Re: [PATCH net-next 2/2] fq_codel: implement L4S style ce_threshold_ect1 marking

[Bob Briscoe <ietf@bobbriscoe.net>]

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@google.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@google.com>
> Cc: Neal Cardwell <ncardwell@google.com>
> Cc: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
> Cc: Tom Henderson <tomh@tomh.org>
> Cc: Bob Briscoe <in@bobbriscoe.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/