From e114a710aa5058c0ba4aa1dfb105132aefeb5e04 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 30 Apr 2014 11:58:13 -0700 Subject: tcp: fix cwnd limited checking to improve congestion control Yuchung discovered tcp_is_cwnd_limited() was returning false in slow start phase even if the application filled the socket write queue. All congestion modules take into account tcp_is_cwnd_limited() before increasing cwnd, so this behavior limits slow start from probing the bandwidth at full speed. The problem is that even if write queue is full (aka we are _not_ application limited), cwnd can be under utilized if TSO should auto defer or TCP Small queues decided to hold packets. So the in_flight can be kept to smaller value, and we can get to the point tcp_is_cwnd_limited() returns false. With TCP Small Queues and FQ/pacing, this issue is more visible. We fix this by having tcp_cwnd_validate(), which is supposed to track such things, take into account unsent_segs, the number of segs that we are not sending at the moment due to TSO or TSQ, but intend to send real soon. Then when we are cwnd-limited, remember this fact while we are processing the window of ACKs that comes back. For example, suppose we have a brand new connection with cwnd=10; we are in slow start, and we send a flight of 9 packets. By the time we have received ACKs for all 9 packets we want our cwnd to be 18. We implement this by setting tp->lsnd_pending to 9, and considering ourselves to be cwnd-limited while cwnd is less than twice tp->lsnd_pending (2*9 -> 18). This makes tcp_is_cwnd_limited() more understandable, by removing the GSO/TSO kludge, that tried to work around the issue. Note the in_flight parameter can be removed in a followup cleanup patch. Signed-off-by: Eric Dumazet Signed-off-by: Neal Cardwell Signed-off-by: Yuchung Cheng Signed-off-by: David S. Miller --- net/ipv4/tcp_cong.c | 20 -------------------- 1 file changed, 20 deletions(-) (limited to 'net/ipv4/tcp_cong.c') diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c index 2b9464c93b88..a93b41ba05ff 100644 --- a/net/ipv4/tcp_cong.c +++ b/net/ipv4/tcp_cong.c @@ -276,26 +276,6 @@ int tcp_set_congestion_control(struct sock *sk, const char *name) return err; } -/* RFC2861 Check whether we are limited by application or congestion window - * This is the inverse of cwnd check in tcp_tso_should_defer - */ -bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight) -{ - const struct tcp_sock *tp = tcp_sk(sk); - u32 left; - - if (in_flight >= tp->snd_cwnd) - return true; - - left = tp->snd_cwnd - in_flight; - if (sk_can_gso(sk) && - left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd && - left < tp->xmit_size_goal_segs) - return true; - return left <= tcp_max_tso_deferred_mss(tp); -} -EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited); - /* Slow start is used when congestion window is no greater than the slow start * threshold. We base on RFC2581 and also handle stretch ACKs properly. * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but -- cgit v1.2.3