From: Eric Dumazet Date: Tue, 6 Aug 2013 03:05:12 +0000 (-0700) Subject: tcp: cubic: fix bug in bictcp_acked() X-Git-Tag: accepted/tizen/common/20141203.182822~1696^2~37 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=cd6b423afd3c08b27e1fed52db828ade0addbc6b;p=platform%2Fkernel%2Flinux-arm64.git tcp: cubic: fix bug in bictcp_acked() 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 --- 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;