net/rxrpc/rtt.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /* RTT/RTO calculation.
   3  *
   4  * Adapted from TCP for AF_RXRPC by David Howells (dhowells@redhat.com)
   5  *
   6  * https://tools.ietf.org/html/rfc6298
   7  * https://tools.ietf.org/html/rfc1122#section-4.2.3.1
   8  * http://ccr.sigcomm.org/archive/1995/jan95/ccr-9501-partridge87.pdf
   9  */
  10
  11 #include <linux/net.h>
  12 #include "ar-internal.h"
  13
  14 #define RXRPC_RTO_MAX   ((unsigned)(120 * HZ))
  15 #define RXRPC_TIMEOUT_INIT ((unsigned)(1*HZ))   /* RFC6298 2.1 initial RTO value        */
  16 #define rxrpc_jiffies32 ((u32)jiffies)          /* As rxrpc_jiffies32 */
  17
  18 static u32 rxrpc_rto_min_us(struct rxrpc_peer *peer)
  19 {
  20         return 200;
  21 }
  22
  23 static u32 __rxrpc_set_rto(const struct rxrpc_peer *peer)
  24 {
  25         return _usecs_to_jiffies((peer->srtt_us >> 3) + peer->rttvar_us);
  26 }
  27
  28 static u32 rxrpc_bound_rto(u32 rto)
  29 {
  30         return min(rto, RXRPC_RTO_MAX);
  31 }
  32
  33 /*
  34  * Called to compute a smoothed rtt estimate. The data fed to this
  35  * routine either comes from timestamps, or from segments that were
  36  * known _not_ to have been retransmitted [see Karn/Partridge
  37  * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
  38  * piece by Van Jacobson.
  39  * NOTE: the next three routines used to be one big routine.
  40  * To save cycles in the RFC 1323 implementation it was better to break
  41  * it up into three procedures. -- erics
  42  */
  43 static void rxrpc_rtt_estimator(struct rxrpc_peer *peer, long sample_rtt_us)
  44 {
  45         long m = sample_rtt_us; /* RTT */
  46         u32 srtt = peer->srtt_us;
  47
  48         /*      The following amusing code comes from Jacobson's
  49          *      article in SIGCOMM '88.  Note that rtt and mdev
  50          *      are scaled versions of rtt and mean deviation.
  51          *      This is designed to be as fast as possible
  52          *      m stands for "measurement".
  53          *
  54          *      On a 1990 paper the rto value is changed to:
  55          *      RTO = rtt + 4 * mdev
  56          *
  57          * Funny. This algorithm seems to be very broken.
  58          * These formulae increase RTO, when it should be decreased, increase
  59          * too slowly, when it should be increased quickly, decrease too quickly
  60          * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
  61          * does not matter how to _calculate_ it. Seems, it was trap
  62          * that VJ failed to avoid. 8)
  63          */
  64         if (srtt != 0) {
  65                 m -= (srtt >> 3);       /* m is now error in rtt est */
  66                 srtt += m;              /* rtt = 7/8 rtt + 1/8 new */
  67                 if (m < 0) {
  68                         m = -m;         /* m is now abs(error) */
  69                         m -= (peer->mdev_us >> 2);   /* similar update on mdev */
  70                         /* This is similar to one of Eifel findings.
  71                          * Eifel blocks mdev updates when rtt decreases.
  72                          * This solution is a bit different: we use finer gain
  73                          * for mdev in this case (alpha*beta).
  74                          * Like Eifel it also prevents growth of rto,
  75                          * but also it limits too fast rto decreases,
  76                          * happening in pure Eifel.
  77                          */
  78                         if (m > 0)
  79                                 m >>= 3;
  80                 } else {
  81                         m -= (peer->mdev_us >> 2);   /* similar update on mdev */
  82                 }
  83
  84                 peer->mdev_us += m;             /* mdev = 3/4 mdev + 1/4 new */
  85                 if (peer->mdev_us > peer->mdev_max_us) {
  86                         peer->mdev_max_us = peer->mdev_us;
  87                         if (peer->mdev_max_us > peer->rttvar_us)
  88                                 peer->rttvar_us = peer->mdev_max_us;
  89                 }
  90         } else {
  91                 /* no previous measure. */
  92                 srtt = m << 3;          /* take the measured time to be rtt */
  93                 peer->mdev_us = m << 1; /* make sure rto = 3*rtt */
  94                 peer->rttvar_us = max(peer->mdev_us, rxrpc_rto_min_us(peer));
  95                 peer->mdev_max_us = peer->rttvar_us;
  96         }
  97
  98         peer->srtt_us = max(1U, srtt);
  99 }
 100
 101 /*
 102  * Calculate rto without backoff.  This is the second half of Van Jacobson's
 103  * routine referred to above.
 104  */
 105 static void rxrpc_set_rto(struct rxrpc_peer *peer)
 106 {
 107         u32 rto;
 108
 109         /* 1. If rtt variance happened to be less 50msec, it is hallucination.
 110          *    It cannot be less due to utterly erratic ACK generation made
 111          *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
 112          *    to do with delayed acks, because at cwnd>2 true delack timeout
 113          *    is invisible. Actually, Linux-2.4 also generates erratic
 114          *    ACKs in some circumstances.
 115          */
 116         rto = __rxrpc_set_rto(peer);
 117
 118         /* 2. Fixups made earlier cannot be right.
 119          *    If we do not estimate RTO correctly without them,
 120          *    all the algo is pure shit and should be replaced
 121          *    with correct one. It is exactly, which we pretend to do.
 122          */
 123
 124         /* NOTE: clamping at RXRPC_RTO_MIN is not required, current algo
 125          * guarantees that rto is higher.
 126          */
 127         peer->rto_j = rxrpc_bound_rto(rto);
 128 }
 129
 130 static void rxrpc_ack_update_rtt(struct rxrpc_peer *peer, long rtt_us)
 131 {
 132         if (rtt_us < 0)
 133                 return;
 134
 135         //rxrpc_update_rtt_min(peer, rtt_us);
 136         rxrpc_rtt_estimator(peer, rtt_us);
 137         rxrpc_set_rto(peer);
 138
 139         /* RFC6298: only reset backoff on valid RTT measurement. */
 140         peer->backoff = 0;
 141 }
 142
 143 /*
 144  * Add RTT information to cache.  This is called in softirq mode and has
 145  * exclusive access to the peer RTT data.
 146  */
 147 void rxrpc_peer_add_rtt(struct rxrpc_call *call, enum rxrpc_rtt_rx_trace why,
 148                         int rtt_slot,
 149                         rxrpc_serial_t send_serial, rxrpc_serial_t resp_serial,
 150                         ktime_t send_time, ktime_t resp_time)
 151 {
 152         struct rxrpc_peer *peer = call->peer;
 153         s64 rtt_us;
 154
 155         rtt_us = ktime_to_us(ktime_sub(resp_time, send_time));
 156         if (rtt_us < 0)
 157                 return;
 158
 159         spin_lock(&peer->rtt_input_lock);
 160         rxrpc_ack_update_rtt(peer, rtt_us);
 161         if (peer->rtt_count < 3)
 162                 peer->rtt_count++;
 163         spin_unlock(&peer->rtt_input_lock);
 164
 165         trace_rxrpc_rtt_rx(call, why, rtt_slot, send_serial, resp_serial,
 166                            peer->srtt_us >> 3, peer->rto_j);
 167 }
 168
 169 /*
 170  * Get the retransmission timeout to set in jiffies, backing it off each time
 171  * we retransmit.
 172  */
 173 unsigned long rxrpc_get_rto_backoff(struct rxrpc_peer *peer, bool retrans)
 174 {
 175         u64 timo_j;
 176         u8 backoff = READ_ONCE(peer->backoff);
 177
 178         timo_j = peer->rto_j;
 179         timo_j <<= backoff;
 180         if (retrans && timo_j * 2 <= RXRPC_RTO_MAX)
 181                 WRITE_ONCE(peer->backoff, backoff + 1);
 182
 183         if (timo_j < 1)
 184                 timo_j = 1;
 185
 186         return timo_j;
 187 }
 188
 189 void rxrpc_peer_init_rtt(struct rxrpc_peer *peer)
 190 {
 191         peer->rto_j     = RXRPC_TIMEOUT_INIT;
 192         peer->mdev_us   = jiffies_to_usecs(RXRPC_TIMEOUT_INIT);
 193         peer->backoff   = 0;
 194         //minmax_reset(&peer->rtt_min, rxrpc_jiffies32, ~0U);
 195 }