net/sched: sch_cake: skip clearing unused tins during rate adjustment

When cake_configure_rates() is called from the dequeue path with
rate_adjust=true, it only needs to update the rate parameters. The
loop that clears the unused tins is both unnecessary and harmful in
this path:

 - cake_clear_tin() overwrites q->cur_tin and q->cur_flow, which are
   actively used by cake_dequeue(), corrupting the dequeue state.
 - iterating over the unused tins and their internal queues to purge
   packets adds needless overhead to the hot path.

Skip the entire loop when rate_adjust is set, as neither
cake_clear_tin() nor the mtu_time update are needed when only the
rate changes.

The clearing loop runs on every rate adjustment from the dequeue path,
clearing (max_tins - cur_tins) tins each time, so the cost grows the
fewer tins the configured mode actually uses. Testing cake_mq over veth
(8 rx/tx queues, 2 Gbit limit) with flent's [1] rrul and tcp_nup tests and
32 TCP upstreams shows a large drop in loaded latency and a throughput
gain, restoring behaviour to pre-15c2715a5264 levels:

  +------------+------+------+-------+-------+---------+
  | kernel     | mode | test |  base |  load |    tput |
  |            |      |      |  (ms) |  (ms) |  (Mbit) |
  +------------+------+------+-------+-------+---------+
  | net-next   | be   | rrul | 0.810 | 11.78 | 1469.67 |
  | net-next   | be   | nup  | 0.637 | 85.71 | 1243.15 |
  | net-next   | ds3  | rrul | 0.397 | 15.28 | 1770.06 |
  | net-next   | ds3  | nup  | 0.351 | 15.98 | 1799.39 |
  +------------+------+------+-------+-------+---------+
  | patched    | be   | rrul | 0.092 |  0.56 | 1873.40 |
  | patched    | be   | nup  | 0.109 |  1.82 | 1869.12 |
  | patched    | ds3  | rrul | 0.097 |  0.98 | 1866.10 |
  | patched    | ds3  | nup  | 0.101 |  0.51 | 1861.79 |
  +------------+------+------+-------+-------+---------+

The same trend holds on real hardware (IPQ8074A, 4 rx/tx queues,
OpenWrt): in besteffort mode the tcp_nup loaded latency drops from
~470 ms to ~4 ms.

[1] https://flent.org

Fixes: 15c2715a5264 ("net/sched: sch_cake: fixup cake_mq rate adjustment for diffserv config")
Signed-off-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
Tested-by: Mike Pham <mikepham4321@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20260720-sch_cake-skip-clearing-tins-v2-1-e6a8b0275c73@tu-berlin.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 505f63fecf64..f64be54ead49 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -2609,9 +2609,11 @@ static void cake_configure_rates(struct Qdisc *sch, u64 rate, bool rate_adjust)
 		break;
 	}
 
-	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;
+	if (!rate_adjust) {
+		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;
+		}
 	}
 
 	qd->rate_ns   = qd->tins[ft].tin_rate_ns;