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

On Fri, Oct 15, 2021 at 5:59 AM Bob Briscoe [off-list ref] wrote:

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.

What you describe about a minimal backlog was already there with
ce_threshold handling ?

Or is it something exclusive to L4S ?

This deserves a separate patch, if anything :)

Bob


On 14/10/2021 18:59, Eric Dumazet wrote:

quoted

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 <redacted>
Cc: Tom Henderson <redacted>
Cc: Bob Briscoe <redacted>
---
  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/