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