openwrt/target/linux/generic/patches-3.3/049-codel-refine-one-condition-to-avoid-a-nul-rec_inv_sqrt.patch

From b8fc328668a74e1314a19266755a54abd875e5a6 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet@google.com>
Date: Sun, 29 Jul 2012 20:52:21 +0000
Subject: [PATCH] codel: refine one condition to avoid a nul rec_inv_sqrt

commit 2359a47671fc4fb0fe5e9945f76c2cb10792c0f8 upstream.

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@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 include/net/codel.h |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -305,6 +305,8 @@ static struct sk_buff *codel_dequeue(str
 			}
 		}
 	} 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(str
 		 * 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.