- cubic fix

diff --git a/kernel-small_fixes.patch b/kernel-small_fixes.patch
index 28517b8ba73a02d99c71733dc738dde08270f500..01e03aa64da497ac17ba88984c47f9e05ec251da 100644 (file)
--- a/kernel-small_fixes.patch
+++ b/kernel-small_fixes.patch
@@ -27,3 +27,73 @@
                        fi
                done
 
+From 30927520dbae297182990bb21d08762bcc35ce1d Mon Sep 17 00:00:00 2001
+From: Eric Dumazet <edumazet@google.com>
+Date: Wed, 9 Sep 2015 21:55:07 -0700
+Subject: [PATCH] tcp_cubic: better follow cubic curve after idle period
+
+Jana Iyengar found an interesting issue on CUBIC :
+
+The epoch is only updated/reset initially and when experiencing losses.
+The delta "t" of now - epoch_start can be arbitrary large after app idle
+as well as the bic_target. Consequentially the slope (inverse of
+ca->cnt) would be really large, and eventually ca->cnt would be
+lower-bounded in the end to 2 to have delayed-ACK slow-start behavior.
+
+This particularly shows up when slow_start_after_idle is disabled
+as a dangerous cwnd inflation (1.5 x RTT) after few seconds of idle
+time.
+
+Jana initial fix was to reset epoch_start if app limited,
+but Neal pointed out it would ask the CUBIC algorithm to recalculate the
+curve so that we again start growing steeply upward from where cwnd is
+now (as CUBIC does just after a loss). Ideally we'd want the cwnd growth
+curve to be the same shape, just shifted later in time by the amount of
+the idle period.
+
+Reported-by: Jana Iyengar <jri@google.com>
+Signed-off-by: Eric Dumazet <edumazet@google.com>
+Signed-off-by: Yuchung Cheng <ycheng@google.com>
+Signed-off-by: Neal Cardwell <ncardwell@google.com>
+Cc: Stephen Hemminger <stephen@networkplumber.org>
+Cc: Sangtae Ha <sangtae.ha@gmail.com>
+Cc: Lawrence Brakmo <lawrence@brakmo.org>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ net/ipv4/tcp_cubic.c | 16 ++++++++++++++++
+ 1 file changed, 16 insertions(+)
+
+diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
+index 28011fb1..c6ded6b 100644
+--- a/net/ipv4/tcp_cubic.c
++++ b/net/ipv4/tcp_cubic.c
+@@ -151,6 +151,21 @@ static void bictcp_init(struct sock *sk)
+               tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
+ }
+ 
++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);
++
++              /* We were application limited (idle) for a while.
++               * Shift epoch_start to keep cwnd growth to cubic curve.
++               */
++              if (ca->epoch_start && delta > 0)
++                      ca->epoch_start += delta;
++              return;
++      }
++}
++
+ /* calculate the cubic root of x using a table lookup followed by one
+  * Newton-Raphson iteration.
+  * Avg err ~= 0.195%
+@@ -450,6 +465,7 @@ static struct tcp_congestion_ops cubictcp __read_mostly = {
+       .cong_avoid     = bictcp_cong_avoid,
+       .set_state      = bictcp_state,
+       .undo_cwnd      = bictcp_undo_cwnd,
++      .cwnd_event     = bictcp_cwnd_event,
+       .pkts_acked     = bictcp_acked,
+       .owner          = THIS_MODULE,
+       .name           = "cubic",