net/ipv4/tcp_cdg.c

   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 = div_u64(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 (tcp_in_slow_start(tp) && 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");