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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: 15c2715a52 ("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>
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