tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance

Commit 814d488c6126 ("tcp: fix the timid additive increase on stretch
ACKs") fixed a bug where tcp_cong_avoid_ai() would either credit a
connection with an increase of snd_cwnd_cnt, or increase snd_cwnd, but
not both, resulting in cwnd increasing by 1 packet on at most every
alternate invocation of tcp_cong_avoid_ai().

Although the commit correctly implemented the CUBIC algorithm, which
can increase cwnd by as much as 1 packet per 1 packet ACKed (2x per
RTT), in practice that could be too aggressive: in tests on network
paths with small buffers, YouTube server retransmission rates nearly
doubled.

This commit restores CUBIC to a maximum cwnd growth rate of 1 packet
per 2 packets ACKed (1.5x per RTT). In YouTube tests this restored
retransmit rates to low levels.

Testing: This patch has been tested in datacenter netperf transfers
and live youtube.com and google.com servers.

Fixes: 9cd981dcf174 ("tcp: fix stretch ACK bugs in CUBIC")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 4b276d1ed9807057986bd3b050e2e901bf1afec0..06d3d665a9fd1bfda5688907a284de83697273f6 100644 (file)
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -306,8 +306,10 @@ tcp_friendliness:
                }
        }
 
-       if (ca->cnt == 0)                       /* cannot be zero */
-               ca->cnt = 1;
+       /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
+        * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
+        */
+       ca->cnt = max(ca->cnt, 2U);
 }
 
 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)