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 most recently sent time and the highest
78 * sequence.
79 */
80 void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
81 struct rate_sample *rs)
82 {
83 struct tcp_sock *tp = tcp_sk(sk);
84 struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
85 u64 tx_tstamp;
86
87 if (!scb->tx.delivered_mstamp)
88 return;
89
90 tx_tstamp = tcp_skb_timestamp_us(skb);
91 if (!rs->prior_delivered ||
92 tcp_skb_sent_after(tx_tstamp, tp->first_tx_mstamp,
93 scb->end_seq, rs->last_end_seq)) {
94 rs->prior_delivered_ce = scb->tx.delivered_ce;
95 rs->prior_delivered = scb->tx.delivered;
96 rs->prior_mstamp = scb->tx.delivered_mstamp;
97 rs->is_app_limited = scb->tx.is_app_limited;
98 rs->is_retrans = scb->sacked & TCPCB_RETRANS;
99 rs->last_end_seq = scb->end_seq;
100
101 /* Record send time of most recently ACKed packet: */
102 tp->first_tx_mstamp = tx_tstamp;
103 /* Find the duration of the "send phase" of this window: */
104 rs->interval_us = tcp_stamp_us_delta(tp->first_tx_mstamp,
105 scb->tx.first_tx_mstamp);
106
107 }
108 /* Mark off the skb delivered once it's sacked to avoid being
109 * used again when it's cumulatively acked. For acked packets
110 * we don't need to reset since it'll be freed soon.
111 */
112 if (scb->sacked & TCPCB_SACKED_ACKED)
113 scb->tx.delivered_mstamp = 0;
114 }
115
116 /* Update the connection delivery information and generate a rate sample. */
117 void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost,
118 bool is_sack_reneg, struct rate_sample *rs)
119 {
120 struct tcp_sock *tp = tcp_sk(sk);
121 u32 snd_us, ack_us;
122
123 /* Clear app limited if bubble is acked and gone. */
124 if (tp->app_limited && after(tp->delivered, tp->app_limited))
125 tp->app_limited = 0;
126
127 /* TODO: there are multiple places throughout tcp_ack() to get
128 * current time. Refactor the code using a new "tcp_acktag_state"
129 * to carry current time, flags, stats like "tcp_sacktag_state".
130 */
131 if (delivered)
132 tp->delivered_mstamp = tp->tcp_mstamp;
133
134 rs->acked_sacked = delivered; /* freshly ACKed or SACKed */
135 rs->losses = lost; /* freshly marked lost */
136 /* Return an invalid sample if no timing information is available or
137 * in recovery from loss with SACK reneging. Rate samples taken during
138 * a SACK reneging event may overestimate bw by including packets that
139 * were SACKed before the reneg.
140 */
141 if (!rs->prior_mstamp || is_sack_reneg) {
142 rs->delivered = -1;
143 rs->interval_us = -1;
144 return;
145 }
146 rs->delivered = tp->delivered - rs->prior_delivered;
147
148 rs->delivered_ce = tp->delivered_ce - rs->prior_delivered_ce;
149 /* delivered_ce occupies less than 32 bits in the skb control block */
150 rs->delivered_ce &= TCPCB_DELIVERED_CE_MASK;
151
152 /* Model sending data and receiving ACKs as separate pipeline phases
153 * for a window. Usually the ACK phase is longer, but with ACK
154 * compression the send phase can be longer. To be safe we use the
155 * longer phase.
156 */
157 snd_us = rs->interval_us; /* send phase */
158 ack_us = tcp_stamp_us_delta(tp->tcp_mstamp,
159 rs->prior_mstamp); /* ack phase */
160 rs->interval_us = max(snd_us, ack_us);
161
162 /* Record both segment send and ack receive intervals */
163 rs->snd_interval_us = snd_us;
164 rs->rcv_interval_us = ack_us;
165
166 /* Normally we expect interval_us >= min-rtt.
167 * Note that rate may still be over-estimated when a spuriously
168 * retransmistted skb was first (s)acked because "interval_us"
169 * is under-estimated (up to an RTT). However continuously
170 * measuring the delivery rate during loss recovery is crucial
171 * for connections suffer heavy or prolonged losses.
172 */
173 if (unlikely(rs->interval_us < tcp_min_rtt(tp))) {
174 if (!rs->is_retrans)
175 pr_debug("tcp rate: %ld %d %u %u %u\n",
176 rs->interval_us, rs->delivered,
177 inet_csk(sk)->icsk_ca_state,
178 tp->rx_opt.sack_ok, tcp_min_rtt(tp));
179 rs->interval_us = -1;
180 return;
181 }
182
183 /* Record the last non-app-limited or the highest app-limited bw */
184 if (!rs->is_app_limited ||
185 ((u64)rs->delivered * tp->rate_interval_us >=
186 (u64)tp->rate_delivered * rs->interval_us)) {
187 tp->rate_delivered = rs->delivered;
188 tp->rate_interval_us = rs->interval_us;
189 tp->rate_app_limited = rs->is_app_limited;
190 }
191 }
192
193 /* If a gap is detected between sends, mark the socket application-limited. */
194 void tcp_rate_check_app_limited(struct sock *sk)
195 {
196 struct tcp_sock *tp = tcp_sk(sk);
197
198 if (/* We have less than one packet to send. */
199 tp->write_seq - tp->snd_nxt < tp->mss_cache &&
200 /* Nothing in sending host's qdisc queues or NIC tx queue. */
201 sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) &&
202 /* We are not limited by CWND. */
203 tcp_packets_in_flight(tp) < tcp_snd_cwnd(tp) &&
204 /* All lost packets have been retransmitted. */
205 tp->lost_out <= tp->retrans_out)
206 tp->app_limited =
207 (tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
208 }
209 EXPORT_SYMBOL_GPL(tcp_rate_check_app_limited);
210
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