Merge branch 'net_sched-fix-hfsc-qlen-backlog-accounting-bug-and-add-selftest'

Cong Wang says:

====================
net_sched: Fix HFSC qlen/backlog accounting bug and add selftest

This series addresses a long-standing bug in the HFSC qdisc where queue length
and backlog accounting could become inconsistent if a packet is dropped during
a peek-induced dequeue operation, and adds a corresponding selftest to tc-testing.
====================

Link: https://patch.msgid.link/20250518222038.58538-1-xiyou.wangcong@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/sched/sch_hfsc.c

@@ -1569,6 +1569,9 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 		return err;
 	}
 
+	sch->qstats.backlog += len;
+	sch->q.qlen++;
+
 	if (first && !cl->cl_nactive) {
 		if (cl->cl_flags & HFSC_RSC)
 			init_ed(cl, len);
@@ -1584,9 +1587,6 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 
 	}
 
-	sch->qstats.backlog += len;
-	sch->q.qlen++;
-
 	return NET_XMIT_SUCCESS;
 }
 

tools/testing/selftests/tc-testing/tc-tests/infra/qdiscs.json

@@ -573,5 +573,32 @@
         "teardown": [
             "$TC qdisc del dev $DEV1 handle 1: root"
         ]
+    },
+    {
+        "id": "831d",
+        "name": "Test HFSC qlen accounting with DRR/NETEM/BLACKHOLE chain",
+        "category": ["qdisc", "hfsc", "drr", "netem", "blackhole"],
+        "plugins": { "requires": ["nsPlugin", "scapyPlugin"] },
+        "setup": [
+            "$IP link set dev $DEV1 up || true",
+            "$TC qdisc add dev $DEV1 root handle 1: drr",
+            "$TC filter add dev $DEV1 parent 1: basic classid 1:1",
+            "$TC class add dev $DEV1 parent 1: classid 1:1 drr",
+            "$TC qdisc add dev $DEV1 parent 1:1 handle 2: hfsc def 1",
+            "$TC class add dev $DEV1 parent 2: classid 2:1 hfsc rt m1 8 d 1 m2 0",
+            "$TC qdisc add dev $DEV1 parent 2:1 handle 3: netem",
+            "$TC qdisc add dev $DEV1 parent 3:1 handle 4: blackhole"
+        ],
+        "scapy": {
+            "iface": "$DEV0",
+            "count": 5,
+            "packet": "Ether()/IP(dst='10.10.10.1', src='10.10.10.10')/ICMP()"
+        },
+        "cmdUnderTest": "$TC -s qdisc show dev $DEV1",
+        "expExitCode": "0",
+        "verifyCmd": "$TC -s qdisc show dev $DEV1",
+        "matchPattern": "qdisc hfsc",
+        "matchCount": "1",
+        "teardown": ["$TC qdisc del dev $DEV1 root handle 1: drr"]
     }
 ]