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 -------------------- net/ipv4/tcp_output.c | 21 ++++++++++++++------- 2 files changed, 14 insertions(+), 27 deletions(-) (limited to 'net') 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 diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 20847de991ea..f9181a133462 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1402,12 +1402,13 @@ static void tcp_cwnd_application_limited(struct sock *sk) tp->snd_cwnd_stamp = tcp_time_stamp; } -/* Congestion window validation. (RFC2861) */ -static void tcp_cwnd_validate(struct sock *sk) +static void tcp_cwnd_validate(struct sock *sk, u32 unsent_segs) { struct tcp_sock *tp = tcp_sk(sk); - if (tp->packets_out >= tp->snd_cwnd) { + tp->lsnd_pending = tp->packets_out + unsent_segs; + + if (tcp_is_cwnd_limited(sk, 0)) { /* Network is feed fully. */ tp->snd_cwnd_used = 0; tp->snd_cwnd_stamp = tcp_time_stamp; @@ -1880,7 +1881,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, { struct tcp_sock *tp = tcp_sk(sk); struct sk_buff *skb; - unsigned int tso_segs, sent_pkts; + unsigned int tso_segs, sent_pkts, unsent_segs = 0; int cwnd_quota; int result; @@ -1924,7 +1925,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, break; } else { if (!push_one && tcp_tso_should_defer(sk, skb)) - break; + goto compute_unsent_segs; } /* TCP Small Queues : @@ -1949,8 +1950,14 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, * there is no smp_mb__after_set_bit() yet */ smp_mb__after_clear_bit(); - if (atomic_read(&sk->sk_wmem_alloc) > limit) + if (atomic_read(&sk->sk_wmem_alloc) > limit) { + u32 unsent_bytes; + +compute_unsent_segs: + unsent_bytes = tp->write_seq - tp->snd_nxt; + unsent_segs = DIV_ROUND_UP(unsent_bytes, mss_now); break; + } } limit = mss_now; @@ -1990,7 +1997,7 @@ repair: /* Send one loss probe per tail loss episode. */ if (push_one != 2) tcp_schedule_loss_probe(sk); - tcp_cwnd_validate(sk); + tcp_cwnd_validate(sk, unsent_segs); return false; } return (push_one == 2) || (!tp->packets_out && tcp_send_head(sk)); -- cgit v1.2.3-59-g8ed1b