codel: refine one condition to avoid a nul rec_inv_sqrt

svn path=/dists/sid/linux/; revision=19403

debian/changelog

@@ -1,3 +1,9 @@
+linux (3.2.30-2) UNRELEASED; urgency=low
+
+  * codel: refine one condition to avoid a nul rec_inv_sqrt
+
+ -- Ben Hutchings <ben@decadent.org.uk>  Sat, 29 Sep 2012 14:19:46 +0200
+
 linux (3.2.30-1) unstable; urgency=low
 
   * New upstream stable update:

debian/patches/features/all/codel/0008-codel-refine-one-condition-to-avoid-a-nul-rec_inv_sq.patch

@@ -0,0 +1,53 @@
+From: Eric Dumazet <edumazet@google.com>
+Date: Sun, 29 Jul 2012 20:52:21 +0000
+Subject: 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(-)
+
+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.

debian/patches/series

@@ -271,6 +271,7 @@ features/all/codel/0004-net-codel-Add-missing-include-linux-prefetch.h.patch
 features/all/codel/0005-net-codel-fix-build-errors.patch
 features/all/codel/0006-codel-use-u16-field-instead-of-31bits-for-rec_inv_sq.patch
 features/all/codel/0007-fq_codel-should-use-qdisc-backlog-as-threshold.patch
+features/all/codel/0008-codel-refine-one-condition-to-avoid-a-nul-rec_inv_sq.patch
 
 # AppArmor userland compatibility.  This had better be gone in wheezy+1!
 features/all/AppArmor-compatibility-patch-for-v5-interface.patch