[PKT_SCHED]: (G)RED: Introduce hard dropping

Introduces a new flag TC_RED_HARDDROP which specifies that if ECN
marking is enabled packets should still be dropped once the
average queue length exceeds the maximum threshold.

This _may_ help to avoid global synchronisation during small
bursts of peers advertising but not caring about ECN. Use this
option very carefully, it does more harm than good if
(qth_max - qth_min) does not cover at least two average burst
cycles.

The difference to the current behaviour, in which we'd run into
the hard queue limit, is that due to the low pass filter of RED
short bursts are less likely to cause a global synchronisation.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

net/sched/sch_gred.c

@@ -146,6 +146,11 @@ static inline int gred_use_ecn(struct gred_sched *t)
 	return t->red_flags & TC_RED_ECN;
 }
 
+static inline int gred_use_harddrop(struct gred_sched *t)
+{
+	return t->red_flags & TC_RED_HARDDROP;
+}
+
 static int gred_enqueue(struct sk_buff *skb, struct Qdisc* sch)
 {
 	struct gred_sched_data *q=NULL;
@@ -214,7 +219,8 @@ static int gred_enqueue(struct sk_buff *skb, struct Qdisc* sch)
 
 		case RED_HARD_MARK:
 			sch->qstats.overlimits++;
-			if (!gred_use_ecn(t) || !INET_ECN_set_ce(skb)) {
+			if (gred_use_harddrop(t) || !gred_use_ecn(t) ||
+			    !INET_ECN_set_ce(skb)) {
 				q->stats.forced_drop++;
 				goto congestion_drop;
 			}