linux-kernel - [ 19/42] cbq: incorrect processing of high limits

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Message-Id: <20130429184754.548083065@linuxfoundation.org>
Date:	Mon, 29 Apr 2013 12:02:01 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Vasily Averin <vvs@...nvz.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [ 19/42] cbq: incorrect processing of high limits

3.8-stable review patch.  If anyone has any objections, please let me know.

------------------


From: Vasily Averin <vvs@...allels.com>

[ Upstream commit f0f6ee1f70c4eaab9d52cf7d255df4bd89f8d1c2 ]

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>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/sched/sch_cbq.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- 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 (;;) {


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