Re: [PATCH net-next v2] net/sched: sch_cake: skip clearing unused tins during rate adjustment

[Toke Høiland-Jørgensen]

On 20 July 2026 23.14.52 CEST, "Jonas Köppeler" <j.koeppeler@xxxxxxxxxxxx> wrote:
>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@xxxxxxxxxxxx>
>Tested-by: Mike Pham <mikepham4321@xxxxxxxxx>

Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxx>