netdev - [PATCH 1/1] cbq: incorrect processing of high limits

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Message-ID: <5159852C.8060003@parallels.com>
Date:	Mon, 01 Apr 2013 17:01:32 +0400
From:	Vasily Averin <vvs@...allels.com>
To:	netdev@...r.kernel.org
CC:	Alexey Kuznetsov <kuznet@...allels.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jamal Hadi Salim <jhs@...atatu.com>
Subject: [PATCH 1/1] cbq: incorrect processing of high limits

currently cbq works incorrectly for limits > 10% real link bandwidth,
and practically does not work for limits > 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->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->now until its synchronization
with real time.

Signed-off-by: Vasily Averin <vvs@...nvz.org>
Reviewed-by: Alexey Kuznetsov <kuznet@....inr.ac.ru>
---
 net/sched/sch_cbq.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
index 6aabd77..5b5c83a 100644
--- 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 (;;) {
--
1.7.5.4
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