Recent kernels cause a lot of TCP retransmissions

[ ID] Interval Transfer Bitrate Retr Cwnd [ 5] 0.00-1.00 sec 2.24 GBytes 19.2 Gbits/sec 2767 442 KBytes [ 5] 1.00-2.00 sec 2.23 GBytes 19.1 Gbits/sec 2312 350 KBytes ^^^^

Replacing the qdisc with pfifo makes retransmissions go away.

It appears that a flow may have a delayed packet with a very near Tx time. Later, we may get busy processing Rx and the target Tx time will pass, but we won't service Tx since the CPU is busy with Rx. If Rx sees an ACK and we try to push more data for the delayed flow we may fastpath the skb, not realizing that there are already "ready to send" packets for this flow sitting in the qdisc.

Don't trust the fastpath if we are "behind" according to the projected Tx time for next flow waiting in the Qdisc. Because we consider anything within the offload window to be okay for fastpath we must consider the entire offload window as "now".

Qdisc config:

qdisc fq 8001: dev eth0 parent 1234:1 limit 10000p flow_limit 100p \ buckets 32768 orphan_mask 1023 bands 3 \ priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1 \ weights 589824 196608 65536 quantum 3028b initial_quantum 15140b \ low_rate_threshold 550Kbit \ refill_delay 40ms timer_slack 10us horizon 10s horizon_drop

For iperf this change seems to do fine, the reordering is gone. The fastpath still gets used most of the time:

gc 0 highprio 0 fastpath 142614 throttled 418309 latency 19.1us xx_behind 2731

where "xx_behind" counts how many times we hit the new "return false".

CC: stable@vger.kernel.org Fixes: 076433bd78d7 ("net_sched: sch_fq: add fast path for mostly idle qdisc") Signed-off-by: Jakub Kicinski kuba@kernel.org --- v2: - use Eric's condition (fix offload, don't care about throttled) - throttled -> delayed - explicitly CC stable, it won't build on 6.12 because of the offload horizon, so make sure they don't just drop this v1: https://lore.kernel.org/20241122162108.2697803-1-kuba@kernel.org

CC: jhs@mojatatu.com CC: xiyou.wangcong@gmail.com CC: jiri@resnulli.us --- net/sched/sch_fq.c | 6 ++++++ 1 file changed, 6 insertions(+)

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c index a97638bef6da..a5e87f9ea986 100644 --- a/net/sched/sch_fq.c +++ b/net/sched/sch_fq.c @@ -332,6 +332,12 @@ static bool fq_fastpath_check(const struct Qdisc *sch, struct sk_buff *skb, */ if (q->internal.qlen >= 8) return false; + + /* Ordering invariants fall apart if some delayed flows + * are ready but we haven't serviced them, yet. + */ + if (q->time_next_delayed_flow <= now + q->offload_horizon) + return false; }

sk = skb->sk;