net/ipv4/tcp_rate.c

   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.is_app_limited      = tp->app_limited ? 1 : 0;
  69 }
  70
  71 /* When an skb is sacked or acked, we fill in the rate sample with the (prior)
  72  * delivery information when the skb was last transmitted.
  73  *
  74  * If an ACK (s)acks multiple skbs (e.g., stretched-acks), this function is
  75  * called multiple times. We favor the information from the most recently
  76  * sent skb, i.e., the skb with the highest prior_delivered count.
  77  */
  78 void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
  79                             struct rate_sample *rs)
  80 {
  81         struct tcp_sock *tp = tcp_sk(sk);
  82         struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  83
  84         if (!scb->tx.delivered_mstamp)
  85                 return;
  86
  87         if (!rs->prior_delivered ||
  88             after(scb->tx.delivered, rs->prior_delivered)) {
  89                 rs->prior_delivered  = scb->tx.delivered;
  90                 rs->prior_mstamp     = scb->tx.delivered_mstamp;
  91                 rs->is_app_limited   = scb->tx.is_app_limited;
  92                 rs->is_retrans       = scb->sacked & TCPCB_RETRANS;
  93
  94                 /* Record send time of most recently ACKed packet: */
  95                 tp->first_tx_mstamp  = tcp_skb_timestamp_us(skb);
  96                 /* Find the duration of the "send phase" of this window: */
  97                 rs->interval_us = tcp_stamp_us_delta(tp->first_tx_mstamp,
  98                                                      scb->tx.first_tx_mstamp);
  99
 100         }
 101         /* Mark off the skb delivered once it's sacked to avoid being
 102          * used again when it's cumulatively acked. For acked packets
 103          * we don't need to reset since it'll be freed soon.
 104          */
 105         if (scb->sacked & TCPCB_SACKED_ACKED)
 106                 scb->tx.delivered_mstamp = 0;
 107 }
 108
 109 /* Update the connection delivery information and generate a rate sample. */
 110 void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost,
 111                   bool is_sack_reneg, struct rate_sample *rs)
 112 {
 113         struct tcp_sock *tp = tcp_sk(sk);
 114         u32 snd_us, ack_us;
 115
 116         /* Clear app limited if bubble is acked and gone. */
 117         if (tp->app_limited && after(tp->delivered, tp->app_limited))
 118                 tp->app_limited = 0;
 119
 120         /* TODO: there are multiple places throughout tcp_ack() to get
 121          * current time. Refactor the code using a new "tcp_acktag_state"
 122          * to carry current time, flags, stats like "tcp_sacktag_state".
 123          */
 124         if (delivered)
 125                 tp->delivered_mstamp = tp->tcp_mstamp;
 126
 127         rs->acked_sacked = delivered;   /* freshly ACKed or SACKed */
 128         rs->losses = lost;              /* freshly marked lost */
 129         /* Return an invalid sample if no timing information is available or
 130          * in recovery from loss with SACK reneging. Rate samples taken during
 131          * a SACK reneging event may overestimate bw by including packets that
 132          * were SACKed before the reneg.
 133          */
 134         if (!rs->prior_mstamp || is_sack_reneg) {
 135                 rs->delivered = -1;
 136                 rs->interval_us = -1;
 137                 return;
 138         }
 139         rs->delivered   = tp->delivered - rs->prior_delivered;
 140
 141         /* Model sending data and receiving ACKs as separate pipeline phases
 142          * for a window. Usually the ACK phase is longer, but with ACK
 143          * compression the send phase can be longer. To be safe we use the
 144          * longer phase.
 145          */
 146         snd_us = rs->interval_us;                               /* send phase */
 147         ack_us = tcp_stamp_us_delta(tp->tcp_mstamp,
 148                                     rs->prior_mstamp); /* ack phase */
 149         rs->interval_us = max(snd_us, ack_us);
 150
 151         /* Record both segment send and ack receive intervals */
 152         rs->snd_interval_us = snd_us;
 153         rs->rcv_interval_us = ack_us;
 154
 155         /* Normally we expect interval_us >= min-rtt.
 156          * Note that rate may still be over-estimated when a spuriously
 157          * retransmistted skb was first (s)acked because "interval_us"
 158          * is under-estimated (up to an RTT). However continuously
 159          * measuring the delivery rate during loss recovery is crucial
 160          * for connections suffer heavy or prolonged losses.
 161          */
 162         if (unlikely(rs->interval_us < tcp_min_rtt(tp))) {
 163                 if (!rs->is_retrans)
 164                         pr_debug("tcp rate: %ld %d %u %u %u\n",
 165                                  rs->interval_us, rs->delivered,
 166                                  inet_csk(sk)->icsk_ca_state,
 167                                  tp->rx_opt.sack_ok, tcp_min_rtt(tp));
 168                 rs->interval_us = -1;
 169                 return;
 170         }
 171
 172         /* Record the last non-app-limited or the highest app-limited bw */
 173         if (!rs->is_app_limited ||
 174             ((u64)rs->delivered * tp->rate_interval_us >=
 175              (u64)tp->rate_delivered * rs->interval_us)) {
 176                 tp->rate_delivered = rs->delivered;
 177                 tp->rate_interval_us = rs->interval_us;
 178                 tp->rate_app_limited = rs->is_app_limited;
 179         }
 180 }
 181
 182 /* If a gap is detected between sends, mark the socket application-limited. */
 183 void tcp_rate_check_app_limited(struct sock *sk)
 184 {
 185         struct tcp_sock *tp = tcp_sk(sk);
 186
 187         if (/* We have less than one packet to send. */
 188             tp->write_seq - tp->snd_nxt < tp->mss_cache &&
 189             /* Nothing in sending host's qdisc queues or NIC tx queue. */
 190             sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) &&
 191             /* We are not limited by CWND. */
 192             tcp_packets_in_flight(tp) < tp->snd_cwnd &&
 193             /* All lost packets have been retransmitted. */
 194             tp->lost_out <= tp->retrans_out)
 195                 tp->app_limited =
 196                         (tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
 197 }
 198 EXPORT_SYMBOL_GPL(tcp_rate_check_app_limited);