[ 3.8.y.z extended stable ] Patch "tcp: cubic: fix bug in bictcp_acked()" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    tcp: cubic: fix bug in bictcp_acked()

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.11.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 9b05dcee7b7917a5aa7552ac9906b2e49b3487f7 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet at google.com>
Date: Mon, 5 Aug 2013 20:05:12 -0700
Subject: tcp: cubic: fix bug in bictcp_acked()

[ Upstream commit cd6b423afd3c08b27e1fed52db828ade0addbc6b ]

While investigating about strange increase of retransmit rates
on hosts ~24 days after boot, Van found hystart was disabled
if ca->epoch_start was 0, as following condition is true
when tcp_time_stamp high order bit is set.

(s32)(tcp_time_stamp - ca->epoch_start) < HZ

Quoting Van :

 At initialization & after every loss ca->epoch_start is set to zero so
 I believe that the above line will turn off hystart as soon as the 2^31
 bit is set in tcp_time_stamp & hystart will stay off for 24 days.
 I think we've observed that cubic's restart is too aggressive without
 hystart so this might account for the higher drop rate we observe.

Diagnosed-by: Van Jacobson <vanj at google.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Cc: Neal Cardwell <ncardwell at google.com>
Cc: Yuchung Cheng <ycheng at google.com>
Acked-by: Neal Cardwell <ncardwell at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/ipv4/tcp_cubic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index b6b591f..b6ae92a 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -416,7 +416,7 @@ static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 		return;

 	/* Discard delay samples right after fast recovery */
-	if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
+	if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 		return;

 	delay = (rtt_us << 3) / USEC_PER_MSEC;
--
1.8.1.2