1 // SPDX-License-Identifier: GPL-2.0-only 2 #include <net/tcp.h> 3 4 /* The bandwidth estimator estimates the rate at which the network 5 * can currently deliver outbound data packets for this flow. At a high 6 * level, it operates by taking a delivery rate sample for each ACK. 7 * 8 * A rate sample records the rate at which the network delivered packets 9 * for this flow, calculated over the time interval between the transmission 10 * of a data packet and the acknowledgment of that packet. 11 * 12 * Specifically, over the interval between each transmit and corresponding ACK, 13 * the estimator generates a delivery rate sample. Typically it uses the rate 14 * at which packets were acknowledged. However, the approach of using only the 15 * acknowledgment rate faces a challenge under the prevalent ACK decimation or 16 * compression: packets can temporarily appear to be delivered much quicker 17 * than the bottleneck rate. Since it is physically impossible to do that in a 18 * sustained fashion, when the estimator notices that the ACK rate is faster 19 * than the transmit rate, it uses the latter: 20 * 21 * send_rate = #pkts_delivered/(last_snd_time - first_snd_time) 22 * ack_rate = #pkts_delivered/(last_ack_time - first_ack_time) 23 * bw = min(send_rate, ack_rate) 24 * 25 * Notice the estimator essentially estimates the goodput, not always the 26 * network bottleneck link rate when the sending or receiving is limited by 27 * other factors like applications or receiver window limits. The estimator 28 * deliberately avoids using the inter-packet spacing approach because that 29 * approach requires a large number of samples and sophisticated filtering. 30 * 31 * TCP flows can often be application-limited in request/response workloads. 32 * The estimator marks a bandwidth sample as application-limited if there 33 * was some moment during the sampled window of packets when there was no data 34 * ready to send in the write queue. 35 */ 36 37 /* Snapshot the current delivery information in the skb, to generate 38 * a rate sample later when the skb is (s)acked in tcp_rate_skb_delivered(). 39 */ 40 void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb) 41 { 42 struct tcp_sock *tp = tcp_sk(sk); 43 44 /* In general we need to start delivery rate samples from the 45 * time we received the most recent ACK, to ensure we include 46 * the full time the network needs to deliver all in-flight 47 * packets. If there are no packets in flight yet, then we 48 * know that any ACKs after now indicate that the network was 49 * able to deliver those packets completely in the sampling 50 * interval between now and the next ACK. 51 * 52 * Note that we use packets_out instead of tcp_packets_in_flight(tp) 53 * because the latter is a guess based on RTO and loss-marking 54 * heuristics. We don't want spurious RTOs or loss markings to cause 55 * a spuriously small time interval, causing a spuriously high 56 * bandwidth estimate. 57 */ 58 if (!tp->packets_out) { 59 u64 tstamp_us = tcp_skb_timestamp_us(skb); 60 61 tp->first_tx_mstamp = tstamp_us; 62 tp->delivered_mstamp = tstamp_us; 63 } 64 65 TCP_SKB_CB(skb)->tx.first_tx_mstamp = tp->first_tx_mstamp; 66 TCP_SKB_CB(skb)->tx.delivered_mstamp = tp->delivered_mstamp; 67 TCP_SKB_CB(skb)->tx.delivered = tp->delivered; 68 TCP_SKB_CB(skb)->tx.delivered_ce = tp->delivered_ce; 69 TCP_SKB_CB(skb)->tx.is_app_limited = tp->app_limited ? 1 : 0; 70 } 71 72 /* When an skb is sacked or acked, we fill in the rate sample with the (prior) 73 * delivery information when the skb was last transmitted. 74 * 75 * If an ACK (s)acks multiple skbs (e.g., stretched-acks), this function is 76 * called multiple times. We favor the information from the most recently 77 * sent skb, i.e., the skb with the highest prior_delivered count. 78 */ 79 void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb, 80 struct rate_sample *rs) 81 { 82 struct tcp_sock *tp = tcp_sk(sk); 83 struct tcp_skb_cb *scb = TCP_SKB_CB(skb); 84 85 if (!scb->tx.delivered_mstamp) 86 return; 87 88 if (!rs->prior_delivered || 89 after(scb->tx.delivered, rs->prior_delivered)) { 90 rs->prior_delivered_ce = scb->tx.delivered_ce; 91 rs->prior_delivered = scb->tx.delivered; 92 rs->prior_mstamp = scb->tx.delivered_mstamp; 93 rs->is_app_limited = scb->tx.is_app_limited; 94 rs->is_retrans = scb->sacked & TCPCB_RETRANS; 95 96 /* Record send time of most recently ACKed packet: */ 97 tp->first_tx_mstamp = tcp_skb_timestamp_us(skb); 98 /* Find the duration of the "send phase" of this window: */ 99 rs->interval_us = tcp_stamp_us_delta(tp->first_tx_mstamp, 100 scb->tx.first_tx_mstamp); 101 102 } 103 /* Mark off the skb delivered once it's sacked to avoid being 104 * used again when it's cumulatively acked. For acked packets 105 * we don't need to reset since it'll be freed soon. 106 */ 107 if (scb->sacked & TCPCB_SACKED_ACKED) 108 scb->tx.delivered_mstamp = 0; 109 } 110 111 /* Update the connection delivery information and generate a rate sample. */ 112 void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost, 113 bool is_sack_reneg, struct rate_sample *rs) 114 { 115 struct tcp_sock *tp = tcp_sk(sk); 116 u32 snd_us, ack_us; 117 118 /* Clear app limited if bubble is acked and gone. */ 119 if (tp->app_limited && after(tp->delivered, tp->app_limited)) 120 tp->app_limited = 0; 121 122 /* TODO: there are multiple places throughout tcp_ack() to get 123 * current time. Refactor the code using a new "tcp_acktag_state" 124 * to carry current time, flags, stats like "tcp_sacktag_state". 125 */ 126 if (delivered) 127 tp->delivered_mstamp = tp->tcp_mstamp; 128 129 rs->acked_sacked = delivered; /* freshly ACKed or SACKed */ 130 rs->losses = lost; /* freshly marked lost */ 131 /* Return an invalid sample if no timing information is available or 132 * in recovery from loss with SACK reneging. Rate samples taken during 133 * a SACK reneging event may overestimate bw by including packets that 134 * were SACKed before the reneg. 135 */ 136 if (!rs->prior_mstamp || is_sack_reneg) { 137 rs->delivered = -1; 138 rs->interval_us = -1; 139 return; 140 } 141 rs->delivered = tp->delivered - rs->prior_delivered; 142 143 rs->delivered_ce = tp->delivered_ce - rs->prior_delivered_ce; 144 /* delivered_ce occupies less than 32 bits in the skb control block */ 145 rs->delivered_ce &= TCPCB_DELIVERED_CE_MASK; 146 147 /* Model sending data and receiving ACKs as separate pipeline phases 148 * for a window. Usually the ACK phase is longer, but with ACK 149 * compression the send phase can be longer. To be safe we use the 150 * longer phase. 151 */ 152 snd_us = rs->interval_us; /* send phase */ 153 ack_us = tcp_stamp_us_delta(tp->tcp_mstamp, 154 rs->prior_mstamp); /* ack phase */ 155 rs->interval_us = max(snd_us, ack_us); 156 157 /* Record both segment send and ack receive intervals */ 158 rs->snd_interval_us = snd_us; 159 rs->rcv_interval_us = ack_us; 160 161 /* Normally we expect interval_us >= min-rtt. 162 * Note that rate may still be over-estimated when a spuriously 163 * retransmistted skb was first (s)acked because "interval_us" 164 * is under-estimated (up to an RTT). However continuously 165 * measuring the delivery rate during loss recovery is crucial 166 * for connections suffer heavy or prolonged losses. 167 */ 168 if (unlikely(rs->interval_us < tcp_min_rtt(tp))) { 169 if (!rs->is_retrans) 170 pr_debug("tcp rate: %ld %d %u %u %u\n", 171 rs->interval_us, rs->delivered, 172 inet_csk(sk)->icsk_ca_state, 173 tp->rx_opt.sack_ok, tcp_min_rtt(tp)); 174 rs->interval_us = -1; 175 return; 176 } 177 178 /* Record the last non-app-limited or the highest app-limited bw */ 179 if (!rs->is_app_limited || 180 ((u64)rs->delivered * tp->rate_interval_us >= 181 (u64)tp->rate_delivered * rs->interval_us)) { 182 tp->rate_delivered = rs->delivered; 183 tp->rate_interval_us = rs->interval_us; 184 tp->rate_app_limited = rs->is_app_limited; 185 } 186 } 187 188 /* If a gap is detected between sends, mark the socket application-limited. */ 189 void tcp_rate_check_app_limited(struct sock *sk) 190 { 191 struct tcp_sock *tp = tcp_sk(sk); 192 193 if (/* We have less than one packet to send. */ 194 tp->write_seq - tp->snd_nxt < tp->mss_cache && 195 /* Nothing in sending host's qdisc queues or NIC tx queue. */ 196 sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) && 197 /* We are not limited by CWND. */ 198 tcp_packets_in_flight(tp) < tp->snd_cwnd && 199 /* All lost packets have been retransmitted. */ 200 tp->lost_out <= tp->retrans_out) 201 tp->app_limited = 202 (tp->delivered + tcp_packets_in_flight(tp)) ? : 1; 203 } 204 EXPORT_SYMBOL_GPL(tcp_rate_check_app_limited); 205