From: Jonas Köppeler Date: Mon, 20 Jul 2026 21:14:52 +0000 (+0200) Subject: net/sched: sch_cake: skip clearing unused tins during rate adjustment X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a3729e0df005a936ceb3c2b0d167f01a2b03f970;p=thirdparty%2Fkernel%2Fstable.git 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 Tested-by: Mike Pham Acked-by: Toke Høiland-Jørgensen Link: https://patch.msgid.link/20260720-sch_cake-skip-clearing-tins-v2-1-e6a8b0275c73@tu-berlin.de Signed-off-by: Jakub Kicinski --- 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;