From: Neal Cardwell Date: Thu, 22 May 2014 14:41:08 +0000 (-0400) Subject: tcp: make cwnd-limited checks measurement-based, and gentler X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=ca8a22634381537c92b5a10308652e1c38fd9edf;p=GitHub%2Fexynos8895%2Fandroid_kernel_samsung_universal8895.git tcp: make cwnd-limited checks measurement-based, and gentler Experience with the recent e114a710aa50 ("tcp: fix cwnd limited checking to improve congestion control") has shown that there are common cases where that commit can cause cwnd to be much larger than necessary. This leads to TSO autosizing cooking skbs that are too large, among other things. The main problems seemed to be: (1) That commit attempted to predict the future behavior of the connection by looking at the write queue (if TSO or TSQ limit sending). That prediction sometimes overestimated future outstanding packets. (2) That commit always allowed cwnd to grow to twice the number of outstanding packets (even in congestion avoidance, where this is not needed). This commit improves both of these, by: (1) Switching to a measurement-based approach where we explicitly track the largest number of packets in flight during the past window ("max_packets_out"), and remember whether we were cwnd-limited at the moment we finished sending that flight. (2) Only allowing cwnd to grow to twice the number of outstanding packets ("max_packets_out") in slow start. In congestion avoidance mode we now only allow cwnd to grow if it was fully utilized. Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- diff --git a/include/linux/tcp.h b/include/linux/tcp.h index bc35e4709e8e..a0513210798f 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -197,7 +197,8 @@ struct tcp_sock { u8 do_early_retrans:1,/* Enable RFC5827 early-retransmit */ syn_data:1, /* SYN includes data */ syn_fastopen:1, /* SYN includes Fast Open option */ - syn_data_acked:1;/* data in SYN is acked by SYN-ACK */ + syn_data_acked:1,/* data in SYN is acked by SYN-ACK */ + is_cwnd_limited:1;/* forward progress limited by snd_cwnd? */ u32 tlp_high_seq; /* snd_nxt at the time of TLP retransmit. */ /* RTT measurement */ @@ -209,6 +210,8 @@ struct tcp_sock { u32 packets_out; /* Packets which are "in flight" */ u32 retrans_out; /* Retransmitted packets out */ + u32 max_packets_out; /* max packets_out in last window */ + u32 max_packets_seq; /* right edge of max_packets_out flight */ u16 urg_data; /* Saved octet of OOB data and control flags */ u8 ecn_flags; /* ECN status bits. */ @@ -230,7 +233,6 @@ struct tcp_sock { u32 snd_cwnd_clamp; /* Do not allow snd_cwnd to grow above this */ u32 snd_cwnd_used; u32 snd_cwnd_stamp; - u32 lsnd_pending; /* packets inflight or unsent since last xmit */ u32 prior_cwnd; /* Congestion window at start of Recovery. */ u32 prr_delivered; /* Number of newly delivered packets to * receiver in Recovery. */ diff --git a/include/net/tcp.h b/include/net/tcp.h index f5d6ca4a9d28..e80abe4486cb 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -971,8 +971,9 @@ static inline u32 tcp_wnd_end(const struct tcp_sock *tp) /* We follow the spirit of RFC2861 to validate cwnd but implement a more * flexible approach. The RFC suggests cwnd should not be raised unless - * it was fully used previously. But we allow cwnd to grow as long as the - * application has used half the cwnd. + * it was fully used previously. And that's exactly what we do in + * congestion avoidance mode. But in slow start we allow cwnd to grow + * as long as the application has used half the cwnd. * Example : * cwnd is 10 (IW10), but application sends 9 frames. * We allow cwnd to reach 18 when all frames are ACKed. @@ -985,7 +986,11 @@ static inline bool tcp_is_cwnd_limited(const struct sock *sk) { const struct tcp_sock *tp = tcp_sk(sk); - return tp->snd_cwnd < 2 * tp->lsnd_pending; + /* If in slow start, ensure cwnd grows to twice what was ACKed. */ + if (tp->snd_cwnd <= tp->snd_ssthresh) + return tp->snd_cwnd < 2 * tp->max_packets_out; + + return tp->is_cwnd_limited; } static inline void tcp_check_probe_timer(struct sock *sk) diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 3d61c52bdf79..d463c35db33d 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1402,11 +1402,19 @@ static void tcp_cwnd_application_limited(struct sock *sk) tp->snd_cwnd_stamp = tcp_time_stamp; } -static void tcp_cwnd_validate(struct sock *sk, u32 unsent_segs) +static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited) { struct tcp_sock *tp = tcp_sk(sk); - tp->lsnd_pending = tp->packets_out + unsent_segs; + /* Track the maximum number of outstanding packets in each + * window, and remember whether we were cwnd-limited then. + */ + if (!before(tp->snd_una, tp->max_packets_seq) || + tp->packets_out > tp->max_packets_out) { + tp->max_packets_out = tp->packets_out; + tp->max_packets_seq = tp->snd_nxt; + tp->is_cwnd_limited = is_cwnd_limited; + } if (tcp_is_cwnd_limited(sk)) { /* Network is feed fully. */ @@ -1660,7 +1668,8 @@ static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len, * * This algorithm is from John Heffner. */ -static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb) +static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb, + bool *is_cwnd_limited) { struct tcp_sock *tp = tcp_sk(sk); const struct inet_connection_sock *icsk = inet_csk(sk); @@ -1724,6 +1733,9 @@ static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb) if (!tp->tso_deferred) tp->tso_deferred = 1 | (jiffies << 1); + if (cong_win < send_win && cong_win < skb->len) + *is_cwnd_limited = true; + return true; send_now: @@ -1881,9 +1893,10 @@ 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, unsent_segs = 0; + unsigned int tso_segs, sent_pkts; int cwnd_quota; int result; + bool is_cwnd_limited = false; sent_pkts = 0; @@ -1908,6 +1921,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, cwnd_quota = tcp_cwnd_test(tp, skb); if (!cwnd_quota) { + is_cwnd_limited = true; if (push_one == 2) /* Force out a loss probe pkt. */ cwnd_quota = 1; @@ -1924,8 +1938,9 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, nonagle : TCP_NAGLE_PUSH)))) break; } else { - if (!push_one && tcp_tso_should_defer(sk, skb)) - goto compute_unsent_segs; + if (!push_one && + tcp_tso_should_defer(sk, skb, &is_cwnd_limited)) + break; } /* TCP Small Queues : @@ -1950,14 +1965,8 @@ 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) { - u32 unsent_bytes; - -compute_unsent_segs: - unsent_bytes = tp->write_seq - tp->snd_nxt; - unsent_segs = DIV_ROUND_UP(unsent_bytes, mss_now); + if (atomic_read(&sk->sk_wmem_alloc) > limit) break; - } } limit = mss_now; @@ -1997,7 +2006,7 @@ repair: /* Send one loss probe per tail loss episode. */ if (push_one != 2) tcp_schedule_loss_probe(sk); - tcp_cwnd_validate(sk, unsent_segs); + tcp_cwnd_validate(sk, is_cwnd_limited); return false; } return (push_one == 2) || (!tp->packets_out && tcp_send_head(sk));