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2b0a8c9e KKJ |
1 | /* |
2 | * CAIA Delay-Gradient (CDG) congestion control | |
3 | * | |
4 | * This implementation is based on the paper: | |
5 | * D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using | |
6 | * delay gradients." In IFIP Networking, pages 328-341. Springer, 2011. | |
7 | * | |
8 | * Scavenger traffic (Less-than-Best-Effort) should disable coexistence | |
9 | * heuristics using parameters use_shadow=0 and use_ineff=0. | |
10 | * | |
11 | * Parameters window, backoff_beta, and backoff_factor are crucial for | |
12 | * throughput and delay. Future work is needed to determine better defaults, | |
13 | * and to provide guidelines for use in different environments/contexts. | |
14 | * | |
15 | * Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/. | |
16 | * Parameter window is only configurable when loading tcp_cdg as a module. | |
17 | * | |
18 | * Notable differences from paper/FreeBSD: | |
19 | * o Using Hybrid Slow start and Proportional Rate Reduction. | |
20 | * o Add toggle for shadow window mechanism. Suggested by David Hayes. | |
21 | * o Add toggle for non-congestion loss tolerance. | |
22 | * o Scaling parameter G is changed to a backoff factor; | |
23 | * conversion is given by: backoff_factor = 1000/(G * window). | |
24 | * o Limit shadow window to 2 * cwnd, or to cwnd when application limited. | |
25 | * o More accurate e^-x. | |
26 | */ | |
27 | #include <linux/kernel.h> | |
28 | #include <linux/random.h> | |
29 | #include <linux/module.h> | |
30 | #include <net/tcp.h> | |
31 | ||
32 | #define HYSTART_ACK_TRAIN 1 | |
33 | #define HYSTART_DELAY 2 | |
34 | ||
35 | static int window __read_mostly = 8; | |
36 | static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */ | |
37 | static unsigned int backoff_factor __read_mostly = 42; | |
38 | static unsigned int hystart_detect __read_mostly = 3; | |
39 | static unsigned int use_ineff __read_mostly = 5; | |
40 | static bool use_shadow __read_mostly = true; | |
41 | static bool use_tolerance __read_mostly; | |
42 | ||
43 | module_param(window, int, 0444); | |
44 | MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)"); | |
45 | module_param(backoff_beta, uint, 0644); | |
46 | MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)"); | |
47 | module_param(backoff_factor, uint, 0644); | |
48 | MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor"); | |
49 | module_param(hystart_detect, uint, 0644); | |
50 | MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start " | |
51 | "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)"); | |
52 | module_param(use_ineff, uint, 0644); | |
53 | MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)"); | |
54 | module_param(use_shadow, bool, 0644); | |
55 | MODULE_PARM_DESC(use_shadow, "use shadow window heuristic"); | |
56 | module_param(use_tolerance, bool, 0644); | |
57 | MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic"); | |
58 | ||
59 | struct minmax { | |
60 | union { | |
61 | struct { | |
62 | s32 min; | |
63 | s32 max; | |
64 | }; | |
65 | u64 v64; | |
66 | }; | |
67 | }; | |
68 | ||
69 | enum cdg_state { | |
70 | CDG_UNKNOWN = 0, | |
71 | CDG_NONFULL = 1, | |
72 | CDG_FULL = 2, | |
73 | CDG_BACKOFF = 3, | |
74 | }; | |
75 | ||
76 | struct cdg { | |
77 | struct minmax rtt; | |
78 | struct minmax rtt_prev; | |
79 | struct minmax *gradients; | |
80 | struct minmax gsum; | |
81 | bool gfilled; | |
82 | u8 tail; | |
83 | u8 state; | |
84 | u8 delack; | |
85 | u32 rtt_seq; | |
86 | u32 undo_cwnd; | |
87 | u32 shadow_wnd; | |
88 | u16 backoff_cnt; | |
89 | u16 sample_cnt; | |
90 | s32 delay_min; | |
91 | u32 last_ack; | |
92 | u32 round_start; | |
93 | }; | |
94 | ||
95 | /** | |
96 | * nexp_u32 - negative base-e exponential | |
97 | * @ux: x in units of micro | |
98 | * | |
99 | * Returns exp(ux * -1e-6) * U32_MAX. | |
100 | */ | |
101 | static u32 __pure nexp_u32(u32 ux) | |
102 | { | |
103 | static const u16 v[] = { | |
104 | /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */ | |
105 | 65535, | |
106 | 65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422, | |
107 | 61378, 57484, 50423, 38795, 22965, 8047, 987, 14, | |
108 | }; | |
109 | u32 msb = ux >> 8; | |
110 | u32 res; | |
111 | int i; | |
112 | ||
113 | /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */ | |
114 | if (msb > U16_MAX) | |
115 | return 0; | |
116 | ||
117 | /* Scale first eight bits linearly: */ | |
118 | res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000); | |
119 | ||
120 | /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */ | |
121 | for (i = 1; msb; i++, msb >>= 1) { | |
122 | u32 y = v[i & -(msb & 1)] + U32_C(1); | |
123 | ||
124 | res = ((u64)res * y) >> 16; | |
125 | } | |
126 | ||
127 | return res; | |
128 | } | |
129 | ||
130 | /* Based on the HyStart algorithm (by Ha et al.) that is implemented in | |
131 | * tcp_cubic. Differences/experimental changes: | |
132 | * o Using Hayes' delayed ACK filter. | |
133 | * o Using a usec clock for the ACK train. | |
134 | * o Reset ACK train when application limited. | |
135 | * o Invoked at any cwnd (i.e. also when cwnd < 16). | |
136 | * o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh). | |
137 | */ | |
138 | static void tcp_cdg_hystart_update(struct sock *sk) | |
139 | { | |
140 | struct cdg *ca = inet_csk_ca(sk); | |
141 | struct tcp_sock *tp = tcp_sk(sk); | |
142 | ||
143 | ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min); | |
144 | if (ca->delay_min == 0) | |
145 | return; | |
146 | ||
147 | if (hystart_detect & HYSTART_ACK_TRAIN) { | |
148 | u32 now_us = local_clock() / NSEC_PER_USEC; | |
149 | ||
150 | if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) { | |
151 | ca->last_ack = now_us; | |
152 | ca->round_start = now_us; | |
153 | } else if (before(now_us, ca->last_ack + 3000)) { | |
154 | u32 base_owd = max(ca->delay_min / 2U, 125U); | |
155 | ||
156 | ca->last_ack = now_us; | |
157 | if (after(now_us, ca->round_start + base_owd)) { | |
158 | NET_INC_STATS_BH(sock_net(sk), | |
159 | LINUX_MIB_TCPHYSTARTTRAINDETECT); | |
160 | NET_ADD_STATS_BH(sock_net(sk), | |
161 | LINUX_MIB_TCPHYSTARTTRAINCWND, | |
162 | tp->snd_cwnd); | |
163 | tp->snd_ssthresh = tp->snd_cwnd; | |
164 | return; | |
165 | } | |
166 | } | |
167 | } | |
168 | ||
169 | if (hystart_detect & HYSTART_DELAY) { | |
170 | if (ca->sample_cnt < 8) { | |
171 | ca->sample_cnt++; | |
172 | } else { | |
173 | s32 thresh = max(ca->delay_min + ca->delay_min / 8U, | |
174 | 125U); | |
175 | ||
176 | if (ca->rtt.min > thresh) { | |
177 | NET_INC_STATS_BH(sock_net(sk), | |
178 | LINUX_MIB_TCPHYSTARTDELAYDETECT); | |
179 | NET_ADD_STATS_BH(sock_net(sk), | |
180 | LINUX_MIB_TCPHYSTARTDELAYCWND, | |
181 | tp->snd_cwnd); | |
182 | tp->snd_ssthresh = tp->snd_cwnd; | |
183 | } | |
184 | } | |
185 | } | |
186 | } | |
187 | ||
188 | static s32 tcp_cdg_grad(struct cdg *ca) | |
189 | { | |
190 | s32 gmin = ca->rtt.min - ca->rtt_prev.min; | |
191 | s32 gmax = ca->rtt.max - ca->rtt_prev.max; | |
192 | s32 grad; | |
193 | ||
194 | if (ca->gradients) { | |
195 | ca->gsum.min += gmin - ca->gradients[ca->tail].min; | |
196 | ca->gsum.max += gmax - ca->gradients[ca->tail].max; | |
197 | ca->gradients[ca->tail].min = gmin; | |
198 | ca->gradients[ca->tail].max = gmax; | |
199 | ca->tail = (ca->tail + 1) & (window - 1); | |
200 | gmin = ca->gsum.min; | |
201 | gmax = ca->gsum.max; | |
202 | } | |
203 | ||
204 | /* We keep sums to ignore gradients during cwnd reductions; | |
205 | * the paper's smoothed gradients otherwise simplify to: | |
206 | * (rtt_latest - rtt_oldest) / window. | |
207 | * | |
208 | * We also drop division by window here. | |
209 | */ | |
210 | grad = gmin > 0 ? gmin : gmax; | |
211 | ||
212 | /* Extrapolate missing values in gradient window: */ | |
213 | if (!ca->gfilled) { | |
214 | if (!ca->gradients && window > 1) | |
215 | grad *= window; /* Memory allocation failed. */ | |
216 | else if (ca->tail == 0) | |
217 | ca->gfilled = true; | |
218 | else | |
219 | grad = (grad * window) / (int)ca->tail; | |
220 | } | |
221 | ||
222 | /* Backoff was effectual: */ | |
223 | if (gmin <= -32 || gmax <= -32) | |
224 | ca->backoff_cnt = 0; | |
225 | ||
226 | if (use_tolerance) { | |
227 | /* Reduce small variations to zero: */ | |
228 | gmin = DIV_ROUND_CLOSEST(gmin, 64); | |
229 | gmax = DIV_ROUND_CLOSEST(gmax, 64); | |
230 | ||
231 | if (gmin > 0 && gmax <= 0) | |
232 | ca->state = CDG_FULL; | |
233 | else if ((gmin > 0 && gmax > 0) || gmax < 0) | |
234 | ca->state = CDG_NONFULL; | |
235 | } | |
236 | return grad; | |
237 | } | |
238 | ||
239 | static bool tcp_cdg_backoff(struct sock *sk, u32 grad) | |
240 | { | |
241 | struct cdg *ca = inet_csk_ca(sk); | |
242 | struct tcp_sock *tp = tcp_sk(sk); | |
243 | ||
244 | if (prandom_u32() <= nexp_u32(grad * backoff_factor)) | |
245 | return false; | |
246 | ||
247 | if (use_ineff) { | |
248 | ca->backoff_cnt++; | |
249 | if (ca->backoff_cnt > use_ineff) | |
250 | return false; | |
251 | } | |
252 | ||
253 | ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd); | |
254 | ca->state = CDG_BACKOFF; | |
255 | tcp_enter_cwr(sk); | |
256 | return true; | |
257 | } | |
258 | ||
259 | /* Not called in CWR or Recovery state. */ | |
260 | static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked) | |
261 | { | |
262 | struct cdg *ca = inet_csk_ca(sk); | |
263 | struct tcp_sock *tp = tcp_sk(sk); | |
264 | u32 prior_snd_cwnd; | |
265 | u32 incr; | |
266 | ||
267 | if (tp->snd_cwnd < tp->snd_ssthresh && hystart_detect) | |
268 | tcp_cdg_hystart_update(sk); | |
269 | ||
270 | if (after(ack, ca->rtt_seq) && ca->rtt.v64) { | |
271 | s32 grad = 0; | |
272 | ||
273 | if (ca->rtt_prev.v64) | |
274 | grad = tcp_cdg_grad(ca); | |
275 | ca->rtt_seq = tp->snd_nxt; | |
276 | ca->rtt_prev = ca->rtt; | |
277 | ca->rtt.v64 = 0; | |
278 | ca->last_ack = 0; | |
279 | ca->sample_cnt = 0; | |
280 | ||
281 | if (grad > 0 && tcp_cdg_backoff(sk, grad)) | |
282 | return; | |
283 | } | |
284 | ||
285 | if (!tcp_is_cwnd_limited(sk)) { | |
286 | ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd); | |
287 | return; | |
288 | } | |
289 | ||
290 | prior_snd_cwnd = tp->snd_cwnd; | |
291 | tcp_reno_cong_avoid(sk, ack, acked); | |
292 | ||
293 | incr = tp->snd_cwnd - prior_snd_cwnd; | |
294 | ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr); | |
295 | } | |
296 | ||
297 | static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us) | |
298 | { | |
299 | struct cdg *ca = inet_csk_ca(sk); | |
300 | struct tcp_sock *tp = tcp_sk(sk); | |
301 | ||
302 | if (rtt_us <= 0) | |
303 | return; | |
304 | ||
305 | /* A heuristic for filtering delayed ACKs, adapted from: | |
306 | * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support | |
307 | * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010. | |
308 | */ | |
309 | if (tp->sacked_out == 0) { | |
310 | if (num_acked == 1 && ca->delack) { | |
311 | /* A delayed ACK is only used for the minimum if it is | |
312 | * provenly lower than an existing non-zero minimum. | |
313 | */ | |
314 | ca->rtt.min = min(ca->rtt.min, rtt_us); | |
315 | ca->delack--; | |
316 | return; | |
317 | } else if (num_acked > 1 && ca->delack < 5) { | |
318 | ca->delack++; | |
319 | } | |
320 | } | |
321 | ||
322 | ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us); | |
323 | ca->rtt.max = max(ca->rtt.max, rtt_us); | |
324 | } | |
325 | ||
326 | static u32 tcp_cdg_ssthresh(struct sock *sk) | |
327 | { | |
328 | struct cdg *ca = inet_csk_ca(sk); | |
329 | struct tcp_sock *tp = tcp_sk(sk); | |
330 | ||
331 | ca->undo_cwnd = tp->snd_cwnd; | |
332 | ||
333 | if (ca->state == CDG_BACKOFF) | |
334 | return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10); | |
335 | ||
336 | if (ca->state == CDG_NONFULL && use_tolerance) | |
337 | return tp->snd_cwnd; | |
338 | ||
339 | ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd); | |
340 | if (use_shadow) | |
341 | return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1); | |
342 | return max(2U, tp->snd_cwnd >> 1); | |
343 | } | |
344 | ||
345 | static u32 tcp_cdg_undo_cwnd(struct sock *sk) | |
346 | { | |
347 | struct cdg *ca = inet_csk_ca(sk); | |
348 | ||
349 | return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd); | |
350 | } | |
351 | ||
352 | static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev) | |
353 | { | |
354 | struct cdg *ca = inet_csk_ca(sk); | |
355 | struct tcp_sock *tp = tcp_sk(sk); | |
356 | struct minmax *gradients; | |
357 | ||
358 | switch (ev) { | |
359 | case CA_EVENT_CWND_RESTART: | |
360 | gradients = ca->gradients; | |
361 | if (gradients) | |
362 | memset(gradients, 0, window * sizeof(gradients[0])); | |
363 | memset(ca, 0, sizeof(*ca)); | |
364 | ||
365 | ca->gradients = gradients; | |
366 | ca->rtt_seq = tp->snd_nxt; | |
367 | ca->shadow_wnd = tp->snd_cwnd; | |
368 | break; | |
369 | case CA_EVENT_COMPLETE_CWR: | |
370 | ca->state = CDG_UNKNOWN; | |
371 | ca->rtt_seq = tp->snd_nxt; | |
372 | ca->rtt_prev = ca->rtt; | |
373 | ca->rtt.v64 = 0; | |
374 | break; | |
375 | default: | |
376 | break; | |
377 | } | |
378 | } | |
379 | ||
380 | static void tcp_cdg_init(struct sock *sk) | |
381 | { | |
382 | struct cdg *ca = inet_csk_ca(sk); | |
383 | struct tcp_sock *tp = tcp_sk(sk); | |
384 | ||
385 | /* We silently fall back to window = 1 if allocation fails. */ | |
386 | if (window > 1) | |
387 | ca->gradients = kcalloc(window, sizeof(ca->gradients[0]), | |
388 | GFP_NOWAIT | __GFP_NOWARN); | |
389 | ca->rtt_seq = tp->snd_nxt; | |
390 | ca->shadow_wnd = tp->snd_cwnd; | |
391 | } | |
392 | ||
393 | static void tcp_cdg_release(struct sock *sk) | |
394 | { | |
395 | struct cdg *ca = inet_csk_ca(sk); | |
396 | ||
397 | kfree(ca->gradients); | |
398 | } | |
399 | ||
400 | struct tcp_congestion_ops tcp_cdg __read_mostly = { | |
401 | .cong_avoid = tcp_cdg_cong_avoid, | |
402 | .cwnd_event = tcp_cdg_cwnd_event, | |
403 | .pkts_acked = tcp_cdg_acked, | |
404 | .undo_cwnd = tcp_cdg_undo_cwnd, | |
405 | .ssthresh = tcp_cdg_ssthresh, | |
406 | .release = tcp_cdg_release, | |
407 | .init = tcp_cdg_init, | |
408 | .owner = THIS_MODULE, | |
409 | .name = "cdg", | |
410 | }; | |
411 | ||
412 | static int __init tcp_cdg_register(void) | |
413 | { | |
414 | if (backoff_beta > 1024 || window < 1 || window > 256) | |
415 | return -ERANGE; | |
416 | if (!is_power_of_2(window)) | |
417 | return -EINVAL; | |
418 | ||
419 | BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE); | |
420 | tcp_register_congestion_control(&tcp_cdg); | |
421 | return 0; | |
422 | } | |
423 | ||
424 | static void __exit tcp_cdg_unregister(void) | |
425 | { | |
426 | tcp_unregister_congestion_control(&tcp_cdg); | |
427 | } | |
428 | ||
429 | module_init(tcp_cdg_register); | |
430 | module_exit(tcp_cdg_unregister); | |
431 | MODULE_AUTHOR("Kenneth Klette Jonassen"); | |
432 | MODULE_LICENSE("GPL"); | |
433 | MODULE_DESCRIPTION("TCP CDG"); |