[PATCH] netem: fix rate extension and drop accounting

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

commit 7bc0f28c7a0c (netem: rate extension) did wrong maths when packet
is enqueued while queue is not empty.

Result is unexpected cumulative delays

# tc qd add dev eth0 root est 1sec 4sec netem delay 200ms rate 100kbit
# ping -i 0.1 172.30.42.18
PING 172.30.42.18 (172.30.42.18) 56(84) bytes of data.
64 bytes from 172.30.42.18: icmp_req=1 ttl=64 time=208 ms
64 bytes from 172.30.42.18: icmp_req=2 ttl=64 time=424 ms
64 bytes from 172.30.42.18: icmp_req=3 ttl=64 time=838 ms
64 bytes from 172.30.42.18: icmp_req=4 ttl=64 time=1142 ms
64 bytes from 172.30.42.18: icmp_req=5 ttl=64 time=1335 ms
64 bytes from 172.30.42.18: icmp_req=6 ttl=64 time=1949 ms
64 bytes from 172.30.42.18: icmp_req=7 ttl=64 time=2450 ms
64 bytes from 172.30.42.18: icmp_req=8 ttl=64 time=2840 ms
64 bytes from 172.30.42.18: icmp_req=9 ttl=64 time=3121 ms
64 bytes from 172.30.42.18: icmp_req=10 ttl=64 time=3291 ms
64 bytes from 172.30.42.18: icmp_req=11 ttl=64 time=3784 ms

This patch also fixes a double drop accounting in case packet is dropped
in tfifo_enqueue()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Andreas Terzis <aterzis@google.com>
Cc: Mark Gordon <msg@google.com>
Cc: Hagen Paul Pfeifer <hagen@jauu.net>
---
 net/sched/sch_netem.c |   14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index a2a95aa..e8b5ac3 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -368,7 +368,6 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 	/* We don't fill cb now as skb_unshare() may invalidate it */
 	struct netem_skb_cb *cb;
 	struct sk_buff *skb2;
-	int ret;
 	int count = 1;
 
 	/* Random duplication */
@@ -443,14 +442,14 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 				 * calculate this time bonus and substract
 				 * from delay.
 				 */
-				delay -= now - netem_skb_cb(skb_peek(list))->time_to_send;
+				delay -= netem_skb_cb(skb_peek(list))->time_to_send - now;
 				now = netem_skb_cb(skb_peek_tail(list))->time_to_send;
 			}
 		}
 
 		cb->time_to_send = now + delay;
 		++q->counter;
-		ret = tfifo_enqueue(skb, sch);
+		return tfifo_enqueue(skb, sch);
 	} else {
 		/*
 		 * Do re-ordering by putting one out of N packets at the front
@@ -462,16 +461,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		__skb_queue_head(&sch->q, skb);
 		sch->qstats.backlog += qdisc_pkt_len(skb);
 		sch->qstats.requeues++;
-		ret = NET_XMIT_SUCCESS;
-	}
-
-	if (ret != NET_XMIT_SUCCESS) {
-		if (net_xmit_drop_count(ret)) {
-			sch->qstats.drops++;
-			return ret;
-		}
 	}
-
 	return NET_XMIT_SUCCESS;
 }