From: Eric Dumazet <edumazet@google.com>

At enqueue time, check sojourn time of packet at head of the queue,
and return NET_XMIT_CN instead of NET_XMIT_SUCCESS if this sejourn
time is above codel @target.

This permits local TCP stack to call tcp_enter_cwr() and reduce its cwnd
without drops (for example if ECN is not enabled for the flow)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Dave Taht <redacted>
Cc: Tom Herbert <redacted>
Cc: Matt Mathis <redacted>
Cc: Yuchung Cheng <redacted>
Cc: Nandita Dukkipati <redacted>
Cc: Neal Cardwell <ncardwell@google.com>
---
 include/linux/pkt_sched.h |    1 +
 include/net/codel.h       |    2 +-
 net/sched/sch_fq_codel.c  |   19 +++++++++++++++----
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/include/linux/pkt_sched.h b/include/linux/pkt_sched.h
index 32aef0a..4d409a5 100644
--- a/include/linux/pkt_sched.h
+++ b/include/linux/pkt_sched.h

@@ -714,6 +714,7 @@ struct tc_fq_codel_qd_stats {

                                */
       __u32   new_flows_len;  /* count of flows in new list */
       __u32   old_flows_len;  /* count of flows in old list */
+       __u32   congestion_count;
 };

 struct tc_fq_codel_cl_stats {

diff --git a/include/net/codel.h b/include/net/codel.h
index 550debf..8c7d6a7 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h

@@ -148,7 +148,7 @@ struct codel_vars {

 * struct codel_stats - contains codel shared variables and stats
 * @maxpacket: largest packet we've seen so far
 * @drop_count:        temp count of dropped packets in dequeue()
- * ecn_mark:   number of packets we ECN marked instead of dropping
+ * @ecn_mark:  number of packets we ECN marked instead of dropping
 */
 struct codel_stats {
       u32             maxpacket;

diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index 9fc1c62..c0485a0 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c

@@ -62,6 +62,7 @@ struct fq_codel_sched_data {

       struct codel_stats cstats;
       u32             drop_overlimit;
       u32             new_flow_count;
+       u32             congestion_count;

       struct list_head new_flows;     /* list of new flows */
       struct list_head old_flows;     /* list of old flows */

@@ -196,16 +197,25 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch)

               flow->deficit = q->quantum;
               flow->dropped = 0;
       }
-       if (++sch->q.qlen < sch->limit)
+       if (++sch->q.qlen < sch->limit) {
+               codel_time_t hdelay = codel_get_enqueue_time(skb) -
+                                     codel_get_enqueue_time(flow->head);
+
+               /* If this flow is congested, tell the caller ! */
+               if (codel_time_after(hdelay, q->cparams.target)) {
+                       q->congestion_count++;
+                       return NET_XMIT_CN;
+               }
               return NET_XMIT_SUCCESS;
-
+       }
       q->drop_overlimit++;
       /* Return Congestion Notification only if we dropped a packet
        * from this flow.
        */
-       if (fq_codel_drop(sch) == idx)
+       if (fq_codel_drop(sch) == idx) {
+               q->congestion_count++;
               return NET_XMIT_CN;
-
+       }
       /* As we dropped a packet, better let upper stack know this */
       qdisc_tree_decrease_qlen(sch, 1);
       return NET_XMIT_SUCCESS;

@@ -467,6 +477,7 @@ static int fq_codel_dump_stats(struct Qdisc *sch, struct gnet_dump *d)

       st.qdisc_stats.maxpacket = q->cstats.maxpacket;
       st.qdisc_stats.drop_overlimit = q->drop_overlimit;
+       st.qdisc_stats.congestion_count = q->congestion_count;
       st.qdisc_stats.ecn_mark = q->cstats.ecn_mark;
       st.qdisc_stats.new_flow_count = q->new_flow_count;