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@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 71d5f2f29fa62b92cb4ae4d4762b5322d017f5d3..0d41c2657f45ff1c359189b3d157deedc951e17f 100644 (file)
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -388,7 +388,7 @@ 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 = usecs_to_jiffies(rtt_us) << 3;