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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}
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
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 _usecs_to_jiffies((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_j = 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_j);
167}
168
169/*
170 * Get the retransmission timeout to set in jiffies, backing it off each time
171 * we retransmit.
172 */
173unsigned 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
189void 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}