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Date: Fri, 15 Oct 2021 13:59:46 +0100
From: Bob Briscoe <ietf@...briscoe.net>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Neal Cardwell <ncardwell@...gle.com>,
Ingemar Johansson S <ingemar.s.johansson@...csson.com>,
Tom Henderson <tomh@...h.org>,
"David S . Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next 2/2] fq_codel: implement L4S style
ce_threshold_ect1 marking
Eric,
Because the threshold is in time units, I suggest the condition for
exceeding it needs to be AND'd with (*backlog > mtu), otherwise you can
get 100% solid marking at low link rates.
When ce_threshold is for DCs, low link rates are unlikely.
However, given ce_threshold_ect1 is mainly for the Internet, during
testing with 1ms threshold we encountered solid marking at low link
rates, so we had to add a 1 packet floor:
https://bobbriscoe.net/projects/latency/dctth_journal_draft20190726.pdf
Sorry to chime in after your patch went to net-next.
Bob
On 14/10/2021 18:59, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> Add TCA_FQ_CODEL_CE_THRESHOLD_ECT1 boolean option to select Low Latency,
> Low Loss, Scalable Throughput (L4S) style marking, along with ce_threshold.
>
> If enabled, only packets with ECT(1) can be transformed to CE
> if their sojourn time is above the ce_threshold.
>
> Note that this new option does not change rules for codel law.
> In particular, if TCA_FQ_CODEL_ECN is left enabled (this is
> the default when fq_codel qdisc is created), ECT(0) packets can
> still get CE if codel law (as governed by limit/target) decides so.
>
> Section 4.3.b of current draft [1] states:
>
> b. A scheduler with per-flow queues such as FQ-CoDel or FQ-PIE can
> be used for L4S. For instance within each queue of an FQ-CoDel
> system, as well as a CoDel AQM, there is typically also ECN
> marking at an immediate (unsmoothed) shallow threshold to support
> use in data centres (see Sec.5.2.7 of [RFC8290]). This can be
> modified so that the shallow threshold is solely applied to
> ECT(1) packets. Then if there is a flow of non-ECN or ECT(0)
> packets in the per-flow-queue, the Classic AQM (e.g. CoDel) is
> applied; while if there is a flow of ECT(1) packets in the queue,
> the shallower (typically sub-millisecond) threshold is applied.
>
> Tested:
>
> tc qd replace dev eth1 root fq_codel ce_threshold_ect1 50usec
>
> netperf ... -t TCP_STREAM -- K dctcp
>
> tc -s -d qd sh dev eth1
> qdisc fq_codel 8022: root refcnt 32 limit 10240p flows 1024 quantum 9212 target 5ms ce_threshold_ect1 49us interval 100ms memory_limit 32Mb ecn drop_batch 64
> Sent 14388596616 bytes 9543449 pkt (dropped 0, overlimits 0 requeues 152013)
> backlog 0b 0p requeues 152013
> maxpacket 68130 drop_overlimit 0 new_flow_count 95678 ecn_mark 0 ce_mark 7639
> new_flows_len 0 old_flows_len 0
>
> [1] L4S current draft:
> https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-l4s-arch
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: Ingemar Johansson S <ingemar.s.johansson@...csson.com>
> Cc: Tom Henderson <tomh@...h.org>
> Cc: Bob Briscoe <in@...briscoe.net>
> ---
> include/net/codel.h | 2 ++
> include/net/codel_impl.h | 18 +++++++++++++++---
> include/uapi/linux/pkt_sched.h | 1 +
> net/mac80211/sta_info.c | 1 +
> net/sched/sch_fq_codel.c | 15 +++++++++++----
> 5 files changed, 30 insertions(+), 7 deletions(-)
>
> diff --git a/include/net/codel.h b/include/net/codel.h
> index a6e428f801350809322aaff08d92904e059c3b5a..5e8b181b76b829d6af3c57809d9bc5f0578dd112 100644
> --- a/include/net/codel.h
> +++ b/include/net/codel.h
> @@ -102,6 +102,7 @@ static inline u32 codel_time_to_us(codel_time_t val)
> * @interval: width of moving time window
> * @mtu: device mtu, or minimal queue backlog in bytes.
> * @ecn: is Explicit Congestion Notification enabled
> + * @ce_threshold_ect1: if ce_threshold only marks ECT(1) packets
> */
> struct codel_params {
> codel_time_t target;
> @@ -109,6 +110,7 @@ struct codel_params {
> codel_time_t interval;
> u32 mtu;
> bool ecn;
> + bool ce_threshold_ect1;
> };
>
> /**
> diff --git a/include/net/codel_impl.h b/include/net/codel_impl.h
> index d289b91dcd65ecdc96dc0c9bf85d4a4be6961022..7af2c3eb3c43c24364519120aad5be77522854a6 100644
> --- a/include/net/codel_impl.h
> +++ b/include/net/codel_impl.h
> @@ -54,6 +54,7 @@ static void codel_params_init(struct codel_params *params)
> params->interval = MS2TIME(100);
> params->target = MS2TIME(5);
> params->ce_threshold = CODEL_DISABLED_THRESHOLD;
> + params->ce_threshold_ect1 = false;
> params->ecn = false;
> }
>
> @@ -246,9 +247,20 @@ static struct sk_buff *codel_dequeue(void *ctx,
> vars->rec_inv_sqrt);
> }
> end:
> - if (skb && codel_time_after(vars->ldelay, params->ce_threshold) &&
> - INET_ECN_set_ce(skb))
> - stats->ce_mark++;
> + if (skb && codel_time_after(vars->ldelay, params->ce_threshold)) {
> + bool set_ce = true;
> +
> + if (params->ce_threshold_ect1) {
> + /* Note: if skb_get_dsfield() returns -1, following
> + * gives INET_ECN_MASK, which is != INET_ECN_ECT_1.
> + */
> + u8 ecn = skb_get_dsfield(skb) & INET_ECN_MASK;
> +
> + set_ce = (ecn == INET_ECN_ECT_1);
> + }
> + if (set_ce && INET_ECN_set_ce(skb))
> + stats->ce_mark++;
> + }
> return skb;
> }
>
> diff --git a/include/uapi/linux/pkt_sched.h b/include/uapi/linux/pkt_sched.h
> index ec88590b3198441f18cc9def7bd40c48f0bc82a1..6be9a84cccfa79bace1f3f7123d02f484b67a25e 100644
> --- a/include/uapi/linux/pkt_sched.h
> +++ b/include/uapi/linux/pkt_sched.h
> @@ -840,6 +840,7 @@ enum {
> TCA_FQ_CODEL_CE_THRESHOLD,
> TCA_FQ_CODEL_DROP_BATCH_SIZE,
> TCA_FQ_CODEL_MEMORY_LIMIT,
> + TCA_FQ_CODEL_CE_THRESHOLD_ECT1,
> __TCA_FQ_CODEL_MAX
> };
>
> diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
> index 2b5acb37587f7068e2d11fe842ec963a556f1eb1..a39830418434d4bb74d238373f63a4858230fce5 100644
> --- a/net/mac80211/sta_info.c
> +++ b/net/mac80211/sta_info.c
> @@ -513,6 +513,7 @@ struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
> sta->cparams.target = MS2TIME(20);
> sta->cparams.interval = MS2TIME(100);
> sta->cparams.ecn = true;
> + sta->cparams.ce_threshold_ect1 = false;
>
> sta_dbg(sdata, "Allocated STA %pM\n", sta->sta.addr);
>
> diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
> index bb0cd6d3d2c2749d54e26368fb2558beedea85c9..033d65d06eb136ff704cddd3ee950a5c3a5d9831 100644
> --- a/net/sched/sch_fq_codel.c
> +++ b/net/sched/sch_fq_codel.c
> @@ -362,6 +362,7 @@ static const struct nla_policy fq_codel_policy[TCA_FQ_CODEL_MAX + 1] = {
> [TCA_FQ_CODEL_CE_THRESHOLD] = { .type = NLA_U32 },
> [TCA_FQ_CODEL_DROP_BATCH_SIZE] = { .type = NLA_U32 },
> [TCA_FQ_CODEL_MEMORY_LIMIT] = { .type = NLA_U32 },
> + [TCA_FQ_CODEL_CE_THRESHOLD_ECT1] = { .type = NLA_U8 },
> };
>
> static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
> @@ -408,6 +409,9 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
> q->cparams.ce_threshold = (val * NSEC_PER_USEC) >> CODEL_SHIFT;
> }
>
> + if (tb[TCA_FQ_CODEL_CE_THRESHOLD_ECT1])
> + q->cparams.ce_threshold_ect1 = !!nla_get_u8(tb[TCA_FQ_CODEL_CE_THRESHOLD_ECT1]);
> +
> if (tb[TCA_FQ_CODEL_INTERVAL]) {
> u64 interval = nla_get_u32(tb[TCA_FQ_CODEL_INTERVAL]);
>
> @@ -544,10 +548,13 @@ static int fq_codel_dump(struct Qdisc *sch, struct sk_buff *skb)
> q->flows_cnt))
> goto nla_put_failure;
>
> - if (q->cparams.ce_threshold != CODEL_DISABLED_THRESHOLD &&
> - nla_put_u32(skb, TCA_FQ_CODEL_CE_THRESHOLD,
> - codel_time_to_us(q->cparams.ce_threshold)))
> - goto nla_put_failure;
> + if (q->cparams.ce_threshold != CODEL_DISABLED_THRESHOLD) {
> + if (nla_put_u32(skb, TCA_FQ_CODEL_CE_THRESHOLD,
> + codel_time_to_us(q->cparams.ce_threshold)))
> + goto nla_put_failure;
> + if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_ECT1, q->cparams.ce_threshold_ect1))
> + goto nla_put_failure;
> + }
>
> return nla_nest_end(skb, opts);
>
--
________________________________________________________________
Bob Briscoe http://bobbriscoe.net/
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