selftests: tc-testing: Add TBF with SKBPRIO queue length corner case …

…test

Add a test case to validate the interaction between TBF and SKBPRIO queueing
disciplines, specifically targeting queue length accounting corner cases.

This test complements the fix for the queue length accounting issue in the
SKBPRIO qdisc. This is still best-effort, as timing and manipulating enqueue
and dequeue from user-space is very hard.

Cc: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/20250329222536.696204-3-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -126,5 +126,37 @@
             "$TC qdisc del dev $DUMMY root handle 1: drr",
             "$IP addr del 10.10.10.10/24 dev $DUMMY"
         ]
-   }
+    },
+    {
+        "id": "c024",
+        "name": "Test TBF with SKBPRIO - catch qlen corner cases",
+        "category": [
+            "qdisc",
+            "tbf",
+            "skbprio"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "$IP link set dev $DUMMY up || true",
+            "$IP addr add 10.10.10.10/24 dev $DUMMY || true",
+            "$TC qdisc add dev $DUMMY handle 1: root tbf rate 100bit burst 2000 limit 1000",
+            "$TC qdisc add dev $DUMMY parent 1: handle 10: skbprio limit 1",
+            "ping -c 1 -W 0.1 -Q 0x00 -s 1400 -I $DUMMY 10.10.10.1 > /dev/null || true",
+            "ping -c 1 -W 0.1 -Q 0x1c -s 1400 -I $DUMMY 10.10.10.1 > /dev/null || true",
+            "ping -c 1 -W 0.1 -Q 0x00 -s 1400 -I $DUMMY 10.10.10.1 > /dev/null || true",
+            "ping -c 1 -W 0.1 -Q 0x1c -s 1400 -I $DUMMY 10.10.10.1 > /dev/null || true",
+            "sleep 0.5"
+        ],
+        "cmdUnderTest": "$TC -s qdisc show dev $DUMMY",
+        "expExitCode": "0",
+        "verifyCmd": "$TC -s qdisc show dev $DUMMY | grep -A 5 'qdisc skbprio'",
+        "matchPattern": "dropped [1-9][0-9]*",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $DUMMY handle 1: root",
+            "$IP addr del 10.10.10.10/24 dev $DUMMY || true"
+        ]
+    }
 ]