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	(120 * USEC_PER_SEC)
15#define RXRPC_TIMEOUT_INIT ((unsigned int)(1 * MSEC_PER_SEC)) /* RFC6298 2.1 initial RTO value */
16#define rxrpc_jiffies32 ((u32)jiffies)		/* As rxrpc_jiffies32 */
17
18static u32 rxrpc_rto_min_us(struct rxrpc_peer *peer)
19{
20	return 200;
21}
22
23static u32 __rxrpc_set_rto(const struct rxrpc_peer *peer)
24{
25	return (peer->srtt_us >> 3) + peer->rttvar_us;
26}
27
28static 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 */
43static 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 */
105static 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_us = rxrpc_bound_rto(rto);
128}
129
130static 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 */
147void 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_us);
167}
168
169/*
170 * Get the retransmission timeout to set in nanoseconds, backing it off each
171 * time we retransmit.
172 */
173ktime_t rxrpc_get_rto_backoff(struct rxrpc_peer *peer, bool retrans)
174{
175	u64 timo_us;
176	u32 backoff = READ_ONCE(peer->backoff);
177
178	timo_us = peer->rto_us;
179	timo_us <<= backoff;
180	if (retrans && timo_us * 2 <= RXRPC_RTO_MAX)
181		WRITE_ONCE(peer->backoff, backoff + 1);
182
183	if (timo_us < 1)
184		timo_us = 1;
185
186	return ns_to_ktime(timo_us * NSEC_PER_USEC);
187}
188
189void rxrpc_peer_init_rtt(struct rxrpc_peer *peer)
190{
191	peer->rto_us	= RXRPC_TIMEOUT_INIT;
192	peer->mdev_us	= RXRPC_TIMEOUT_INIT;
193	peer->backoff	= 0;
194	//minmax_reset(&peer->rtt_min, rxrpc_jiffies32, ~0U);
195}
196