Kenneth Klette Jonassen | 2b0a8c9 | 2015-06-10 19:08:17 +0200 | [diff] [blame] | 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) { |
Kenneth Klette Jonassen | 758f0d4 | 2015-06-12 17:24:03 +0200 | [diff] [blame] | 148 | u32 now_us = div_u64(local_clock(), NSEC_PER_USEC); |
Kenneth Klette Jonassen | 2b0a8c9 | 2015-06-10 19:08:17 +0200 | [diff] [blame] | 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"); |