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09c434b8 | 1 | // SPDX-License-Identifier: GPL-2.0-only |
2b0a8c9e KKJ |
2 | /* |
3 | * CAIA Delay-Gradient (CDG) congestion control | |
4 | * | |
5 | * This implementation is based on the paper: | |
6 | * D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using | |
7 | * delay gradients." In IFIP Networking, pages 328-341. Springer, 2011. | |
8 | * | |
9 | * Scavenger traffic (Less-than-Best-Effort) should disable coexistence | |
10 | * heuristics using parameters use_shadow=0 and use_ineff=0. | |
11 | * | |
12 | * Parameters window, backoff_beta, and backoff_factor are crucial for | |
13 | * throughput and delay. Future work is needed to determine better defaults, | |
14 | * and to provide guidelines for use in different environments/contexts. | |
15 | * | |
16 | * Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/. | |
17 | * Parameter window is only configurable when loading tcp_cdg as a module. | |
18 | * | |
19 | * Notable differences from paper/FreeBSD: | |
20 | * o Using Hybrid Slow start and Proportional Rate Reduction. | |
21 | * o Add toggle for shadow window mechanism. Suggested by David Hayes. | |
22 | * o Add toggle for non-congestion loss tolerance. | |
23 | * o Scaling parameter G is changed to a backoff factor; | |
24 | * conversion is given by: backoff_factor = 1000/(G * window). | |
25 | * o Limit shadow window to 2 * cwnd, or to cwnd when application limited. | |
26 | * o More accurate e^-x. | |
27 | */ | |
28 | #include <linux/kernel.h> | |
29 | #include <linux/random.h> | |
30 | #include <linux/module.h> | |
e6017571 IM |
31 | #include <linux/sched/clock.h> |
32 | ||
2b0a8c9e KKJ |
33 | #include <net/tcp.h> |
34 | ||
35 | #define HYSTART_ACK_TRAIN 1 | |
36 | #define HYSTART_DELAY 2 | |
37 | ||
38 | static int window __read_mostly = 8; | |
39 | static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */ | |
40 | static unsigned int backoff_factor __read_mostly = 42; | |
41 | static unsigned int hystart_detect __read_mostly = 3; | |
42 | static unsigned int use_ineff __read_mostly = 5; | |
43 | static bool use_shadow __read_mostly = true; | |
44 | static bool use_tolerance __read_mostly; | |
45 | ||
46 | module_param(window, int, 0444); | |
47 | MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)"); | |
48 | module_param(backoff_beta, uint, 0644); | |
49 | MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)"); | |
50 | module_param(backoff_factor, uint, 0644); | |
51 | MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor"); | |
52 | module_param(hystart_detect, uint, 0644); | |
53 | MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start " | |
54 | "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)"); | |
55 | module_param(use_ineff, uint, 0644); | |
56 | MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)"); | |
57 | module_param(use_shadow, bool, 0644); | |
58 | MODULE_PARM_DESC(use_shadow, "use shadow window heuristic"); | |
59 | module_param(use_tolerance, bool, 0644); | |
60 | MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic"); | |
61 | ||
f78e73e2 | 62 | struct cdg_minmax { |
2b0a8c9e KKJ |
63 | union { |
64 | struct { | |
65 | s32 min; | |
66 | s32 max; | |
67 | }; | |
68 | u64 v64; | |
69 | }; | |
70 | }; | |
71 | ||
72 | enum cdg_state { | |
73 | CDG_UNKNOWN = 0, | |
74 | CDG_NONFULL = 1, | |
75 | CDG_FULL = 2, | |
76 | CDG_BACKOFF = 3, | |
77 | }; | |
78 | ||
79 | struct cdg { | |
f78e73e2 SHY |
80 | struct cdg_minmax rtt; |
81 | struct cdg_minmax rtt_prev; | |
82 | struct cdg_minmax *gradients; | |
83 | struct cdg_minmax gsum; | |
2b0a8c9e KKJ |
84 | bool gfilled; |
85 | u8 tail; | |
86 | u8 state; | |
87 | u8 delack; | |
88 | u32 rtt_seq; | |
2b0a8c9e KKJ |
89 | u32 shadow_wnd; |
90 | u16 backoff_cnt; | |
91 | u16 sample_cnt; | |
92 | s32 delay_min; | |
93 | u32 last_ack; | |
94 | u32 round_start; | |
95 | }; | |
96 | ||
97 | /** | |
98 | * nexp_u32 - negative base-e exponential | |
99 | * @ux: x in units of micro | |
100 | * | |
101 | * Returns exp(ux * -1e-6) * U32_MAX. | |
102 | */ | |
103 | static u32 __pure nexp_u32(u32 ux) | |
104 | { | |
105 | static const u16 v[] = { | |
106 | /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */ | |
107 | 65535, | |
108 | 65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422, | |
109 | 61378, 57484, 50423, 38795, 22965, 8047, 987, 14, | |
110 | }; | |
111 | u32 msb = ux >> 8; | |
112 | u32 res; | |
113 | int i; | |
114 | ||
115 | /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */ | |
116 | if (msb > U16_MAX) | |
117 | return 0; | |
118 | ||
119 | /* Scale first eight bits linearly: */ | |
120 | res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000); | |
121 | ||
122 | /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */ | |
123 | for (i = 1; msb; i++, msb >>= 1) { | |
124 | u32 y = v[i & -(msb & 1)] + U32_C(1); | |
125 | ||
126 | res = ((u64)res * y) >> 16; | |
127 | } | |
128 | ||
129 | return res; | |
130 | } | |
131 | ||
132 | /* Based on the HyStart algorithm (by Ha et al.) that is implemented in | |
133 | * tcp_cubic. Differences/experimental changes: | |
134 | * o Using Hayes' delayed ACK filter. | |
135 | * o Using a usec clock for the ACK train. | |
136 | * o Reset ACK train when application limited. | |
137 | * o Invoked at any cwnd (i.e. also when cwnd < 16). | |
138 | * o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh). | |
139 | */ | |
140 | static void tcp_cdg_hystart_update(struct sock *sk) | |
141 | { | |
142 | struct cdg *ca = inet_csk_ca(sk); | |
143 | struct tcp_sock *tp = tcp_sk(sk); | |
144 | ||
145 | ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min); | |
146 | if (ca->delay_min == 0) | |
147 | return; | |
148 | ||
149 | if (hystart_detect & HYSTART_ACK_TRAIN) { | |
825e1c52 | 150 | u32 now_us = tp->tcp_mstamp; |
2b0a8c9e KKJ |
151 | |
152 | if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) { | |
153 | ca->last_ack = now_us; | |
154 | ca->round_start = now_us; | |
155 | } else if (before(now_us, ca->last_ack + 3000)) { | |
156 | u32 base_owd = max(ca->delay_min / 2U, 125U); | |
157 | ||
158 | ca->last_ack = now_us; | |
159 | if (after(now_us, ca->round_start + base_owd)) { | |
c10d9310 ED |
160 | NET_INC_STATS(sock_net(sk), |
161 | LINUX_MIB_TCPHYSTARTTRAINDETECT); | |
162 | NET_ADD_STATS(sock_net(sk), | |
163 | LINUX_MIB_TCPHYSTARTTRAINCWND, | |
40570375 ED |
164 | tcp_snd_cwnd(tp)); |
165 | tp->snd_ssthresh = tcp_snd_cwnd(tp); | |
2b0a8c9e KKJ |
166 | return; |
167 | } | |
168 | } | |
169 | } | |
170 | ||
171 | if (hystart_detect & HYSTART_DELAY) { | |
172 | if (ca->sample_cnt < 8) { | |
173 | ca->sample_cnt++; | |
174 | } else { | |
175 | s32 thresh = max(ca->delay_min + ca->delay_min / 8U, | |
176 | 125U); | |
177 | ||
178 | if (ca->rtt.min > thresh) { | |
c10d9310 ED |
179 | NET_INC_STATS(sock_net(sk), |
180 | LINUX_MIB_TCPHYSTARTDELAYDETECT); | |
181 | NET_ADD_STATS(sock_net(sk), | |
182 | LINUX_MIB_TCPHYSTARTDELAYCWND, | |
40570375 ED |
183 | tcp_snd_cwnd(tp)); |
184 | tp->snd_ssthresh = tcp_snd_cwnd(tp); | |
2b0a8c9e KKJ |
185 | } |
186 | } | |
187 | } | |
188 | } | |
189 | ||
190 | static s32 tcp_cdg_grad(struct cdg *ca) | |
191 | { | |
192 | s32 gmin = ca->rtt.min - ca->rtt_prev.min; | |
193 | s32 gmax = ca->rtt.max - ca->rtt_prev.max; | |
194 | s32 grad; | |
195 | ||
196 | if (ca->gradients) { | |
197 | ca->gsum.min += gmin - ca->gradients[ca->tail].min; | |
198 | ca->gsum.max += gmax - ca->gradients[ca->tail].max; | |
199 | ca->gradients[ca->tail].min = gmin; | |
200 | ca->gradients[ca->tail].max = gmax; | |
201 | ca->tail = (ca->tail + 1) & (window - 1); | |
202 | gmin = ca->gsum.min; | |
203 | gmax = ca->gsum.max; | |
204 | } | |
205 | ||
206 | /* We keep sums to ignore gradients during cwnd reductions; | |
207 | * the paper's smoothed gradients otherwise simplify to: | |
208 | * (rtt_latest - rtt_oldest) / window. | |
209 | * | |
210 | * We also drop division by window here. | |
211 | */ | |
212 | grad = gmin > 0 ? gmin : gmax; | |
213 | ||
214 | /* Extrapolate missing values in gradient window: */ | |
215 | if (!ca->gfilled) { | |
216 | if (!ca->gradients && window > 1) | |
217 | grad *= window; /* Memory allocation failed. */ | |
218 | else if (ca->tail == 0) | |
219 | ca->gfilled = true; | |
220 | else | |
221 | grad = (grad * window) / (int)ca->tail; | |
222 | } | |
223 | ||
224 | /* Backoff was effectual: */ | |
225 | if (gmin <= -32 || gmax <= -32) | |
226 | ca->backoff_cnt = 0; | |
227 | ||
228 | if (use_tolerance) { | |
229 | /* Reduce small variations to zero: */ | |
230 | gmin = DIV_ROUND_CLOSEST(gmin, 64); | |
231 | gmax = DIV_ROUND_CLOSEST(gmax, 64); | |
232 | ||
233 | if (gmin > 0 && gmax <= 0) | |
234 | ca->state = CDG_FULL; | |
235 | else if ((gmin > 0 && gmax > 0) || gmax < 0) | |
236 | ca->state = CDG_NONFULL; | |
237 | } | |
238 | return grad; | |
239 | } | |
240 | ||
241 | static bool tcp_cdg_backoff(struct sock *sk, u32 grad) | |
242 | { | |
243 | struct cdg *ca = inet_csk_ca(sk); | |
244 | struct tcp_sock *tp = tcp_sk(sk); | |
245 | ||
a251c17a | 246 | if (get_random_u32() <= nexp_u32(grad * backoff_factor)) |
2b0a8c9e KKJ |
247 | return false; |
248 | ||
249 | if (use_ineff) { | |
250 | ca->backoff_cnt++; | |
251 | if (ca->backoff_cnt > use_ineff) | |
252 | return false; | |
253 | } | |
254 | ||
40570375 | 255 | ca->shadow_wnd = max(ca->shadow_wnd, tcp_snd_cwnd(tp)); |
2b0a8c9e KKJ |
256 | ca->state = CDG_BACKOFF; |
257 | tcp_enter_cwr(sk); | |
258 | return true; | |
259 | } | |
260 | ||
261 | /* Not called in CWR or Recovery state. */ | |
262 | static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked) | |
263 | { | |
264 | struct cdg *ca = inet_csk_ca(sk); | |
265 | struct tcp_sock *tp = tcp_sk(sk); | |
266 | u32 prior_snd_cwnd; | |
267 | u32 incr; | |
268 | ||
76174004 | 269 | if (tcp_in_slow_start(tp) && hystart_detect) |
2b0a8c9e KKJ |
270 | tcp_cdg_hystart_update(sk); |
271 | ||
272 | if (after(ack, ca->rtt_seq) && ca->rtt.v64) { | |
273 | s32 grad = 0; | |
274 | ||
275 | if (ca->rtt_prev.v64) | |
276 | grad = tcp_cdg_grad(ca); | |
277 | ca->rtt_seq = tp->snd_nxt; | |
278 | ca->rtt_prev = ca->rtt; | |
279 | ca->rtt.v64 = 0; | |
280 | ca->last_ack = 0; | |
281 | ca->sample_cnt = 0; | |
282 | ||
283 | if (grad > 0 && tcp_cdg_backoff(sk, grad)) | |
284 | return; | |
285 | } | |
286 | ||
287 | if (!tcp_is_cwnd_limited(sk)) { | |
40570375 | 288 | ca->shadow_wnd = min(ca->shadow_wnd, tcp_snd_cwnd(tp)); |
2b0a8c9e KKJ |
289 | return; |
290 | } | |
291 | ||
40570375 | 292 | prior_snd_cwnd = tcp_snd_cwnd(tp); |
2b0a8c9e KKJ |
293 | tcp_reno_cong_avoid(sk, ack, acked); |
294 | ||
40570375 | 295 | incr = tcp_snd_cwnd(tp) - prior_snd_cwnd; |
2b0a8c9e KKJ |
296 | ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr); |
297 | } | |
298 | ||
756ee172 | 299 | static void tcp_cdg_acked(struct sock *sk, const struct ack_sample *sample) |
2b0a8c9e KKJ |
300 | { |
301 | struct cdg *ca = inet_csk_ca(sk); | |
302 | struct tcp_sock *tp = tcp_sk(sk); | |
303 | ||
756ee172 | 304 | if (sample->rtt_us <= 0) |
2b0a8c9e KKJ |
305 | return; |
306 | ||
307 | /* A heuristic for filtering delayed ACKs, adapted from: | |
308 | * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support | |
309 | * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010. | |
310 | */ | |
311 | if (tp->sacked_out == 0) { | |
756ee172 | 312 | if (sample->pkts_acked == 1 && ca->delack) { |
2b0a8c9e KKJ |
313 | /* A delayed ACK is only used for the minimum if it is |
314 | * provenly lower than an existing non-zero minimum. | |
315 | */ | |
756ee172 | 316 | ca->rtt.min = min(ca->rtt.min, sample->rtt_us); |
2b0a8c9e KKJ |
317 | ca->delack--; |
318 | return; | |
756ee172 | 319 | } else if (sample->pkts_acked > 1 && ca->delack < 5) { |
2b0a8c9e KKJ |
320 | ca->delack++; |
321 | } | |
322 | } | |
323 | ||
756ee172 LB |
324 | ca->rtt.min = min_not_zero(ca->rtt.min, sample->rtt_us); |
325 | ca->rtt.max = max(ca->rtt.max, sample->rtt_us); | |
2b0a8c9e KKJ |
326 | } |
327 | ||
328 | static u32 tcp_cdg_ssthresh(struct sock *sk) | |
329 | { | |
330 | struct cdg *ca = inet_csk_ca(sk); | |
331 | struct tcp_sock *tp = tcp_sk(sk); | |
332 | ||
2b0a8c9e | 333 | if (ca->state == CDG_BACKOFF) |
40570375 | 334 | return max(2U, (tcp_snd_cwnd(tp) * min(1024U, backoff_beta)) >> 10); |
2b0a8c9e KKJ |
335 | |
336 | if (ca->state == CDG_NONFULL && use_tolerance) | |
40570375 | 337 | return tcp_snd_cwnd(tp); |
2b0a8c9e | 338 | |
40570375 | 339 | ca->shadow_wnd = min(ca->shadow_wnd >> 1, tcp_snd_cwnd(tp)); |
2b0a8c9e | 340 | if (use_shadow) |
40570375 ED |
341 | return max3(2U, ca->shadow_wnd, tcp_snd_cwnd(tp) >> 1); |
342 | return max(2U, tcp_snd_cwnd(tp) >> 1); | |
2b0a8c9e KKJ |
343 | } |
344 | ||
2b0a8c9e KKJ |
345 | static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev) |
346 | { | |
347 | struct cdg *ca = inet_csk_ca(sk); | |
348 | struct tcp_sock *tp = tcp_sk(sk); | |
f78e73e2 | 349 | struct cdg_minmax *gradients; |
2b0a8c9e KKJ |
350 | |
351 | switch (ev) { | |
352 | case CA_EVENT_CWND_RESTART: | |
353 | gradients = ca->gradients; | |
354 | if (gradients) | |
355 | memset(gradients, 0, window * sizeof(gradients[0])); | |
356 | memset(ca, 0, sizeof(*ca)); | |
357 | ||
358 | ca->gradients = gradients; | |
359 | ca->rtt_seq = tp->snd_nxt; | |
40570375 | 360 | ca->shadow_wnd = tcp_snd_cwnd(tp); |
2b0a8c9e KKJ |
361 | break; |
362 | case CA_EVENT_COMPLETE_CWR: | |
363 | ca->state = CDG_UNKNOWN; | |
364 | ca->rtt_seq = tp->snd_nxt; | |
365 | ca->rtt_prev = ca->rtt; | |
366 | ca->rtt.v64 = 0; | |
367 | break; | |
368 | default: | |
369 | break; | |
370 | } | |
371 | } | |
372 | ||
373 | static void tcp_cdg_init(struct sock *sk) | |
374 | { | |
375 | struct cdg *ca = inet_csk_ca(sk); | |
376 | struct tcp_sock *tp = tcp_sk(sk); | |
377 | ||
72e560cb | 378 | ca->gradients = NULL; |
2b0a8c9e KKJ |
379 | /* We silently fall back to window = 1 if allocation fails. */ |
380 | if (window > 1) | |
381 | ca->gradients = kcalloc(window, sizeof(ca->gradients[0]), | |
382 | GFP_NOWAIT | __GFP_NOWARN); | |
383 | ca->rtt_seq = tp->snd_nxt; | |
40570375 | 384 | ca->shadow_wnd = tcp_snd_cwnd(tp); |
2b0a8c9e KKJ |
385 | } |
386 | ||
387 | static void tcp_cdg_release(struct sock *sk) | |
388 | { | |
389 | struct cdg *ca = inet_csk_ca(sk); | |
390 | ||
391 | kfree(ca->gradients); | |
72e560cb | 392 | ca->gradients = NULL; |
2b0a8c9e KKJ |
393 | } |
394 | ||
d85969f1 | 395 | static struct tcp_congestion_ops tcp_cdg __read_mostly = { |
2b0a8c9e KKJ |
396 | .cong_avoid = tcp_cdg_cong_avoid, |
397 | .cwnd_event = tcp_cdg_cwnd_event, | |
398 | .pkts_acked = tcp_cdg_acked, | |
f1722a1b | 399 | .undo_cwnd = tcp_reno_undo_cwnd, |
2b0a8c9e KKJ |
400 | .ssthresh = tcp_cdg_ssthresh, |
401 | .release = tcp_cdg_release, | |
402 | .init = tcp_cdg_init, | |
403 | .owner = THIS_MODULE, | |
404 | .name = "cdg", | |
405 | }; | |
406 | ||
407 | static int __init tcp_cdg_register(void) | |
408 | { | |
409 | if (backoff_beta > 1024 || window < 1 || window > 256) | |
410 | return -ERANGE; | |
411 | if (!is_power_of_2(window)) | |
412 | return -EINVAL; | |
413 | ||
414 | BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE); | |
415 | tcp_register_congestion_control(&tcp_cdg); | |
416 | return 0; | |
417 | } | |
418 | ||
419 | static void __exit tcp_cdg_unregister(void) | |
420 | { | |
421 | tcp_unregister_congestion_control(&tcp_cdg); | |
422 | } | |
423 | ||
424 | module_init(tcp_cdg_register); | |
425 | module_exit(tcp_cdg_unregister); | |
426 | MODULE_AUTHOR("Kenneth Klette Jonassen"); | |
427 | MODULE_LICENSE("GPL"); | |
428 | MODULE_DESCRIPTION("TCP CDG"); |