mapping and
  xps_rxqs, and says why no queue is recorded instead of queue 0.
- Subject: "on user port receive" -> "on receive".
v1: https://lore.kernel.org/all/20261025072203.1519561-1-dencity7@xxxxxxxxx/

Claude assisted with diagnosis, the patch and runtime harness. Codex
independently reviewed the code, claims and raw results, and corrected
the test descriptions.

Testing on net/main (5d7f3375c), x86_64 under QEMU: a 4-queue virtio-net
conduit with dsa_loop, which for this test only was changed to use the
Marvell DSA tagger and to accept LAG offload (lan2 + lan3 in a
balance-xor bond with 2 RX queues). The host injects DSA-tagged UDP on
chosen conduit RX queues; rps_cpus on lan1 and on bond0 rx-0 points to
an otherwise idle CPU. 200 datagrams per queue and path. The RPS columns
say on which conduit queues the receiving socket saw the RPS CPU
(SO_INCOMING_CPU, which holds the socket's latest receive CPU, so this
is a per-socket observation rather than a per-packet one):

                       lan1 RPS     bond0 RPS    WARN_ONCE   eth1 TX
                       (q0-q3)      (q0-q3)                  per queue
  unpatched            q0 only      q0 only      yes         800/0/1/0
  v1                   all          q0 only      yes         201/100/150/350
  v2                   all          all          no          201/100/350/150
  skb_record_rx_queue  all          all          no          800/0/0/1
  (skb, 0) instead

(Queue 2's NAPI happened to run on the RPS CPU, so q2 does not tell the
cases apart; q1 and q3 do.) The last column is the eth1 TX packet delta
per queue while 16 UDP flows (800 datagrams), all from conduit RX
queue 0, were routed out a 4-queue virtio-net with XPS cleared; each
total is 801 because one other packet was also sent. The aggregate
counters are consistent with fallback TX selection favoring queue 0
when RX queue 0 is recorded, and flow-hash distribution
when none is recorded; the unclassified extra packet is not attributed
to a test flow. W=1 build of net/dsa is clean.

On the original hardware (Netgear RAX120 v1, IPQ8074, OpenWrt Linux
6.18, four-ring conduit), v1 was tested as described in v1. v2 has not
run there yet, and I have no switch with LAG offload to test the trunk
path on real hardware.

 net/dsa/tag.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/dsa/tag.c b/net/dsa/tag.c
index 6a5aa7fc3..e53dc3f3c 100644
--- a/net/dsa/tag.c
+++ b/net/dsa/tag.c
@@ -95,6 +95,13 @@ static int dsa_switch_rcv(struct sk_buff *skb,
struct net_device *dev,
 	skb->pkt_type = PACKET_HOST;
 	skb->protocol = eth_type_trans(skb, skb->dev);

+	/* The RX queue the conduit recorded is an index into the conduit's
+	 * queues. It means nothing on skb->dev, a user port with a single RX
+	 * queue or a bond/team with its own queue count, and get_rps_cpu()
+	 * would check it against that device. Leave no queue recorded.
+	 */
+	skb_set_queue_mapping(skb, 0);
+
 	if (unlikely(!dsa_user_dev_check(skb->dev))) {
 		/* Packet is to be injected directly on an upper
 		 * device, e.g. a team/bond, so skip all DSA-port
-- 
2.47.3