1 // SPDX-License-Identifier: GPL-2.0-only 2 /* 3 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3 4 * Home page: 5 * http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC 6 * This is from the implementation of CUBIC TCP in 7 * Sangtae Ha, Injong Rhee and Lisong Xu, 8 * "CUBIC: A New TCP-Friendly High-Speed TCP Variant" 9 * in ACM SIGOPS Operating System Review, July 2008. 10 * Available from: 11 * http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf 12 * 13 * CUBIC integrates a new slow start algorithm, called HyStart. 14 * The details of HyStart are presented in 15 * Sangtae Ha and Injong Rhee, 16 * "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008. 17 * Available from: 18 * http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf 19 * 20 * All testing results are available from: 21 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing 22 * 23 * Unless CUBIC is enabled and congestion window is large 24 * this behaves the same as the original Reno. 25 */ 26 27 #include <linux/mm.h> 28 #include <linux/module.h> 29 #include <linux/math64.h> 30 #include <net/tcp.h> 31 32 #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation 33 * max_cwnd = snd_cwnd * beta 34 */ 35 #define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */ 36 37 /* Two methods of hybrid slow start */ 38 #define HYSTART_ACK_TRAIN 0x1 39 #define HYSTART_DELAY 0x2 40 41 /* Number of delay samples for detecting the increase of delay */ 42 #define HYSTART_MIN_SAMPLES 8 43 #define HYSTART_DELAY_MIN (4000U) /* 4 ms */ 44 #define HYSTART_DELAY_MAX (16000U) /* 16 ms */ 45 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX) 46 47 static int fast_convergence __read_mostly = 1; 48 static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */ 49 static int initial_ssthresh __read_mostly; 50 static int bic_scale __read_mostly = 41; 51 static int tcp_friendliness __read_mostly = 1; 52 53 static int hystart __read_mostly = 1; 54 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY; 55 static int hystart_low_window __read_mostly = 16; 56 static int hystart_ack_delta_us __read_mostly = 2000; 57 58 static u32 cube_rtt_scale __read_mostly; 59 static u32 beta_scale __read_mostly; 60 static u64 cube_factor __read_mostly; 61 62 /* Note parameters that are used for precomputing scale factors are read-only */ 63 module_param(fast_convergence, int, 0644); 64 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); 65 module_param(beta, int, 0644); 66 MODULE_PARM_DESC(beta, "beta for multiplicative increase"); 67 module_param(initial_ssthresh, int, 0644); 68 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); 69 module_param(bic_scale, int, 0444); 70 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); 71 module_param(tcp_friendliness, int, 0644); 72 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); 73 module_param(hystart, int, 0644); 74 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm"); 75 module_param(hystart_detect, int, 0644); 76 MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms" 77 " 1: packet-train 2: delay 3: both packet-train and delay"); 78 module_param(hystart_low_window, int, 0644); 79 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start"); 80 module_param(hystart_ack_delta_us, int, 0644); 81 MODULE_PARM_DESC(hystart_ack_delta_us, "spacing between ack's indicating train (usecs)"); 82 83 /* BIC TCP Parameters */ 84 struct bictcp { 85 u32 cnt; /* increase cwnd by 1 after ACKs */ 86 u32 last_max_cwnd; /* last maximum snd_cwnd */ 87 u32 last_cwnd; /* the last snd_cwnd */ 88 u32 last_time; /* time when updated last_cwnd */ 89 u32 bic_origin_point;/* origin point of bic function */ 90 u32 bic_K; /* time to origin point 91 from the beginning of the current epoch */ 92 u32 delay_min; /* min delay (usec) */ 93 u32 epoch_start; /* beginning of an epoch */ 94 u32 ack_cnt; /* number of acks */ 95 u32 tcp_cwnd; /* estimated tcp cwnd */ 96 u16 unused; 97 u8 sample_cnt; /* number of samples to decide curr_rtt */ 98 u8 found; /* the exit point is found? */ 99 u32 round_start; /* beginning of each round */ 100 u32 end_seq; /* end_seq of the round */ 101 u32 last_ack; /* last time when the ACK spacing is close */ 102 u32 curr_rtt; /* the minimum rtt of current round */ 103 }; 104 105 static inline void bictcp_reset(struct bictcp *ca) 106 { 107 memset(ca, 0, offsetof(struct bictcp, unused)); 108 ca->found = 0; 109 } 110 111 static inline u32 bictcp_clock_us(const struct sock *sk) 112 { 113 return tcp_sk(sk)->tcp_mstamp; 114 } 115 116 static inline void bictcp_hystart_reset(struct sock *sk) 117 { 118 struct tcp_sock *tp = tcp_sk(sk); 119 struct bictcp *ca = inet_csk_ca(sk); 120 121 ca->round_start = ca->last_ack = bictcp_clock_us(sk); 122 ca->end_seq = tp->snd_nxt; 123 ca->curr_rtt = ~0U; 124 ca->sample_cnt = 0; 125 } 126 127 static void bictcp_init(struct sock *sk) 128 { 129 struct bictcp *ca = inet_csk_ca(sk); 130 131 bictcp_reset(ca); 132 133 if (hystart) 134 bictcp_hystart_reset(sk); 135 136 if (!hystart && initial_ssthresh) 137 tcp_sk(sk)->snd_ssthresh = initial_ssthresh; 138 } 139 140 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event) 141 { 142 if (event == CA_EVENT_TX_START) { 143 struct bictcp *ca = inet_csk_ca(sk); 144 u32 now = tcp_jiffies32; 145 s32 delta; 146 147 delta = now - tcp_sk(sk)->lsndtime; 148 149 /* We were application limited (idle) for a while. 150 * Shift epoch_start to keep cwnd growth to cubic curve. 151 */ 152 if (ca->epoch_start && delta > 0) { 153 ca->epoch_start += delta; 154 if (after(ca->epoch_start, now)) 155 ca->epoch_start = now; 156 } 157 return; 158 } 159 } 160 161 /* calculate the cubic root of x using a table lookup followed by one 162 * Newton-Raphson iteration. 163 * Avg err ~= 0.195% 164 */ 165 static u32 cubic_root(u64 a) 166 { 167 u32 x, b, shift; 168 /* 169 * cbrt(x) MSB values for x MSB values in [0..63]. 170 * Precomputed then refined by hand - Willy Tarreau 171 * 172 * For x in [0..63], 173 * v = cbrt(x << 18) - 1 174 * cbrt(x) = (v[x] + 10) >> 6 175 */ 176 static const u8 v[] = { 177 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, 178 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, 179 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, 180 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, 181 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, 182 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, 183 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, 184 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, 185 }; 186 187 b = fls64(a); 188 if (b < 7) { 189 /* a in [0..63] */ 190 return ((u32)v[(u32)a] + 35) >> 6; 191 } 192 193 b = ((b * 84) >> 8) - 1; 194 shift = (a >> (b * 3)); 195 196 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; 197 198 /* 199 * Newton-Raphson iteration 200 * 2 201 * x = ( 2 * x + a / x ) / 3 202 * k+1 k k 203 */ 204 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1))); 205 x = ((x * 341) >> 10); 206 return x; 207 } 208 209 /* 210 * Compute congestion window to use. 211 */ 212 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked) 213 { 214 u32 delta, bic_target, max_cnt; 215 u64 offs, t; 216 217 ca->ack_cnt += acked; /* count the number of ACKed packets */ 218 219 if (ca->last_cwnd == cwnd && 220 (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32) 221 return; 222 223 /* The CUBIC function can update ca->cnt at most once per jiffy. 224 * On all cwnd reduction events, ca->epoch_start is set to 0, 225 * which will force a recalculation of ca->cnt. 226 */ 227 if (ca->epoch_start && tcp_jiffies32 == ca->last_time) 228 goto tcp_friendliness; 229 230 ca->last_cwnd = cwnd; 231 ca->last_time = tcp_jiffies32; 232 233 if (ca->epoch_start == 0) { 234 ca->epoch_start = tcp_jiffies32; /* record beginning */ 235 ca->ack_cnt = acked; /* start counting */ 236 ca->tcp_cwnd = cwnd; /* syn with cubic */ 237 238 if (ca->last_max_cwnd <= cwnd) { 239 ca->bic_K = 0; 240 ca->bic_origin_point = cwnd; 241 } else { 242 /* Compute new K based on 243 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 244 */ 245 ca->bic_K = cubic_root(cube_factor 246 * (ca->last_max_cwnd - cwnd)); 247 ca->bic_origin_point = ca->last_max_cwnd; 248 } 249 } 250 251 /* cubic function - calc*/ 252 /* calculate c * time^3 / rtt, 253 * while considering overflow in calculation of time^3 254 * (so time^3 is done by using 64 bit) 255 * and without the support of division of 64bit numbers 256 * (so all divisions are done by using 32 bit) 257 * also NOTE the unit of those veriables 258 * time = (t - K) / 2^bictcp_HZ 259 * c = bic_scale >> 10 260 * rtt = (srtt >> 3) / HZ 261 * !!! The following code does not have overflow problems, 262 * if the cwnd < 1 million packets !!! 263 */ 264 265 t = (s32)(tcp_jiffies32 - ca->epoch_start); 266 t += usecs_to_jiffies(ca->delay_min); 267 /* change the unit from HZ to bictcp_HZ */ 268 t <<= BICTCP_HZ; 269 do_div(t, HZ); 270 271 if (t < ca->bic_K) /* t - K */ 272 offs = ca->bic_K - t; 273 else 274 offs = t - ca->bic_K; 275 276 /* c/rtt * (t-K)^3 */ 277 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 278 if (t < ca->bic_K) /* below origin*/ 279 bic_target = ca->bic_origin_point - delta; 280 else /* above origin*/ 281 bic_target = ca->bic_origin_point + delta; 282 283 /* cubic function - calc bictcp_cnt*/ 284 if (bic_target > cwnd) { 285 ca->cnt = cwnd / (bic_target - cwnd); 286 } else { 287 ca->cnt = 100 * cwnd; /* very small increment*/ 288 } 289 290 /* 291 * The initial growth of cubic function may be too conservative 292 * when the available bandwidth is still unknown. 293 */ 294 if (ca->last_max_cwnd == 0 && ca->cnt > 20) 295 ca->cnt = 20; /* increase cwnd 5% per RTT */ 296 297 tcp_friendliness: 298 /* TCP Friendly */ 299 if (tcp_friendliness) { 300 u32 scale = beta_scale; 301 302 delta = (cwnd * scale) >> 3; 303 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 304 ca->ack_cnt -= delta; 305 ca->tcp_cwnd++; 306 } 307 308 if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */ 309 delta = ca->tcp_cwnd - cwnd; 310 max_cnt = cwnd / delta; 311 if (ca->cnt > max_cnt) 312 ca->cnt = max_cnt; 313 } 314 } 315 316 /* The maximum rate of cwnd increase CUBIC allows is 1 packet per 317 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT. 318 */ 319 ca->cnt = max(ca->cnt, 2U); 320 } 321 322 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked) 323 { 324 struct tcp_sock *tp = tcp_sk(sk); 325 struct bictcp *ca = inet_csk_ca(sk); 326 327 if (!tcp_is_cwnd_limited(sk)) 328 return; 329 330 if (tcp_in_slow_start(tp)) { 331 if (hystart && after(ack, ca->end_seq)) 332 bictcp_hystart_reset(sk); 333 acked = tcp_slow_start(tp, acked); 334 if (!acked) 335 return; 336 } 337 bictcp_update(ca, tp->snd_cwnd, acked); 338 tcp_cong_avoid_ai(tp, ca->cnt, acked); 339 } 340 341 static u32 bictcp_recalc_ssthresh(struct sock *sk) 342 { 343 const struct tcp_sock *tp = tcp_sk(sk); 344 struct bictcp *ca = inet_csk_ca(sk); 345 346 ca->epoch_start = 0; /* end of epoch */ 347 348 /* Wmax and fast convergence */ 349 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 350 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 351 / (2 * BICTCP_BETA_SCALE); 352 else 353 ca->last_max_cwnd = tp->snd_cwnd; 354 355 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 356 } 357 358 static void bictcp_state(struct sock *sk, u8 new_state) 359 { 360 if (new_state == TCP_CA_Loss) { 361 bictcp_reset(inet_csk_ca(sk)); 362 bictcp_hystart_reset(sk); 363 } 364 } 365 366 /* Account for TSO/GRO delays. 367 * Otherwise short RTT flows could get too small ssthresh, since during 368 * slow start we begin with small TSO packets and ca->delay_min would 369 * not account for long aggregation delay when TSO packets get bigger. 370 * Ideally even with a very small RTT we would like to have at least one 371 * TSO packet being sent and received by GRO, and another one in qdisc layer. 372 * We apply another 100% factor because @rate is doubled at this point. 373 * We cap the cushion to 1ms. 374 */ 375 static u32 hystart_ack_delay(struct sock *sk) 376 { 377 unsigned long rate; 378 379 rate = READ_ONCE(sk->sk_pacing_rate); 380 if (!rate) 381 return 0; 382 return min_t(u64, USEC_PER_MSEC, 383 div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate)); 384 } 385 386 static void hystart_update(struct sock *sk, u32 delay) 387 { 388 struct tcp_sock *tp = tcp_sk(sk); 389 struct bictcp *ca = inet_csk_ca(sk); 390 u32 threshold; 391 392 if (hystart_detect & HYSTART_ACK_TRAIN) { 393 u32 now = bictcp_clock_us(sk); 394 395 /* first detection parameter - ack-train detection */ 396 if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) { 397 ca->last_ack = now; 398 399 threshold = ca->delay_min + hystart_ack_delay(sk); 400 401 /* Hystart ack train triggers if we get ack past 402 * ca->delay_min/2. 403 * Pacing might have delayed packets up to RTT/2 404 * during slow start. 405 */ 406 if (sk->sk_pacing_status == SK_PACING_NONE) 407 threshold >>= 1; 408 409 if ((s32)(now - ca->round_start) > threshold) { 410 ca->found = 1; 411 pr_debug("hystart_ack_train (%u > %u) delay_min %u (+ ack_delay %u) cwnd %u\n", 412 now - ca->round_start, threshold, 413 ca->delay_min, hystart_ack_delay(sk), tp->snd_cwnd); 414 NET_INC_STATS(sock_net(sk), 415 LINUX_MIB_TCPHYSTARTTRAINDETECT); 416 NET_ADD_STATS(sock_net(sk), 417 LINUX_MIB_TCPHYSTARTTRAINCWND, 418 tp->snd_cwnd); 419 tp->snd_ssthresh = tp->snd_cwnd; 420 } 421 } 422 } 423 424 if (hystart_detect & HYSTART_DELAY) { 425 /* obtain the minimum delay of more than sampling packets */ 426 if (ca->curr_rtt > delay) 427 ca->curr_rtt = delay; 428 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) { 429 ca->sample_cnt++; 430 } else { 431 if (ca->curr_rtt > ca->delay_min + 432 HYSTART_DELAY_THRESH(ca->delay_min >> 3)) { 433 ca->found = 1; 434 NET_INC_STATS(sock_net(sk), 435 LINUX_MIB_TCPHYSTARTDELAYDETECT); 436 NET_ADD_STATS(sock_net(sk), 437 LINUX_MIB_TCPHYSTARTDELAYCWND, 438 tp->snd_cwnd); 439 tp->snd_ssthresh = tp->snd_cwnd; 440 } 441 } 442 } 443 } 444 445 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) 446 { 447 const struct tcp_sock *tp = tcp_sk(sk); 448 struct bictcp *ca = inet_csk_ca(sk); 449 u32 delay; 450 451 /* Some calls are for duplicates without timetamps */ 452 if (sample->rtt_us < 0) 453 return; 454 455 /* Discard delay samples right after fast recovery */ 456 if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ) 457 return; 458 459 delay = sample->rtt_us; 460 if (delay == 0) 461 delay = 1; 462 463 /* first time call or link delay decreases */ 464 if (ca->delay_min == 0 || ca->delay_min > delay) 465 ca->delay_min = delay; 466 467 /* hystart triggers when cwnd is larger than some threshold */ 468 if (!ca->found && tcp_in_slow_start(tp) && hystart && 469 tp->snd_cwnd >= hystart_low_window) 470 hystart_update(sk, delay); 471 } 472 473 static struct tcp_congestion_ops cubictcp __read_mostly = { 474 .init = bictcp_init, 475 .ssthresh = bictcp_recalc_ssthresh, 476 .cong_avoid = bictcp_cong_avoid, 477 .set_state = bictcp_state, 478 .undo_cwnd = tcp_reno_undo_cwnd, 479 .cwnd_event = bictcp_cwnd_event, 480 .pkts_acked = bictcp_acked, 481 .owner = THIS_MODULE, 482 .name = "cubic", 483 }; 484 485 static int __init cubictcp_register(void) 486 { 487 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 488 489 /* Precompute a bunch of the scaling factors that are used per-packet 490 * based on SRTT of 100ms 491 */ 492 493 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3 494 / (BICTCP_BETA_SCALE - beta); 495 496 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ 497 498 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 499 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 500 * the unit of K is bictcp_HZ=2^10, not HZ 501 * 502 * c = bic_scale >> 10 503 * rtt = 100ms 504 * 505 * the following code has been designed and tested for 506 * cwnd < 1 million packets 507 * RTT < 100 seconds 508 * HZ < 1,000,00 (corresponding to 10 nano-second) 509 */ 510 511 /* 1/c * 2^2*bictcp_HZ * srtt */ 512 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 513 514 /* divide by bic_scale and by constant Srtt (100ms) */ 515 do_div(cube_factor, bic_scale * 10); 516 517 return tcp_register_congestion_control(&cubictcp); 518 } 519 520 static void __exit cubictcp_unregister(void) 521 { 522 tcp_unregister_congestion_control(&cubictcp); 523 } 524 525 module_init(cubictcp_register); 526 module_exit(cubictcp_unregister); 527 528 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 529 MODULE_LICENSE("GPL"); 530 MODULE_DESCRIPTION("CUBIC TCP"); 531 MODULE_VERSION("2.3"); 532