net/ipv4/tcp_cdg.c

   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(now_us, 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                                               tp->snd_cwnd);
 165                                 tp->snd_ssthresh = tp->snd_cwnd;
 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                                               tp->snd_cwnd);
 184                                 tp->snd_ssthresh = tp->snd_cwnd;
 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 (prandom_u32() <= nexp_u32(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, tp->snd_cwnd);
 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, tp->snd_cwnd);
 289                 return;
 290         }
 291
 292         prior_snd_cwnd = tp->snd_cwnd;
 293         tcp_reno_cong_avoid(sk, ack, acked);
 294
 295         incr = tp->snd_cwnd - 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, (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 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 = tp->snd_cwnd;
 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         /* We silently fall back to window = 1 if allocation fails. */
 379         if (window > 1)
 380                 ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
 381                                         GFP_NOWAIT | __GFP_NOWARN);
 382         ca->rtt_seq = tp->snd_nxt;
 383         ca->shadow_wnd = tp->snd_cwnd;
 384 }
 385
 386 static void tcp_cdg_release(struct sock *sk)
 387 {
 388         struct cdg *ca = inet_csk_ca(sk);
 389
 390         kfree(ca->gradients);
 391 }
 392
 393 static struct tcp_congestion_ops tcp_cdg __read_mostly = {
 394         .cong_avoid = tcp_cdg_cong_avoid,
 395         .cwnd_event = tcp_cdg_cwnd_event,
 396         .pkts_acked = tcp_cdg_acked,
 397         .undo_cwnd = tcp_reno_undo_cwnd,
 398         .ssthresh = tcp_cdg_ssthresh,
 399         .release = tcp_cdg_release,
 400         .init = tcp_cdg_init,
 401         .owner = THIS_MODULE,
 402         .name = "cdg",
 403 };
 404
 405 static int __init tcp_cdg_register(void)
 406 {
 407         if (backoff_beta > 1024 || window < 1 || window > 256)
 408                 return -ERANGE;
 409         if (!is_power_of_2(window))
 410                 return -EINVAL;
 411
 412         BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
 413         tcp_register_congestion_control(&tcp_cdg);
 414         return 0;
 415 }
 416
 417 static void __exit tcp_cdg_unregister(void)
 418 {
 419         tcp_unregister_congestion_control(&tcp_cdg);
 420 }
 421
 422 module_init(tcp_cdg_register);
 423 module_exit(tcp_cdg_unregister);
 424 MODULE_AUTHOR("Kenneth Klette Jonassen");
 425 MODULE_LICENSE("GPL");
 426 MODULE_DESCRIPTION("TCP CDG");