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 ca->cnt = 0; 108 ca->last_max_cwnd = 0; 109 ca->last_cwnd = 0; 110 ca->last_time = 0; 111 ca->bic_origin_point = 0; 112 ca->bic_K = 0; 113 ca->delay_min = 0; 114 ca->epoch_start = 0; 115 ca->ack_cnt = 0; 116 ca->tcp_cwnd = 0; 117 ca->found = 0; 118 } 119 120 static inline u32 bictcp_clock_us(const struct sock *sk) 121 { 122 return tcp_sk(sk)->tcp_mstamp; 123 } 124 125 static inline void bictcp_hystart_reset(struct sock *sk) 126 { 127 struct tcp_sock *tp = tcp_sk(sk); 128 struct bictcp *ca = inet_csk_ca(sk); 129 130 ca->round_start = ca->last_ack = bictcp_clock_us(sk); 131 ca->end_seq = tp->snd_nxt; 132 ca->curr_rtt = ~0U; 133 ca->sample_cnt = 0; 134 } 135 136 static void bictcp_init(struct sock *sk) 137 { 138 struct bictcp *ca = inet_csk_ca(sk); 139 140 bictcp_reset(ca); 141 142 if (hystart) 143 bictcp_hystart_reset(sk); 144 145 if (!hystart && initial_ssthresh) 146 tcp_sk(sk)->snd_ssthresh = initial_ssthresh; 147 } 148 149 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event) 150 { 151 if (event == CA_EVENT_TX_START) { 152 struct bictcp *ca = inet_csk_ca(sk); 153 u32 now = tcp_jiffies32; 154 s32 delta; 155 156 delta = now - tcp_sk(sk)->lsndtime; 157 158 /* We were application limited (idle) for a while. 159 * Shift epoch_start to keep cwnd growth to cubic curve. 160 */ 161 if (ca->epoch_start && delta > 0) { 162 ca->epoch_start += delta; 163 if (after(ca->epoch_start, now)) 164 ca->epoch_start = now; 165 } 166 return; 167 } 168 } 169 170 /* calculate the cubic root of x using a table lookup followed by one 171 * Newton-Raphson iteration. 172 * Avg err ~= 0.195% 173 */ 174 static u32 cubic_root(u64 a) 175 { 176 u32 x, b, shift; 177 /* 178 * cbrt(x) MSB values for x MSB values in [0..63]. 179 * Precomputed then refined by hand - Willy Tarreau 180 * 181 * For x in [0..63], 182 * v = cbrt(x << 18) - 1 183 * cbrt(x) = (v[x] + 10) >> 6 184 */ 185 static const u8 v[] = { 186 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, 187 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, 188 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, 189 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, 190 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, 191 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, 192 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, 193 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, 194 }; 195 196 b = fls64(a); 197 if (b < 7) { 198 /* a in [0..63] */ 199 return ((u32)v[(u32)a] + 35) >> 6; 200 } 201 202 b = ((b * 84) >> 8) - 1; 203 shift = (a >> (b * 3)); 204 205 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; 206 207 /* 208 * Newton-Raphson iteration 209 * 2 210 * x = ( 2 * x + a / x ) / 3 211 * k+1 k k 212 */ 213 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1))); 214 x = ((x * 341) >> 10); 215 return x; 216 } 217 218 /* 219 * Compute congestion window to use. 220 */ 221 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked) 222 { 223 u32 delta, bic_target, max_cnt; 224 u64 offs, t; 225 226 ca->ack_cnt += acked; /* count the number of ACKed packets */ 227 228 if (ca->last_cwnd == cwnd && 229 (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32) 230 return; 231 232 /* The CUBIC function can update ca->cnt at most once per jiffy. 233 * On all cwnd reduction events, ca->epoch_start is set to 0, 234 * which will force a recalculation of ca->cnt. 235 */ 236 if (ca->epoch_start && tcp_jiffies32 == ca->last_time) 237 goto tcp_friendliness; 238 239 ca->last_cwnd = cwnd; 240 ca->last_time = tcp_jiffies32; 241 242 if (ca->epoch_start == 0) { 243 ca->epoch_start = tcp_jiffies32; /* record beginning */ 244 ca->ack_cnt = acked; /* start counting */ 245 ca->tcp_cwnd = cwnd; /* syn with cubic */ 246 247 if (ca->last_max_cwnd <= cwnd) { 248 ca->bic_K = 0; 249 ca->bic_origin_point = cwnd; 250 } else { 251 /* Compute new K based on 252 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 253 */ 254 ca->bic_K = cubic_root(cube_factor 255 * (ca->last_max_cwnd - cwnd)); 256 ca->bic_origin_point = ca->last_max_cwnd; 257 } 258 } 259 260 /* cubic function - calc*/ 261 /* calculate c * time^3 / rtt, 262 * while considering overflow in calculation of time^3 263 * (so time^3 is done by using 64 bit) 264 * and without the support of division of 64bit numbers 265 * (so all divisions are done by using 32 bit) 266 * also NOTE the unit of those veriables 267 * time = (t - K) / 2^bictcp_HZ 268 * c = bic_scale >> 10 269 * rtt = (srtt >> 3) / HZ 270 * !!! The following code does not have overflow problems, 271 * if the cwnd < 1 million packets !!! 272 */ 273 274 t = (s32)(tcp_jiffies32 - ca->epoch_start); 275 t += usecs_to_jiffies(ca->delay_min); 276 /* change the unit from HZ to bictcp_HZ */ 277 t <<= BICTCP_HZ; 278 do_div(t, HZ); 279 280 if (t < ca->bic_K) /* t - K */ 281 offs = ca->bic_K - t; 282 else 283 offs = t - ca->bic_K; 284 285 /* c/rtt * (t-K)^3 */ 286 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 287 if (t < ca->bic_K) /* below origin*/ 288 bic_target = ca->bic_origin_point - delta; 289 else /* above origin*/ 290 bic_target = ca->bic_origin_point + delta; 291 292 /* cubic function - calc bictcp_cnt*/ 293 if (bic_target > cwnd) { 294 ca->cnt = cwnd / (bic_target - cwnd); 295 } else { 296 ca->cnt = 100 * cwnd; /* very small increment*/ 297 } 298 299 /* 300 * The initial growth of cubic function may be too conservative 301 * when the available bandwidth is still unknown. 302 */ 303 if (ca->last_max_cwnd == 0 && ca->cnt > 20) 304 ca->cnt = 20; /* increase cwnd 5% per RTT */ 305 306 tcp_friendliness: 307 /* TCP Friendly */ 308 if (tcp_friendliness) { 309 u32 scale = beta_scale; 310 311 delta = (cwnd * scale) >> 3; 312 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 313 ca->ack_cnt -= delta; 314 ca->tcp_cwnd++; 315 } 316 317 if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */ 318 delta = ca->tcp_cwnd - cwnd; 319 max_cnt = cwnd / delta; 320 if (ca->cnt > max_cnt) 321 ca->cnt = max_cnt; 322 } 323 } 324 325 /* The maximum rate of cwnd increase CUBIC allows is 1 packet per 326 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT. 327 */ 328 ca->cnt = max(ca->cnt, 2U); 329 } 330 331 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked) 332 { 333 struct tcp_sock *tp = tcp_sk(sk); 334 struct bictcp *ca = inet_csk_ca(sk); 335 336 if (!tcp_is_cwnd_limited(sk)) 337 return; 338 339 if (tcp_in_slow_start(tp)) { 340 if (hystart && after(ack, ca->end_seq)) 341 bictcp_hystart_reset(sk); 342 acked = tcp_slow_start(tp, acked); 343 if (!acked) 344 return; 345 } 346 bictcp_update(ca, tp->snd_cwnd, acked); 347 tcp_cong_avoid_ai(tp, ca->cnt, acked); 348 } 349 350 static u32 bictcp_recalc_ssthresh(struct sock *sk) 351 { 352 const struct tcp_sock *tp = tcp_sk(sk); 353 struct bictcp *ca = inet_csk_ca(sk); 354 355 ca->epoch_start = 0; /* end of epoch */ 356 357 /* Wmax and fast convergence */ 358 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 359 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 360 / (2 * BICTCP_BETA_SCALE); 361 else 362 ca->last_max_cwnd = tp->snd_cwnd; 363 364 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 365 } 366 367 static void bictcp_state(struct sock *sk, u8 new_state) 368 { 369 if (new_state == TCP_CA_Loss) { 370 bictcp_reset(inet_csk_ca(sk)); 371 bictcp_hystart_reset(sk); 372 } 373 } 374 375 /* Account for TSO/GRO delays. 376 * Otherwise short RTT flows could get too small ssthresh, since during 377 * slow start we begin with small TSO packets and ca->delay_min would 378 * not account for long aggregation delay when TSO packets get bigger. 379 * Ideally even with a very small RTT we would like to have at least one 380 * TSO packet being sent and received by GRO, and another one in qdisc layer. 381 * We apply another 100% factor because @rate is doubled at this point. 382 * We cap the cushion to 1ms. 383 */ 384 static u32 hystart_ack_delay(struct sock *sk) 385 { 386 unsigned long rate; 387 388 rate = READ_ONCE(sk->sk_pacing_rate); 389 if (!rate) 390 return 0; 391 return min_t(u64, USEC_PER_MSEC, 392 div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate)); 393 } 394 395 static void hystart_update(struct sock *sk, u32 delay) 396 { 397 struct tcp_sock *tp = tcp_sk(sk); 398 struct bictcp *ca = inet_csk_ca(sk); 399 u32 threshold; 400 401 if (hystart_detect & HYSTART_ACK_TRAIN) { 402 u32 now = bictcp_clock_us(sk); 403 404 /* first detection parameter - ack-train detection */ 405 if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) { 406 ca->last_ack = now; 407 408 threshold = ca->delay_min + hystart_ack_delay(sk); 409 410 /* Hystart ack train triggers if we get ack past 411 * ca->delay_min/2. 412 * Pacing might have delayed packets up to RTT/2 413 * during slow start. 414 */ 415 if (sk->sk_pacing_status == SK_PACING_NONE) 416 threshold >>= 1; 417 418 if ((s32)(now - ca->round_start) > threshold) { 419 ca->found = 1; 420 pr_debug("hystart_ack_train (%u > %u) delay_min %u (+ ack_delay %u) cwnd %u\n", 421 now - ca->round_start, threshold, 422 ca->delay_min, hystart_ack_delay(sk), tp->snd_cwnd); 423 NET_INC_STATS(sock_net(sk), 424 LINUX_MIB_TCPHYSTARTTRAINDETECT); 425 NET_ADD_STATS(sock_net(sk), 426 LINUX_MIB_TCPHYSTARTTRAINCWND, 427 tp->snd_cwnd); 428 tp->snd_ssthresh = tp->snd_cwnd; 429 } 430 } 431 } 432 433 if (hystart_detect & HYSTART_DELAY) { 434 /* obtain the minimum delay of more than sampling packets */ 435 if (ca->curr_rtt > delay) 436 ca->curr_rtt = delay; 437 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) { 438 ca->sample_cnt++; 439 } else { 440 if (ca->curr_rtt > ca->delay_min + 441 HYSTART_DELAY_THRESH(ca->delay_min >> 3)) { 442 ca->found = 1; 443 NET_INC_STATS(sock_net(sk), 444 LINUX_MIB_TCPHYSTARTDELAYDETECT); 445 NET_ADD_STATS(sock_net(sk), 446 LINUX_MIB_TCPHYSTARTDELAYCWND, 447 tp->snd_cwnd); 448 tp->snd_ssthresh = tp->snd_cwnd; 449 } 450 } 451 } 452 } 453 454 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) 455 { 456 const struct tcp_sock *tp = tcp_sk(sk); 457 struct bictcp *ca = inet_csk_ca(sk); 458 u32 delay; 459 460 /* Some calls are for duplicates without timetamps */ 461 if (sample->rtt_us < 0) 462 return; 463 464 /* Discard delay samples right after fast recovery */ 465 if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ) 466 return; 467 468 delay = sample->rtt_us; 469 if (delay == 0) 470 delay = 1; 471 472 /* first time call or link delay decreases */ 473 if (ca->delay_min == 0 || ca->delay_min > delay) 474 ca->delay_min = delay; 475 476 /* hystart triggers when cwnd is larger than some threshold */ 477 if (!ca->found && tcp_in_slow_start(tp) && hystart && 478 tp->snd_cwnd >= hystart_low_window) 479 hystart_update(sk, delay); 480 } 481 482 static struct tcp_congestion_ops cubictcp __read_mostly = { 483 .init = bictcp_init, 484 .ssthresh = bictcp_recalc_ssthresh, 485 .cong_avoid = bictcp_cong_avoid, 486 .set_state = bictcp_state, 487 .undo_cwnd = tcp_reno_undo_cwnd, 488 .cwnd_event = bictcp_cwnd_event, 489 .pkts_acked = bictcp_acked, 490 .owner = THIS_MODULE, 491 .name = "cubic", 492 }; 493 494 static int __init cubictcp_register(void) 495 { 496 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 497 498 /* Precompute a bunch of the scaling factors that are used per-packet 499 * based on SRTT of 100ms 500 */ 501 502 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3 503 / (BICTCP_BETA_SCALE - beta); 504 505 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ 506 507 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 508 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 509 * the unit of K is bictcp_HZ=2^10, not HZ 510 * 511 * c = bic_scale >> 10 512 * rtt = 100ms 513 * 514 * the following code has been designed and tested for 515 * cwnd < 1 million packets 516 * RTT < 100 seconds 517 * HZ < 1,000,00 (corresponding to 10 nano-second) 518 */ 519 520 /* 1/c * 2^2*bictcp_HZ * srtt */ 521 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 522 523 /* divide by bic_scale and by constant Srtt (100ms) */ 524 do_div(cube_factor, bic_scale * 10); 525 526 return tcp_register_congestion_control(&cubictcp); 527 } 528 529 static void __exit cubictcp_unregister(void) 530 { 531 tcp_unregister_congestion_control(&cubictcp); 532 } 533 534 module_init(cubictcp_register); 535 module_exit(cubictcp_unregister); 536 537 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 538 MODULE_LICENSE("GPL"); 539 MODULE_DESCRIPTION("CUBIC TCP"); 540 MODULE_VERSION("2.3"); 541