1 | // SPDX-License-Identifier: GPL-2.0-only |
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> |
31 | #include <linux/sched/clock.h> |
32 | |
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 | |
62 | struct cdg_minmax { |
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 { |
80 | struct cdg_minmax rtt; |
81 | struct cdg_minmax rtt_prev; |
82 | struct cdg_minmax *gradients; |
83 | struct cdg_minmax gsum; |
84 | bool gfilled; |
85 | u8 tail; |
86 | u8 state; |
87 | u8 delack; |
88 | u32 rtt_seq; |
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) { |
150 | u32 now_us = tp->tcp_mstamp; |
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(seq1: now_us, seq2: 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)) { |
160 | NET_INC_STATS(sock_net(sk), |
161 | LINUX_MIB_TCPHYSTARTTRAINDETECT); |
162 | NET_ADD_STATS(sock_net(sk), |
163 | LINUX_MIB_TCPHYSTARTTRAINCWND, |
164 | tcp_snd_cwnd(tp)); |
165 | tp->snd_ssthresh = tcp_snd_cwnd(tp); |
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) { |
179 | NET_INC_STATS(sock_net(sk), |
180 | LINUX_MIB_TCPHYSTARTDELAYDETECT); |
181 | NET_ADD_STATS(sock_net(sk), |
182 | LINUX_MIB_TCPHYSTARTDELAYCWND, |
183 | tcp_snd_cwnd(tp)); |
184 | tp->snd_ssthresh = tcp_snd_cwnd(tp); |
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 | |
246 | if (get_random_u32() <= nexp_u32(ux: grad * backoff_factor)) |
247 | return false; |
248 | |
249 | if (use_ineff) { |
250 | ca->backoff_cnt++; |
251 | if (ca->backoff_cnt > use_ineff) |
252 | return false; |
253 | } |
254 | |
255 | ca->shadow_wnd = max(ca->shadow_wnd, tcp_snd_cwnd(tp)); |
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 | |
269 | if (tcp_in_slow_start(tp) && hystart_detect) |
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)) { |
288 | ca->shadow_wnd = min(ca->shadow_wnd, tcp_snd_cwnd(tp)); |
289 | return; |
290 | } |
291 | |
292 | prior_snd_cwnd = tcp_snd_cwnd(tp); |
293 | tcp_reno_cong_avoid(sk, ack, acked); |
294 | |
295 | incr = tcp_snd_cwnd(tp) - prior_snd_cwnd; |
296 | ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr); |
297 | } |
298 | |
299 | static void tcp_cdg_acked(struct sock *sk, const struct ack_sample *sample) |
300 | { |
301 | struct cdg *ca = inet_csk_ca(sk); |
302 | struct tcp_sock *tp = tcp_sk(sk); |
303 | |
304 | if (sample->rtt_us <= 0) |
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) { |
312 | if (sample->pkts_acked == 1 && ca->delack) { |
313 | /* A delayed ACK is only used for the minimum if it is |
314 | * provenly lower than an existing non-zero minimum. |
315 | */ |
316 | ca->rtt.min = min(ca->rtt.min, sample->rtt_us); |
317 | ca->delack--; |
318 | return; |
319 | } else if (sample->pkts_acked > 1 && ca->delack < 5) { |
320 | ca->delack++; |
321 | } |
322 | } |
323 | |
324 | ca->rtt.min = min_not_zero(ca->rtt.min, sample->rtt_us); |
325 | ca->rtt.max = max(ca->rtt.max, sample->rtt_us); |
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 | |
333 | if (ca->state == CDG_BACKOFF) |
334 | return max(2U, (tcp_snd_cwnd(tp) * min(1024U, backoff_beta)) >> 10); |
335 | |
336 | if (ca->state == CDG_NONFULL && use_tolerance) |
337 | return tcp_snd_cwnd(tp); |
338 | |
339 | ca->shadow_wnd = min(ca->shadow_wnd >> 1, tcp_snd_cwnd(tp)); |
340 | if (use_shadow) |
341 | return max3(2U, ca->shadow_wnd, tcp_snd_cwnd(tp) >> 1); |
342 | return max(2U, tcp_snd_cwnd(tp) >> 1); |
343 | } |
344 | |
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); |
349 | struct cdg_minmax *gradients; |
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; |
360 | ca->shadow_wnd = tcp_snd_cwnd(tp); |
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 | |
378 | ca->gradients = NULL; |
379 | /* We silently fall back to window = 1 if allocation fails. */ |
380 | if (window > 1) |
381 | ca->gradients = kcalloc(n: window, size: sizeof(ca->gradients[0]), |
382 | GFP_NOWAIT | __GFP_NOWARN); |
383 | ca->rtt_seq = tp->snd_nxt; |
384 | ca->shadow_wnd = tcp_snd_cwnd(tp); |
385 | } |
386 | |
387 | static void tcp_cdg_release(struct sock *sk) |
388 | { |
389 | struct cdg *ca = inet_csk_ca(sk); |
390 | |
391 | kfree(objp: ca->gradients); |
392 | ca->gradients = NULL; |
393 | } |
394 | |
395 | static struct tcp_congestion_ops tcp_cdg __read_mostly = { |
396 | .cong_avoid = tcp_cdg_cong_avoid, |
397 | .cwnd_event = tcp_cdg_cwnd_event, |
398 | .pkts_acked = tcp_cdg_acked, |
399 | .undo_cwnd = tcp_reno_undo_cwnd, |
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(n: window)) |
412 | return -EINVAL; |
413 | |
414 | BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE); |
415 | tcp_register_congestion_control(type: &tcp_cdg); |
416 | return 0; |
417 | } |
418 | |
419 | static void __exit tcp_cdg_unregister(void) |
420 | { |
421 | tcp_unregister_congestion_control(type: &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" ); |
429 | |