[PATCH net-next 2/4] net/sched/sch_cake: reduce parent backlog when clearing a tin

[Jamal Hadi Salim <jhs@mojatatu.com>]

CAKE has purged unused tins on DiffServ reconfiguration since commit
83f8fd69af4f ("sch_cake: Add DiffServ handling") without propagating the
resulting backlog reduction to classful parents. The issue was noticed
while reviewing 7cbfb180945c, but predates that change.

cake_clear_tin() drops every packet from a tin with
kfree_skb_reason() but never calls qdisc_tree_reduce_backlog(), so a
CAKE qdisc under a classful parent (e.g. HTB) leaves the dropped packets
in the parent qdisc's and class's qlen/backlog accounting forever. The
normal dequeue path does reduce the parent; only the live reconfigure
purge misses it.

Tins are cleared from cake_reset() and, on reconfigure, from
cake_configure_rates(). The qdisc core already owns the parent delta for
the reset path: qdisc_purge_queue() snapshots qlen/backlog, runs
qdisc_reset(), then reduces the tree, so propagating the reduction from
cake_clear_tin() itself would subtract the same backlog twice. Snapshot
CAKE's qlen/backlog around only the reconfiguration purge in
cake_configure_rates() and reduce the parent once afterwards, matching
the delta pattern already used by cake_enqueue().

Conditions to recreate the bug: put CAKE (diffserv8) under an HTB class,
flood DSCP-classified traffic into a high tin so it backlogs, then change
CAKE to diffserv3. The HTB parent keeps the purged packets in its qlen
and backlog; a kernel with the fix shows the parent at the leaf's backlog
(426b/5p) instead of retaining the purged packets.

Reported-by: Sashiko (gemini) <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260816012109.2865223-1-ooonea@gmail.com
Link: https://lore.kernel.org/netdev/20260816012109.2865223-1-ooonea@gmail.com/
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_cake.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 1c29695dc928..2bad0b6eb13f 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -2611,10 +2611,17 @@ static void cake_configure_rates(struct Qdisc *sch, u64 rate, bool rate_adjust)
 	}
 
 	if (!rate_adjust) {
+		u32 backlog = sch->qstats.backlog, qlen = sch->q.qlen;
+
 		for (c = qd->tin_cnt; c < CAKE_MAX_TINS; c++) {
 			cake_clear_tin(sch, c);
 			qd->tins[c].cparams.mtu_time = qd->tins[ft].cparams.mtu_time;
 		}
+
+		backlog -= sch->qstats.backlog;
+		qlen -= sch->q.qlen;
+		if (qlen)
+			qdisc_tree_reduce_backlog(sch, qlen, backlog);
 	}
 
 	qd->rate_ns   = qd->tins[ft].tin_rate_ns;
-- 
2.43.0