netdev - [PATCH net-next v2 3/6] net: sched: RED: Introduce an ECN tail-dropping mode

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Date:   Wed, 11 Mar 2020 19:33:53 +0200
From:   Petr Machata <petrm@...lanox.com>
To:     netdev@...r.kernel.org
Cc:     Petr Machata <petrm@...lanox.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Roman Mashak <mrv@...atatu.com>, jhs@...atatu.com,
        xiyou.wangcong@...il.com, davem@...emloft.net, jiri@...lanox.com,
        mlxsw@...lanox.com
Subject: [PATCH net-next v2 3/6] net: sched: RED: Introduce an ECN tail-dropping mode

When the RED Qdisc is currently configured to enable ECN, the RED algorithm
is used to decide whether a certain SKB should be marked. If that SKB is
not ECN-capable, it is early-dropped.

It is also possible to keep all traffic in the queue, and just mark the
ECN-capable subset of it, as appropriate under the RED algorithm. Some
switches support this mode, and some installations make use of it.

To that end, add a new RED flag, TC_RED_TAILDROP. When the Qdisc is
configured with this flag, non-ECT traffic is enqueued (and tail-dropped
when the queue size is exhausted) instead of being early-dropped.

Signed-off-by: Petr Machata <petrm@...lanox.com>
---

Notes:
    v2:
    - Fix red_use_taildrop() condition in red_enqueue switch for
      probabilistic case.

 include/net/pkt_cls.h          |  1 +
 include/net/red.h              |  5 +++++
 include/uapi/linux/pkt_sched.h |  1 +
 net/sched/sch_red.c            | 31 +++++++++++++++++++++++++------
 4 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
index 341a66af8d59..9ad369aba678 100644
--- a/include/net/pkt_cls.h
+++ b/include/net/pkt_cls.h
@@ -727,6 +727,7 @@ struct tc_red_qopt_offload_params {
 	u32 limit;
 	bool is_ecn;
 	bool is_harddrop;
+	bool is_taildrop;
 	struct gnet_stats_queue *qstats;
 };
 
diff --git a/include/net/red.h b/include/net/red.h
index 5718d2b25637..372b4988118c 100644
--- a/include/net/red.h
+++ b/include/net/red.h
@@ -200,6 +200,11 @@ static inline bool red_get_flags(unsigned char flags,
 		return false;
 	}
 
+	if ((*p_flags & TC_RED_TAILDROP) && !(*p_flags & TC_RED_ECN)) {
+		NL_SET_ERR_MSG_MOD(extack, "taildrop mode is only meaningful with ECN");
+		return false;
+	}
+
 	*p_userbits = flags & ~historic_mask;
 	return true;
 }
diff --git a/include/uapi/linux/pkt_sched.h b/include/uapi/linux/pkt_sched.h
index 277df546e1a9..45d1c0e6444e 100644
--- a/include/uapi/linux/pkt_sched.h
+++ b/include/uapi/linux/pkt_sched.h
@@ -286,6 +286,7 @@ struct tc_red_qopt {
 #define TC_RED_ECN		1
 #define TC_RED_HARDDROP		2
 #define TC_RED_ADAPTATIVE	4
+#define TC_RED_TAILDROP		8
 };
 
 #define TC_RED_HISTORIC_FLAGS (TC_RED_ECN | TC_RED_HARDDROP | TC_RED_ADAPTATIVE)
diff --git a/net/sched/sch_red.c b/net/sched/sch_red.c
index 61d7c5a61279..1474f973ec6d 100644
--- a/net/sched/sch_red.c
+++ b/net/sched/sch_red.c
@@ -48,7 +48,7 @@ struct red_sched_data {
 	struct Qdisc		*qdisc;
 };
 
-#define RED_SUPPORTED_FLAGS TC_RED_HISTORIC_FLAGS
+#define RED_SUPPORTED_FLAGS (TC_RED_HISTORIC_FLAGS | TC_RED_TAILDROP)
 
 static inline int red_use_ecn(struct red_sched_data *q)
 {
@@ -60,6 +60,11 @@ static inline int red_use_harddrop(struct red_sched_data *q)
 	return q->flags & TC_RED_HARDDROP;
 }
 
+static inline int red_use_taildrop(struct red_sched_data *q)
+{
+	return q->flags & TC_RED_TAILDROP;
+}
+
 static int red_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		       struct sk_buff **to_free)
 {
@@ -80,23 +85,36 @@ static int red_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 
 	case RED_PROB_MARK:
 		qdisc_qstats_overlimit(sch);
-		if (!red_use_ecn(q) || !INET_ECN_set_ce(skb)) {
+		if (!red_use_ecn(q)) {
 			q->stats.prob_drop++;
 			goto congestion_drop;
 		}
 
-		q->stats.prob_mark++;
+		if (INET_ECN_set_ce(skb)) {
+			q->stats.prob_mark++;
+		} else if (!red_use_taildrop(q)) {
+			q->stats.prob_drop++;
+			goto congestion_drop;
+		}
+
+		/* Non-ECT packet in ECN taildrop mode: queue it. */
 		break;
 
 	case RED_HARD_MARK:
 		qdisc_qstats_overlimit(sch);
-		if (red_use_harddrop(q) || !red_use_ecn(q) ||
-		    !INET_ECN_set_ce(skb)) {
+		if (red_use_harddrop(q) || !red_use_ecn(q)) {
 			q->stats.forced_drop++;
 			goto congestion_drop;
 		}
 
-		q->stats.forced_mark++;
+		if (INET_ECN_set_ce(skb)) {
+			q->stats.forced_mark++;
+		} else if (!red_use_taildrop(q)) {
+			q->stats.forced_drop++;
+			goto congestion_drop;
+		}
+
+		/* Non-ECT packet in ECN taildrop mode: queue it. */
 		break;
 	}
 
@@ -171,6 +189,7 @@ static int red_offload(struct Qdisc *sch, bool enable)
 		opt.set.limit = q->limit;
 		opt.set.is_ecn = red_use_ecn(q);
 		opt.set.is_harddrop = red_use_harddrop(q);
+		opt.set.is_taildrop = red_use_taildrop(q);
 		opt.set.qstats = &sch->qstats;
 	} else {
 		opt.command = TC_RED_DESTROY;
-- 
2.20.1