From 9e45a3e36b363cc4c79c70f2b4f994e66543a219 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:41 -0700 Subject: [PATCH 1/7] tcp: apply Kern's check on RTTs used for congestion control Currently ca_seq_rtt_us does not use Kern's check. Fix that by checking if any packet acked is a retransmit, for both RTT used for RTT estimation and congestion control. Fixes: 5b08e47ca ("tcp: prefer packet timing to TS-ECR for RTT") Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 944eaca69115..62ee71efd1ce 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2925,9 +2925,6 @@ static inline bool tcp_ack_update_rtt(struct sock *sk, const int flag, * Karn's algorithm forbids taking RTT if some retransmitted data * is acked (RFC6298). */ - if (flag & FLAG_RETRANS_DATA_ACKED) - seq_rtt_us = -1L; - if (seq_rtt_us < 0) seq_rtt_us = sack_rtt_us; @@ -3169,7 +3166,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, flag |= FLAG_SACK_RENEGING; skb_mstamp_get(&now); - if (likely(first_ackt.v64)) { + if (likely(first_ackt.v64) && !(flag & FLAG_RETRANS_DATA_ACKED)) { seq_rtt_us = skb_mstamp_us_delta(&now, &first_ackt); ca_rtt_us = skb_mstamp_us_delta(&now, &last_ackt); } From f672258391b42a5c7cc2732c9c063e56a85c8dbe Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:42 -0700 Subject: [PATCH 2/7] tcp: track min RTT using windowed min-filter Kathleen Nichols' algorithm for tracking the minimum RTT of a data stream over some measurement window. It uses constant space and constant time per update. Yet it almost always delivers the same minimum as an implementation that has to keep all the data in the window. The measurement window is tunable via sysctl.net.ipv4.tcp_min_rtt_wlen with a default value of 5 minutes. The algorithm keeps track of the best, 2nd best & 3rd best min values, maintaining an invariant that the measurement time of the n'th best >= n-1'th best. It also makes sure that the three values are widely separated in the time window since that bounds the worse case error when that data is monotonically increasing over the window. Upon getting a new min, we can forget everything earlier because it has no value - the new min is less than everything else in the window by definition and it's the most recent. So we restart fresh on every new min and overwrites the 2nd & 3rd choices. The same property holds for the 2nd & 3rd best. Therefore we have to maintain two invariants to maximize the information in the samples, one on values (1st.v <= 2nd.v <= 3rd.v) and the other on times (now-win <=1st.t <= 2nd.t <= 3rd.t <= now). These invariants determine the structure of the code The RTT input to the windowed filter is the minimum RTT measured from ACK or SACK, or as the last resort from TCP timestamps. The accessor tcp_min_rtt() returns the minimum RTT seen in the window. ~0U indicates it is not available. The minimum is 1usec even if the true RTT is below that. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- Documentation/networking/ip-sysctl.txt | 8 +++ include/linux/tcp.h | 3 + include/net/tcp.h | 7 +++ net/ipv4/sysctl_net_ipv4.c | 7 +++ net/ipv4/tcp.c | 1 + net/ipv4/tcp_input.c | 78 ++++++++++++++++++++++++-- net/ipv4/tcp_minisocks.c | 1 + 7 files changed, 100 insertions(+), 5 deletions(-) diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt index ebe94f2cab98..502d6a572b4f 100644 --- a/Documentation/networking/ip-sysctl.txt +++ b/Documentation/networking/ip-sysctl.txt @@ -384,6 +384,14 @@ tcp_mem - vector of 3 INTEGERs: min, pressure, max Defaults are calculated at boot time from amount of available memory. +tcp_min_rtt_wlen - INTEGER + The window length of the windowed min filter to track the minimum RTT. + A shorter window lets a flow more quickly pick up new (higher) + minimum RTT when it is moved to a longer path (e.g., due to traffic + engineering). A longer window makes the filter more resistant to RTT + inflations such as transient congestion. The unit is seconds. + Default: 300 + tcp_moderate_rcvbuf - BOOLEAN If set, TCP performs receive buffer auto-tuning, attempting to automatically size the buffer (no greater than tcp_rmem[2]) to diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 86a7edaa6797..90edef5508f9 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -217,6 +217,9 @@ struct tcp_sock { u32 mdev_max_us; /* maximal mdev for the last rtt period */ u32 rttvar_us; /* smoothed mdev_max */ u32 rtt_seq; /* sequence number to update rttvar */ + struct rtt_meas { + u32 rtt, ts; /* RTT in usec and sampling time in jiffies. */ + } rtt_min[3]; u32 packets_out; /* Packets which are "in flight" */ u32 retrans_out; /* Retransmitted packets out */ diff --git a/include/net/tcp.h b/include/net/tcp.h index eed94fc355c1..4a43152229ea 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -279,6 +279,7 @@ extern int sysctl_tcp_limit_output_bytes; extern int sysctl_tcp_challenge_ack_limit; extern unsigned int sysctl_tcp_notsent_lowat; extern int sysctl_tcp_min_tso_segs; +extern int sysctl_tcp_min_rtt_wlen; extern int sysctl_tcp_autocorking; extern int sysctl_tcp_invalid_ratelimit; extern int sysctl_tcp_pacing_ss_ratio; @@ -671,6 +672,12 @@ static inline bool tcp_ca_dst_locked(const struct dst_entry *dst) return dst_metric_locked(dst, RTAX_CC_ALGO); } +/* Minimum RTT in usec. ~0 means not available. */ +static inline u32 tcp_min_rtt(const struct tcp_sock *tp) +{ + return tp->rtt_min[0].rtt; +} + /* Compute the actual receive window we are currently advertising. * Rcv_nxt can be after the window if our peer push more data * than the offered window. diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index 894da3a70aff..13ab434c2909 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -576,6 +576,13 @@ static struct ctl_table ipv4_table[] = { .mode = 0644, .proc_handler = proc_dointvec }, + { + .procname = "tcp_min_rtt_wlen", + .data = &sysctl_tcp_min_rtt_wlen, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec + }, { .procname = "tcp_low_latency", .data = &sysctl_tcp_low_latency, diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index ac1bdbb50352..0cfa7c0c1e80 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -388,6 +388,7 @@ void tcp_init_sock(struct sock *sk) icsk->icsk_rto = TCP_TIMEOUT_INIT; tp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT); + tp->rtt_min[0].rtt = ~0U; /* So many TCP implementations out there (incorrectly) count the * initial SYN frame in their delayed-ACK and congestion control diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 62ee71efd1ce..eedb25db3947 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -95,6 +95,7 @@ int sysctl_tcp_stdurg __read_mostly; int sysctl_tcp_rfc1337 __read_mostly; int sysctl_tcp_max_orphans __read_mostly = NR_FILE; int sysctl_tcp_frto __read_mostly = 2; +int sysctl_tcp_min_rtt_wlen __read_mostly = 300; int sysctl_tcp_thin_dupack __read_mostly; @@ -2915,8 +2916,69 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked, tcp_xmit_retransmit_queue(sk); } +/* Kathleen Nichols' algorithm for tracking the minimum value of + * a data stream over some fixed time interval. (E.g., the minimum + * RTT over the past five minutes.) It uses constant space and constant + * time per update yet almost always delivers the same minimum as an + * implementation that has to keep all the data in the window. + * + * The algorithm keeps track of the best, 2nd best & 3rd best min + * values, maintaining an invariant that the measurement time of the + * n'th best >= n-1'th best. It also makes sure that the three values + * are widely separated in the time window since that bounds the worse + * case error when that data is monotonically increasing over the window. + * + * Upon getting a new min, we can forget everything earlier because it + * has no value - the new min is <= everything else in the window by + * definition and it's the most recent. So we restart fresh on every new min + * and overwrites 2nd & 3rd choices. The same property holds for 2nd & 3rd + * best. + */ +static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us) +{ + const u32 now = tcp_time_stamp, wlen = sysctl_tcp_min_rtt_wlen * HZ; + struct rtt_meas *m = tcp_sk(sk)->rtt_min; + struct rtt_meas rttm = { .rtt = (rtt_us ? : 1), .ts = now }; + u32 elapsed; + + /* Check if the new measurement updates the 1st, 2nd, or 3rd choices */ + if (unlikely(rttm.rtt <= m[0].rtt)) + m[0] = m[1] = m[2] = rttm; + else if (rttm.rtt <= m[1].rtt) + m[1] = m[2] = rttm; + else if (rttm.rtt <= m[2].rtt) + m[2] = rttm; + + elapsed = now - m[0].ts; + if (unlikely(elapsed > wlen)) { + /* Passed entire window without a new min so make 2nd choice + * the new min & 3rd choice the new 2nd. So forth and so on. + */ + m[0] = m[1]; + m[1] = m[2]; + m[2] = rttm; + if (now - m[0].ts > wlen) { + m[0] = m[1]; + m[1] = rttm; + if (now - m[0].ts > wlen) + m[0] = rttm; + } + } else if (m[1].ts == m[0].ts && elapsed > wlen / 4) { + /* Passed a quarter of the window without a new min so + * take 2nd choice from the 2nd quarter of the window. + */ + m[2] = m[1] = rttm; + } else if (m[2].ts == m[1].ts && elapsed > wlen / 2) { + /* Passed half the window without a new min so take the 3rd + * choice from the last half of the window. + */ + m[2] = rttm; + } +} + static inline bool tcp_ack_update_rtt(struct sock *sk, const int flag, - long seq_rtt_us, long sack_rtt_us) + long seq_rtt_us, long sack_rtt_us, + long ca_rtt_us) { const struct tcp_sock *tp = tcp_sk(sk); @@ -2936,11 +2998,16 @@ static inline bool tcp_ack_update_rtt(struct sock *sk, const int flag, */ if (seq_rtt_us < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr && flag & FLAG_ACKED) - seq_rtt_us = jiffies_to_usecs(tcp_time_stamp - tp->rx_opt.rcv_tsecr); - + seq_rtt_us = ca_rtt_us = jiffies_to_usecs(tcp_time_stamp - + tp->rx_opt.rcv_tsecr); if (seq_rtt_us < 0) return false; + /* ca_rtt_us >= 0 is counting on the invariant that ca_rtt_us is + * always taken together with ACK, SACK, or TS-opts. Any negative + * values will be skipped with the seq_rtt_us < 0 check above. + */ + tcp_update_rtt_min(sk, ca_rtt_us); tcp_rtt_estimator(sk, seq_rtt_us); tcp_set_rto(sk); @@ -2961,7 +3028,7 @@ void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req) rtt_us = skb_mstamp_us_delta(&now, &tcp_rsk(req)->snt_synack); } - tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L); + tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L, rtt_us); } @@ -3175,7 +3242,8 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, ca_rtt_us = skb_mstamp_us_delta(&now, &sack->last_sackt); } - rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us); + rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us, + ca_rtt_us); if (flag & FLAG_ACKED) { tcp_rearm_rto(sk); diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index 41828bdc5d32..b875c288daaa 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -470,6 +470,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk, newtp->srtt_us = 0; newtp->mdev_us = jiffies_to_usecs(TCP_TIMEOUT_INIT); + newtp->rtt_min[0].rtt = ~0U; newicsk->icsk_rto = TCP_TIMEOUT_INIT; newtp->packets_out = 0; From af82f4e84866ecd360a53f770d6217637116e6c1 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:43 -0700 Subject: [PATCH 3/7] tcp: remove tcp_mark_lost_retrans() Remove the existing lost retransmit detection because RACK subsumes it completely. This also stops the overloading the ack_seq field of the skb control block. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- include/linux/tcp.h | 2 -- net/ipv4/tcp_input.c | 65 ------------------------------------------- net/ipv4/tcp_output.c | 6 ---- 3 files changed, 73 deletions(-) diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 90edef5508f9..8c54863dfc38 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -283,8 +283,6 @@ struct tcp_sock { int lost_cnt_hint; u32 retransmit_high; /* L-bits may be on up to this seqno */ - u32 lost_retrans_low; /* Sent seq after any rxmit (lowest) */ - u32 prior_ssthresh; /* ssthresh saved at recovery start */ u32 high_seq; /* snd_nxt at onset of congestion */ diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index eedb25db3947..5a776897a8c7 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1048,70 +1048,6 @@ static bool tcp_is_sackblock_valid(struct tcp_sock *tp, bool is_dsack, return !before(start_seq, end_seq - tp->max_window); } -/* Check for lost retransmit. This superb idea is borrowed from "ratehalving". - * Event "B". Later note: FACK people cheated me again 8), we have to account - * for reordering! Ugly, but should help. - * - * Search retransmitted skbs from write_queue that were sent when snd_nxt was - * less than what is now known to be received by the other end (derived from - * highest SACK block). Also calculate the lowest snd_nxt among the remaining - * retransmitted skbs to avoid some costly processing per ACKs. - */ -static void tcp_mark_lost_retrans(struct sock *sk, int *flag) -{ - const struct inet_connection_sock *icsk = inet_csk(sk); - struct tcp_sock *tp = tcp_sk(sk); - struct sk_buff *skb; - int cnt = 0; - u32 new_low_seq = tp->snd_nxt; - u32 received_upto = tcp_highest_sack_seq(tp); - - if (!tcp_is_fack(tp) || !tp->retrans_out || - !after(received_upto, tp->lost_retrans_low) || - icsk->icsk_ca_state != TCP_CA_Recovery) - return; - - tcp_for_write_queue(skb, sk) { - u32 ack_seq = TCP_SKB_CB(skb)->ack_seq; - - if (skb == tcp_send_head(sk)) - break; - if (cnt == tp->retrans_out) - break; - if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una)) - continue; - - if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)) - continue; - - /* TODO: We would like to get rid of tcp_is_fack(tp) only - * constraint here (see above) but figuring out that at - * least tp->reordering SACK blocks reside between ack_seq - * and received_upto is not easy task to do cheaply with - * the available datastructures. - * - * Whether FACK should check here for tp->reordering segs - * in-between one could argue for either way (it would be - * rather simple to implement as we could count fack_count - * during the walk and do tp->fackets_out - fack_count). - */ - if (after(received_upto, ack_seq)) { - TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS; - tp->retrans_out -= tcp_skb_pcount(skb); - *flag |= FLAG_LOST_RETRANS; - tcp_skb_mark_lost_uncond_verify(tp, skb); - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT); - } else { - if (before(ack_seq, new_low_seq)) - new_low_seq = ack_seq; - cnt += tcp_skb_pcount(skb); - } - } - - if (tp->retrans_out) - tp->lost_retrans_low = new_low_seq; -} - static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb, struct tcp_sack_block_wire *sp, int num_sacks, u32 prior_snd_una) @@ -1838,7 +1774,6 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb, ((inet_csk(sk)->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker)) tcp_update_reordering(sk, tp->fackets_out - state->reord, 0); - tcp_mark_lost_retrans(sk, &state->flag); tcp_verify_left_out(tp); out: diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 19adedb8c5cc..f6f7f9b4901b 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2655,8 +2655,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb) net_dbg_ratelimited("retrans_out leaked\n"); } #endif - if (!tp->retrans_out) - tp->lost_retrans_low = tp->snd_nxt; TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS; tp->retrans_out += tcp_skb_pcount(skb); @@ -2664,10 +2662,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb) if (!tp->retrans_stamp) tp->retrans_stamp = tcp_skb_timestamp(skb); - /* snd_nxt is stored to detect loss of retransmitted segment, - * see tcp_input.c tcp_sacktag_write_queue(). - */ - TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt; } else if (err != -EBUSY) { NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL); } From 77c631273dc1305a89698929decafa6e43bea645 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:44 -0700 Subject: [PATCH 4/7] tcp: add tcp_tsopt_ecr_before helper a helper to prepare the main RACK patch Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 5a776897a8c7..1e97e73e5ecf 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2250,14 +2250,19 @@ static inline void tcp_moderate_cwnd(struct tcp_sock *tp) tp->snd_cwnd_stamp = tcp_time_stamp; } +static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when) +{ + return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr && + before(tp->rx_opt.rcv_tsecr, when); +} + /* Nothing was retransmitted or returned timestamp is less * than timestamp of the first retransmission. */ static inline bool tcp_packet_delayed(const struct tcp_sock *tp) { return !tp->retrans_stamp || - (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr && - before(tp->rx_opt.rcv_tsecr, tp->retrans_stamp)); + tcp_tsopt_ecr_before(tp, tp->retrans_stamp); } /* Undo procedures. */ From 625a5e109a3ed6f36a1008a43069a3462b44a424 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:45 -0700 Subject: [PATCH 5/7] tcp: skb_mstamp_after helper a helper to prepare the first main RACK patch. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- include/linux/skbuff.h | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 4398411236f1..24f4dfd94c51 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -463,6 +463,15 @@ static inline u32 skb_mstamp_us_delta(const struct skb_mstamp *t1, return delta_us; } +static inline bool skb_mstamp_after(const struct skb_mstamp *t1, + const struct skb_mstamp *t0) +{ + s32 diff = t1->stamp_jiffies - t0->stamp_jiffies; + + if (!diff) + diff = t1->stamp_us - t0->stamp_us; + return diff > 0; +} /** * struct sk_buff - socket buffer From 659a8ad56f490279f0efee43a62ffa1ac914a4e0 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:46 -0700 Subject: [PATCH 6/7] tcp: track the packet timings in RACK This patch is the first half of the RACK loss recovery. RACK loss recovery uses the notion of time instead of packet sequence (FACK) or counts (dupthresh). It's inspired by the previous FACK heuristic in tcp_mark_lost_retrans(): when a limited transmit (new data packet) is sacked, then current retransmitted sequence below the newly sacked sequence must been lost, since at least one round trip time has elapsed. But it has several limitations: 1) can't detect tail drops since it depends on limited transmit 2) is disabled upon reordering (assumes no reordering) 3) only enabled in fast recovery ut not timeout recovery RACK (Recently ACK) addresses these limitations with the notion of time instead: a packet P1 is lost if a later packet P2 is s/acked, as at least one round trip has passed. Since RACK cares about the time sequence instead of the data sequence of packets, it can detect tail drops when later retransmission is s/acked while FACK or dupthresh can't. For reordering RACK uses a dynamically adjusted reordering window ("reo_wnd") to reduce false positives on ever (small) degree of reordering. This patch implements tcp_advanced_rack() which tracks the most recent transmission time among the packets that have been delivered (ACKed or SACKed) in tp->rack.mstamp. This timestamp is the key to determine which packet has been lost. Consider an example that the sender sends six packets: T1: P1 (lost) T2: P2 T3: P3 T4: P4 T100: sack of P2. rack.mstamp = T2 T101: retransmit P1 T102: sack of P2,P3,P4. rack.mstamp = T4 T205: ACK of P4 since the hole is repaired. rack.mstamp = T101 We need to be careful about spurious retransmission because it may falsely advance tp->rack.mstamp by an RTT or an RTO, causing RACK to falsely mark all packets lost, just like a spurious timeout. We identify spurious retransmission by the ACK's TS echo value. If TS option is not applicable but the retransmission is acknowledged less than min-RTT ago, it is likely to be spurious. We refrain from using the transmission time of these spurious retransmissions. The second half is implemented in the next patch that marks packet lost using RACK timestamp. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- include/linux/tcp.h | 6 ++++++ include/net/tcp.h | 5 +++++ net/ipv4/Makefile | 1 + net/ipv4/tcp_input.c | 14 ++++++++++++++ net/ipv4/tcp_minisocks.c | 2 ++ net/ipv4/tcp_recovery.c | 32 ++++++++++++++++++++++++++++++++ 6 files changed, 60 insertions(+) create mode 100644 net/ipv4/tcp_recovery.c diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 8c54863dfc38..5dce9705fe84 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -194,6 +194,12 @@ struct tcp_sock { u32 window_clamp; /* Maximal window to advertise */ u32 rcv_ssthresh; /* Current window clamp */ + /* Information of the most recently (s)acked skb */ + struct tcp_rack { + struct skb_mstamp mstamp; /* (Re)sent time of the skb */ + u8 advanced; /* mstamp advanced since last lost marking */ + u8 reord; /* reordering detected */ + } rack; u16 advmss; /* Advertised MSS */ u8 unused; u8 nonagle : 4,/* Disable Nagle algorithm? */ diff --git a/include/net/tcp.h b/include/net/tcp.h index 4a43152229ea..3c3a9fe057d3 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1750,6 +1750,11 @@ int tcpv4_offload_init(void); void tcp_v4_init(void); void tcp_init(void); +/* tcp_recovery.c */ + +extern void tcp_rack_advance(struct tcp_sock *tp, + const struct skb_mstamp *xmit_time, u8 sacked); + /* * Save and compile IPv4 options, return a pointer to it */ diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile index 89aacb630a53..c29809f765dc 100644 --- a/net/ipv4/Makefile +++ b/net/ipv4/Makefile @@ -8,6 +8,7 @@ obj-y := route.o inetpeer.o protocol.o \ inet_timewait_sock.o inet_connection_sock.o \ tcp.o tcp_input.o tcp_output.o tcp_timer.o tcp_ipv4.o \ tcp_minisocks.o tcp_cong.o tcp_metrics.o tcp_fastopen.o \ + tcp_recovery.o \ tcp_offload.o datagram.o raw.o udp.o udplite.o \ udp_offload.o arp.o icmp.o devinet.o af_inet.o igmp.o \ fib_frontend.o fib_semantics.o fib_trie.o \ diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 1e97e73e5ecf..ce8370525832 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1173,6 +1173,8 @@ static u8 tcp_sacktag_one(struct sock *sk, return sacked; if (!(sacked & TCPCB_SACKED_ACKED)) { + tcp_rack_advance(tp, xmit_time, sacked); + if (sacked & TCPCB_SACKED_RETRANS) { /* If the segment is not tagged as lost, * we do not clear RETRANS, believing @@ -2256,6 +2258,16 @@ static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when) before(tp->rx_opt.rcv_tsecr, when); } +/* skb is spurious retransmitted if the returned timestamp echo + * reply is prior to the skb transmission time + */ +static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp, + const struct sk_buff *skb) +{ + return (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS) && + tcp_tsopt_ecr_before(tp, tcp_skb_timestamp(skb)); +} + /* Nothing was retransmitted or returned timestamp is less * than timestamp of the first retransmission. */ @@ -3135,6 +3147,8 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, if (sacked & TCPCB_SACKED_ACKED) tp->sacked_out -= acked_pcount; + else if (tcp_is_sack(tp) && !tcp_skb_spurious_retrans(tp, skb)) + tcp_rack_advance(tp, &skb->skb_mstamp, sacked); if (sacked & TCPCB_LOST) tp->lost_out -= acked_pcount; diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index b875c288daaa..1fd5d413a664 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -548,6 +548,8 @@ struct sock *tcp_create_openreq_child(const struct sock *sk, tcp_ecn_openreq_child(newtp, req); newtp->fastopen_rsk = NULL; newtp->syn_data_acked = 0; + newtp->rack.mstamp.v64 = 0; + newtp->rack.advanced = 0; newtp->saved_syn = req->saved_syn; req->saved_syn = NULL; diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c new file mode 100644 index 000000000000..8f66a6584845 --- /dev/null +++ b/net/ipv4/tcp_recovery.c @@ -0,0 +1,32 @@ +#include +#include + +/* Record the most recently (re)sent time among the (s)acked packets */ +void tcp_rack_advance(struct tcp_sock *tp, + const struct skb_mstamp *xmit_time, u8 sacked) +{ + if (tp->rack.mstamp.v64 && + !skb_mstamp_after(xmit_time, &tp->rack.mstamp)) + return; + + if (sacked & TCPCB_RETRANS) { + struct skb_mstamp now; + + /* If the sacked packet was retransmitted, it's ambiguous + * whether the retransmission or the original (or the prior + * retransmission) was sacked. + * + * If the original is lost, there is no ambiguity. Otherwise + * we assume the original can be delayed up to aRTT + min_rtt. + * the aRTT term is bounded by the fast recovery or timeout, + * so it's at least one RTT (i.e., retransmission is at least + * an RTT later). + */ + skb_mstamp_get(&now); + if (skb_mstamp_us_delta(&now, xmit_time) < tcp_min_rtt(tp)) + return; + } + + tp->rack.mstamp = *xmit_time; + tp->rack.advanced = 1; +} From 4f41b1c58a32537542f14c1150099131613a5e8a Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Fri, 16 Oct 2015 21:57:47 -0700 Subject: [PATCH 7/7] tcp: use RACK to detect losses This patch implements the second half of RACK that uses the the most recent transmit time among all delivered packets to detect losses. tcp_rack_mark_lost() is called upon receiving a dubious ACK. It then checks if an not-yet-sacked packet was sent at least "reo_wnd" prior to the sent time of the most recently delivered. If so the packet is deemed lost. The "reo_wnd" reordering window starts with 1msec for fast loss detection and changes to min-RTT/4 when reordering is observed. We found 1msec accommodates well on tiny degree of reordering (<3 pkts) on faster links. We use min-RTT instead of SRTT because reordering is more of a path property but SRTT can be inflated by self-inflicated congestion. The factor of 4 is borrowed from the delayed early retransmit and seems to work reasonably well. Since RACK is still experimental, it is now used as a supplemental loss detection on top of existing algorithms. It is only effective after the fast recovery starts or after the timeout occurs. The fast recovery is still triggered by FACK and/or dupack threshold instead of RACK. We introduce a new sysctl net.ipv4.tcp_recovery for future experiments of loss recoveries. For now RACK can be disabled by setting it to 0. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- Documentation/networking/ip-sysctl.txt | 9 +++ include/net/tcp.h | 9 +++ net/ipv4/sysctl_net_ipv4.c | 7 +++ net/ipv4/tcp_input.c | 9 ++- net/ipv4/tcp_recovery.c | 77 ++++++++++++++++++++++++++ 5 files changed, 109 insertions(+), 2 deletions(-) diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt index 502d6a572b4f..85752c81c5ec 100644 --- a/Documentation/networking/ip-sysctl.txt +++ b/Documentation/networking/ip-sysctl.txt @@ -433,6 +433,15 @@ tcp_orphan_retries - INTEGER you should think about lowering this value, such sockets may consume significant resources. Cf. tcp_max_orphans. +tcp_recovery - INTEGER + This value is a bitmap to enable various experimental loss recovery + features. + + RACK: 0x1 enables the RACK loss detection for fast detection of lost + retransmissions and tail drops. + + Default: 0x1 + tcp_reordering - INTEGER Initial reordering level of packets in a TCP stream. TCP stack can then dynamically adjust flow reordering level diff --git a/include/net/tcp.h b/include/net/tcp.h index 3c3a9fe057d3..11e320412216 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -567,6 +567,7 @@ void tcp_resume_early_retransmit(struct sock *sk); void tcp_rearm_rto(struct sock *sk); void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req); void tcp_reset(struct sock *sk); +void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb); /* tcp_timer.c */ void tcp_init_xmit_timers(struct sock *); @@ -1752,6 +1753,14 @@ void tcp_init(void); /* tcp_recovery.c */ +/* Flags to enable various loss recovery features. See below */ +extern int sysctl_tcp_recovery; + +/* Use TCP RACK to detect (some) tail and retransmit losses */ +#define TCP_RACK_LOST_RETRANS 0x1 + +extern int tcp_rack_mark_lost(struct sock *sk); + extern void tcp_rack_advance(struct tcp_sock *tp, const struct skb_mstamp *xmit_time, u8 sacked); diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index 13ab434c2909..25300c5e283b 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -495,6 +495,13 @@ static struct ctl_table ipv4_table[] = { .mode = 0644, .proc_handler = proc_dointvec }, + { + .procname = "tcp_recovery", + .data = &sysctl_tcp_recovery, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec, + }, { .procname = "tcp_reordering", .data = &sysctl_tcp_reordering, diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index ce8370525832..fdd88c3803a6 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -881,6 +881,7 @@ static void tcp_update_reordering(struct sock *sk, const int metric, if (metric > 0) tcp_disable_early_retrans(tp); + tp->rack.reord = 1; } /* This must be called before lost_out is incremented */ @@ -906,8 +907,7 @@ static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb) } } -static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, - struct sk_buff *skb) +void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb) { tcp_verify_retransmit_hint(tp, skb); @@ -2806,6 +2806,11 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked, } } + /* Use RACK to detect loss */ + if (sysctl_tcp_recovery & TCP_RACK_LOST_RETRANS && + tcp_rack_mark_lost(sk)) + flag |= FLAG_LOST_RETRANS; + /* E. Process state. */ switch (icsk->icsk_ca_state) { case TCP_CA_Recovery: diff --git a/net/ipv4/tcp_recovery.c b/net/ipv4/tcp_recovery.c index 8f66a6584845..5353085fd0b2 100644 --- a/net/ipv4/tcp_recovery.c +++ b/net/ipv4/tcp_recovery.c @@ -1,6 +1,83 @@ #include #include +int sysctl_tcp_recovery __read_mostly = TCP_RACK_LOST_RETRANS; + +/* Marks a packet lost, if some packet sent later has been (s)acked. + * The underlying idea is similar to the traditional dupthresh and FACK + * but they look at different metrics: + * + * dupthresh: 3 OOO packets delivered (packet count) + * FACK: sequence delta to highest sacked sequence (sequence space) + * RACK: sent time delta to the latest delivered packet (time domain) + * + * The advantage of RACK is it applies to both original and retransmitted + * packet and therefore is robust against tail losses. Another advantage + * is being more resilient to reordering by simply allowing some + * "settling delay", instead of tweaking the dupthresh. + * + * The current version is only used after recovery starts but can be + * easily extended to detect the first loss. + */ +int tcp_rack_mark_lost(struct sock *sk) +{ + struct tcp_sock *tp = tcp_sk(sk); + struct sk_buff *skb; + u32 reo_wnd, prior_retrans = tp->retrans_out; + + if (inet_csk(sk)->icsk_ca_state < TCP_CA_Recovery || !tp->rack.advanced) + return 0; + + /* Reset the advanced flag to avoid unnecessary queue scanning */ + tp->rack.advanced = 0; + + /* To be more reordering resilient, allow min_rtt/4 settling delay + * (lower-bounded to 1000uS). We use min_rtt instead of the smoothed + * RTT because reordering is often a path property and less related + * to queuing or delayed ACKs. + * + * TODO: measure and adapt to the observed reordering delay, and + * use a timer to retransmit like the delayed early retransmit. + */ + reo_wnd = 1000; + if (tp->rack.reord && tcp_min_rtt(tp) != ~0U) + reo_wnd = max(tcp_min_rtt(tp) >> 2, reo_wnd); + + tcp_for_write_queue(skb, sk) { + struct tcp_skb_cb *scb = TCP_SKB_CB(skb); + + if (skb == tcp_send_head(sk)) + break; + + /* Skip ones already (s)acked */ + if (!after(scb->end_seq, tp->snd_una) || + scb->sacked & TCPCB_SACKED_ACKED) + continue; + + if (skb_mstamp_after(&tp->rack.mstamp, &skb->skb_mstamp)) { + + if (skb_mstamp_us_delta(&tp->rack.mstamp, + &skb->skb_mstamp) <= reo_wnd) + continue; + + /* skb is lost if packet sent later is sacked */ + tcp_skb_mark_lost_uncond_verify(tp, skb); + if (scb->sacked & TCPCB_SACKED_RETRANS) { + scb->sacked &= ~TCPCB_SACKED_RETRANS; + tp->retrans_out -= tcp_skb_pcount(skb); + NET_INC_STATS_BH(sock_net(sk), + LINUX_MIB_TCPLOSTRETRANSMIT); + } + } else if (!(scb->sacked & TCPCB_RETRANS)) { + /* Original data are sent sequentially so stop early + * b/c the rest are all sent after rack_sent + */ + break; + } + } + return prior_retrans - tp->retrans_out; +} + /* Record the most recently (re)sent time among the (s)acked packets */ void tcp_rack_advance(struct tcp_sock *tp, const struct skb_mstamp *xmit_time, u8 sacked)