netdev - [PATCH] codel: refine one condition to avoid a nul rec_inv

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Message-ID: <1343631141.2626.13293.camel@edumazet-glaptop>
Date:	Mon, 30 Jul 2012 08:52:21 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>, Anton Mich <lp2s1h@...il.com>
Subject: [PATCH] codel: refine one condition to avoid a nul rec_inv_sqrt

From: Eric Dumazet <edumazet@...gle.com>

One condition before codel_Newton_step() was not good if
we never left the dropping state for a flow. As a result
rec_inv_sqrt was 0, instead of the ~0 initial value.

codel control law was then set to a very aggressive mode, dropping
many packets before reaching 'target' and recovering from this problem.

To keep codel_vars_init() as efficient as possible, refine
the condition to make sure rec_inv_sqrt initial value is correct

Many thanks to Anton Mich for discovering the issue and suggesting
a fix.

Reported-by: Anton Mich <lp2s1h@...il.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/codel.h |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/net/codel.h b/include/net/codel.h
index 550debf..389cf62 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -305,6 +305,8 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
 			}
 		}
 	} else if (drop) {
+		u32 delta;
+
 		if (params->ecn && INET_ECN_set_ce(skb)) {
 			stats->ecn_mark++;
 		} else {
@@ -320,9 +322,11 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
 		 * assume that the drop rate that controlled the queue on the
 		 * last cycle is a good starting point to control it now.
 		 */
-		if (codel_time_before(now - vars->drop_next,
+		delta = vars->count - vars->lastcount;
+		if (delta > 1 &&
+		    codel_time_before(now - vars->drop_next,
 				      16 * params->interval)) {
-			vars->count = (vars->count - vars->lastcount) | 1;
+			vars->count = delta;
 			/* we dont care if rec_inv_sqrt approximation
 			 * is not very precise :
 			 * Next Newton steps will correct it quadratically.


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