// SPDX-License-Identifier: GPL-2.0 /* RTT/RTO calculation. * * Adapted from TCP for AF_RXRPC by David Howells (dhowells@redhat.com) * * https://tools.ietf.org/html/rfc6298 * https://tools.ietf.org/html/rfc1122#section-4.2.3.1 * http://ccr.sigcomm.org/archive/1995/jan95/ccr-9501-partridge87.pdf */ #include <linux/net.h> #include "ar-internal.h" #define RXRPC_RTO_MAX (120 * USEC_PER_SEC) #define RXRPC_TIMEOUT_INIT ((unsigned int)(1 * USEC_PER_SEC)) /* RFC6298 2.1 initial RTO value */ #define rxrpc_jiffies32 ((u32)jiffies) /* As rxrpc_jiffies32 */ static u32 rxrpc_rto_min_us(struct rxrpc_call *call) { return 200; } static u32 __rxrpc_set_rto(const struct rxrpc_call *call) { return (call->srtt_us >> 3) + call->rttvar_us; } static u32 rxrpc_bound_rto(u32 rto) { return clamp(200000, rto + 100000, RXRPC_RTO_MAX); } /* * Called to compute a smoothed rtt estimate. The data fed to this * routine either comes from timestamps, or from segments that were * known _not_ to have been retransmitted [see Karn/Partridge * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88 * piece by Van Jacobson. * NOTE: the next three routines used to be one big routine. * To save cycles in the RFC 1323 implementation it was better to break * it up into three procedures. -- erics */ static void rxrpc_rtt_estimator(struct rxrpc_call *call, long sample_rtt_us) { long m = sample_rtt_us; /* RTT */ u32 srtt = call->srtt_us; /* The following amusing code comes from Jacobson's * article in SIGCOMM '88. Note that rtt and mdev * are scaled versions of rtt and mean deviation. * This is designed to be as fast as possible * m stands for "measurement". * * On a 1990 paper the rto value is changed to: * RTO = rtt + 4 * mdev * * Funny. This algorithm seems to be very broken. * These formulae increase RTO, when it should be decreased, increase * too slowly, when it should be increased quickly, decrease too quickly * etc. I guess in BSD RTO takes ONE value, so that it is absolutely * does not matter how to _calculate_ it. Seems, it was trap * that VJ failed to avoid. 8) */ if (srtt != 0) { m -= (srtt >> 3); /* m is now error in rtt est */ srtt += m; /* rtt = 7/8 rtt + 1/8 new */ if (m < 0) { m = -m; /* m is now abs(error) */ m -= (call->mdev_us >> 2); /* similar update on mdev */ /* This is similar to one of Eifel findings. * Eifel blocks mdev updates when rtt decreases. * This solution is a bit different: we use finer gain * for mdev in this case (alpha*beta). * Like Eifel it also prevents growth of rto, * but also it limits too fast rto decreases, * happening in pure Eifel. */ if (m > 0) m >>= 3; } else { m -= (call->mdev_us >> 2); /* similar update on mdev */ } call->mdev_us += m; /* mdev = 3/4 mdev + 1/4 new */ if (call->mdev_us > call->mdev_max_us) { call->mdev_max_us = call->mdev_us; if (call->mdev_max_us > call->rttvar_us) call->rttvar_us = call->mdev_max_us; } } else { /* no previous measure. */ srtt = m << 3; /* take the measured time to be rtt */ call->mdev_us = m << 1; /* make sure rto = 3*rtt */ call->rttvar_us = umax(call->mdev_us, rxrpc_rto_min_us(call)); call->mdev_max_us = call->rttvar_us; } call->srtt_us = umax(srtt, 1); } /* * Calculate rto without backoff. This is the second half of Van Jacobson's * routine referred to above. */ static void rxrpc_set_rto(struct rxrpc_call *call) { u32 rto; /* 1. If rtt variance happened to be less 50msec, it is hallucination. * It cannot be less due to utterly erratic ACK generation made * at least by solaris and freebsd. "Erratic ACKs" has _nothing_ * to do with delayed acks, because at cwnd>2 true delack timeout * is invisible. Actually, Linux-2.4 also generates erratic * ACKs in some circumstances. */ rto = __rxrpc_set_rto(call); /* 2. Fixups made earlier cannot be right. * If we do not estimate RTO correctly without them, * all the algo is pure shit and should be replaced * with correct one. It is exactly, which we pretend to do. */ /* NOTE: clamping at RXRPC_RTO_MIN is not required, current algo * guarantees that rto is higher. */ call->rto_us = rxrpc_bound_rto(rto); } static void rxrpc_update_rtt_min(struct rxrpc_call *call, ktime_t resp_time, long rtt_us) { /* Window size 5mins in approx usec (ipv4.sysctl_tcp_min_rtt_wlen) */ u32 wlen_us = 5ULL * NSEC_PER_SEC / 1024; minmax_running_min(&call->min_rtt, wlen_us, resp_time / 1024, (u32)rtt_us ? : jiffies_to_usecs(1)); } static void rxrpc_ack_update_rtt(struct rxrpc_call *call, ktime_t resp_time, long rtt_us) { if (rtt_us < 0) return; /* Update RACK min RTT [RFC8985 6.1 Step 1]. */ rxrpc_update_rtt_min(call, resp_time, rtt_us); rxrpc_rtt_estimator(call, rtt_us); rxrpc_set_rto(call); /* Only reset backoff on valid RTT measurement [RFC6298]. */ call->backoff = 0; } /* * Add RTT information to cache. This is called in softirq mode and has * exclusive access to the call RTT data. */ void rxrpc_call_add_rtt(struct rxrpc_call *call, enum rxrpc_rtt_rx_trace why, int rtt_slot, rxrpc_serial_t send_serial, rxrpc_serial_t resp_serial, ktime_t send_time, ktime_t resp_time) { s64 rtt_us; rtt_us = ktime_to_us(ktime_sub(resp_time, send_time)); if (rtt_us < 0) return; rxrpc_ack_update_rtt(call, resp_time, rtt_us); if (call->rtt_count < 3) call->rtt_count++; call->rtt_taken++; WRITE_ONCE(call->peer->recent_srtt_us, call->srtt_us / 8); WRITE_ONCE(call->peer->recent_rto_us, call->rto_us); trace_rxrpc_rtt_rx(call, why, rtt_slot, send_serial, resp_serial, rtt_us, call->srtt_us, call->rto_us); } /* * Get the retransmission timeout to set in nanoseconds, backing it off each * time we retransmit. */ ktime_t rxrpc_get_rto_backoff(struct rxrpc_call *call, bool retrans) { u64 timo_us; u32 backoff = READ_ONCE(call->backoff); timo_us = call->rto_us; timo_us <<= backoff; if (retrans && timo_us * 2 <= RXRPC_RTO_MAX) WRITE_ONCE(call->backoff, backoff + 1); if (timo_us < 1) timo_us = 1; return ns_to_ktime(timo_us * NSEC_PER_USEC); } void rxrpc_call_init_rtt(struct rxrpc_call *call) { call->rtt_last_req = KTIME_MIN; call->rto_us = RXRPC_TIMEOUT_INIT; call->mdev_us = RXRPC_TIMEOUT_INIT; call->backoff = 0; //minmax_reset(&call->rtt_min, rxrpc_jiffies32, ~0U); }
