From: Eric Dumazet Date: Tue, 6 Aug 2013 03:05:12 +0000 (-0700) Subject: tcp: cubic: fix bug in bictcp_acked() X-Git-Tag: v3.0.96~15 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=dd3004635fe0607f64ca9ceca51f209fbc0a09a6;p=thirdparty%2Fkernel%2Fstable.git 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 Signed-off-by: Eric Dumazet Cc: Neal Cardwell Cc: Yuchung Cheng Acked-by: Neal Cardwell Signed-off-by: David S. Miller Signed-off-by: Greg Kroah-Hartman --- diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index b9d22e62074b9..b78eac2b70796 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -414,7 +414,7 @@ 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;