net_sched: sch_fq: add dctcp-like marking

Similar to 80ba92fa ("codel: add ce_threshold attribute")

After EDT adoption, it became easier to implement DCTCP-like CE marking.

In many cases, queues are not building in the network fabric but on
the hosts themselves.

If packets leaving fq missed their Earliest Departure Time by XXX usec,
we mark them with ECN CE. This gives a feedback (after one RTT) to
the sender to slow down and find better operating mode.

Example :

tc qd replace dev eth0 root fq ce_threshold 2.5ms
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/pkt_sched.h

@@ -864,6 +864,8 @@ enum {
 
 	TCA_FQ_LOW_RATE_THRESHOLD, /* per packet delay under this rate */
 
+	TCA_FQ_CE_THRESHOLD,	/* DCTCP-like CE-marking threshold */
+
 	__TCA_FQ_MAX
 };
 
@@ -882,6 +884,7 @@ struct tc_fq_qd_stats {
 	__u32	inactive_flows;
 	__u32	throttled_flows;
 	__u32	unthrottle_latency_ns;
+	__u64	ce_mark;		/* packets above ce_threshold */
 };
 
 /* Heavy-Hitter Filter */

net/sched/sch_fq.c

@@ -94,6 +94,7 @@ struct fq_sched_data {
 	u32		flow_refill_delay;
 	u32		flow_plimit;	/* max packets per flow */
 	unsigned long	flow_max_rate;	/* optional max rate per flow */
+	u64		ce_threshold;
 	u32		orphan_mask;	/* mask for orphaned skb */
 	u32		low_rate_threshold;
 	struct rb_root	*fq_root;
@@ -107,6 +108,7 @@ struct fq_sched_data {
 	u64		stat_gc_flows;
 	u64		stat_internal_packets;
 	u64		stat_throttled;
+	u64		stat_ce_mark;
 	u64		stat_flows_plimit;
 	u64		stat_pkts_too_long;
 	u64		stat_allocation_errors;
@@ -454,6 +456,11 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 			fq_flow_set_throttled(q, f);
 			goto begin;
 		}
+		if (time_next_packet &&
+		    (s64)(now - time_next_packet - q->ce_threshold) > 0) {
+			INET_ECN_set_ce(skb);
+			q->stat_ce_mark++;
+		}
 	}
 
 	skb = fq_dequeue_head(sch, f);
@@ -650,6 +657,7 @@ static const struct nla_policy fq_policy[TCA_FQ_MAX + 1] = {
 	[TCA_FQ_BUCKETS_LOG]		= { .type = NLA_U32 },
 	[TCA_FQ_FLOW_REFILL_DELAY]	= { .type = NLA_U32 },
 	[TCA_FQ_LOW_RATE_THRESHOLD]	= { .type = NLA_U32 },
+	[TCA_FQ_CE_THRESHOLD]		= { .type = NLA_U32 },
 };
 
 static int fq_change(struct Qdisc *sch, struct nlattr *opt,
@@ -729,6 +737,10 @@ static int fq_change(struct Qdisc *sch, struct nlattr *opt,
 	if (tb[TCA_FQ_ORPHAN_MASK])
 		q->orphan_mask = nla_get_u32(tb[TCA_FQ_ORPHAN_MASK]);
 
+	if (tb[TCA_FQ_CE_THRESHOLD])
+		q->ce_threshold = (u64)NSEC_PER_USEC *
+				  nla_get_u32(tb[TCA_FQ_CE_THRESHOLD]);
+
 	if (!err) {
 		sch_tree_unlock(sch);
 		err = fq_resize(sch, fq_log);
@@ -779,6 +791,10 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt,
 	q->fq_trees_log		= ilog2(1024);
 	q->orphan_mask		= 1024 - 1;
 	q->low_rate_threshold	= 550000 / 8;
+
+	/* Default ce_threshold of 4294 seconds */
+	q->ce_threshold		= (u64)NSEC_PER_USEC * ~0U;
+
 	qdisc_watchdog_init_clockid(&q->watchdog, sch, CLOCK_MONOTONIC);
 
 	if (opt)
@@ -792,6 +808,7 @@ static int fq_init(struct Qdisc *sch, struct nlattr *opt,
 static int fq_dump(struct Qdisc *sch, struct sk_buff *skb)
 {
 	struct fq_sched_data *q = qdisc_priv(sch);
+	u64 ce_threshold = q->ce_threshold;
 	struct nlattr *opts;
 
 	opts = nla_nest_start(skb, TCA_OPTIONS);
@@ -800,6 +817,8 @@ static int fq_dump(struct Qdisc *sch, struct sk_buff *skb)
 
 	/* TCA_FQ_FLOW_DEFAULT_RATE is not used anymore */
 
+	do_div(ce_threshold, NSEC_PER_USEC);
+
 	if (nla_put_u32(skb, TCA_FQ_PLIMIT, sch->limit) ||
 	    nla_put_u32(skb, TCA_FQ_FLOW_PLIMIT, q->flow_plimit) ||
 	    nla_put_u32(skb, TCA_FQ_QUANTUM, q->quantum) ||
@@ -812,6 +831,7 @@ static int fq_dump(struct Qdisc *sch, struct sk_buff *skb)
 	    nla_put_u32(skb, TCA_FQ_ORPHAN_MASK, q->orphan_mask) ||
 	    nla_put_u32(skb, TCA_FQ_LOW_RATE_THRESHOLD,
 			q->low_rate_threshold) ||
+	    nla_put_u32(skb, TCA_FQ_CE_THRESHOLD, (u32)ce_threshold) ||
 	    nla_put_u32(skb, TCA_FQ_BUCKETS_LOG, q->fq_trees_log))
 		goto nla_put_failure;
 
@@ -841,6 +861,7 @@ static int fq_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 	st.throttled_flows	  = q->throttled_flows;
 	st.unthrottle_latency_ns  = min_t(unsigned long,
 					  q->unthrottle_latency_ns, ~0U);
+	st.ce_mark		  = q->stat_ce_mark;
 	sch_tree_unlock(sch);
 
 	return gnet_stats_copy_app(d, &st, sizeof(st));