From c2e7204d180f8efc80f27959ca9cf16fa17f67db Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 17 Sep 2015 08:38:00 -0700 Subject: [PATCH] tcp_cubic: do not set epoch_start in the future Tracking idle time in bictcp_cwnd_event() is imprecise, as epoch_start is normally set at ACK processing time, not at send time. Doing a proper fix would need to add an additional state variable, and does not seem worth the trouble, given CUBIC bug has been there forever before Jana noticed it. Let's simply not set epoch_start in the future, otherwise bictcp_update() could overflow and CUBIC would again grow cwnd too fast. This was detected thanks to a packetdrill test Neal wrote that was flaky before applying this fix. Fixes: 30927520dbae ("tcp_cubic: better follow cubic curve after idle period") Signed-off-by: Eric Dumazet Signed-off-by: Neal Cardwell Signed-off-by: Yuchung Cheng Cc: Jana Iyengar Signed-off-by: David S. Miller --- net/ipv4/tcp_cubic.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index c6ded6b2a79f..448c2615fece 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -154,14 +154,20 @@ static void bictcp_init(struct sock *sk) static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event) { if (event == CA_EVENT_TX_START) { - s32 delta = tcp_time_stamp - tcp_sk(sk)->lsndtime; struct bictcp *ca = inet_csk_ca(sk); + u32 now = tcp_time_stamp; + s32 delta; + + delta = now - tcp_sk(sk)->lsndtime; /* We were application limited (idle) for a while. * Shift epoch_start to keep cwnd growth to cubic curve. */ - if (ca->epoch_start && delta > 0) + if (ca->epoch_start && delta > 0) { ca->epoch_start += delta; + if (after(ca->epoch_start, now)) + ca->epoch_start = now; + } return; } } -- 2.20.1