cbq: incorrect processing of high limits

VasilyAverin · davem330 · commit f0f6ee1f70c4 · 2013-04-02T14:29:20.000-04:00
currently cbq works incorrectly for limits &gt; 10% real link bandwidth,
and practically does not work for limits &gt; 50% real link bandwidth.
Below are results of experiments taken on 1 Gbit link

 In shaper | Actual Result
-----------+---------------
  100M     | 108 Mbps
  200M     | 244 Mbps
  300M     | 412 Mbps
  500M     | 893 Mbps

This happen because of q-&gt;now changes incorrectly in cbq_dequeue():
when it is called before real end of packet transmitting,
L2T is greater than real time delay, q_now gets an extra boost
but never compensate it.

To fix this problem we prevent change of q-&gt;now until its synchronization
with real time.

Signed-off-by: Vasily Averin &lt;vvs@openvz.org&gt;
Reviewed-by: Alexey Kuznetsov &lt;kuznet@ms2.inr.ac.ru&gt;
Acked-by: Eric Dumazet &lt;edumazet@google.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
@@ -962,8 +962,11 @@ cbq_dequeue(struct Qdisc *sch)
 		cbq_update(q);
 		if ((incr -= incr2) < 0)
 			incr = 0;
+		q->now += incr;
+	} else {
+		if (now > q->now)
+			q->now = now;
 	}
-	q->now += incr;
 	q->now_rt = now;
 
 	for (;;) {
