linux/net/ipv4/tcp_cdg.c
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   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 <linux/sched/clock.h>
  31
  32#include <net/tcp.h>
  33
  34#define HYSTART_ACK_TRAIN       1
  35#define HYSTART_DELAY           2
  36
  37static int window __read_mostly = 8;
  38static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
  39static unsigned int backoff_factor __read_mostly = 42;
  40static unsigned int hystart_detect __read_mostly = 3;
  41static unsigned int use_ineff __read_mostly = 5;
  42static bool use_shadow __read_mostly = true;
  43static bool use_tolerance __read_mostly;
  44
  45module_param(window, int, 0444);
  46MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
  47module_param(backoff_beta, uint, 0644);
  48MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
  49module_param(backoff_factor, uint, 0644);
  50MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
  51module_param(hystart_detect, uint, 0644);
  52MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
  53                 "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
  54module_param(use_ineff, uint, 0644);
  55MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
  56module_param(use_shadow, bool, 0644);
  57MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
  58module_param(use_tolerance, bool, 0644);
  59MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
  60
  61struct cdg_minmax {
  62        union {
  63                struct {
  64                        s32 min;
  65                        s32 max;
  66                };
  67                u64 v64;
  68        };
  69};
  70
  71enum cdg_state {
  72        CDG_UNKNOWN = 0,
  73        CDG_NONFULL = 1,
  74        CDG_FULL    = 2,
  75        CDG_BACKOFF = 3,
  76};
  77
  78struct cdg {
  79        struct cdg_minmax rtt;
  80        struct cdg_minmax rtt_prev;
  81        struct cdg_minmax *gradients;
  82        struct cdg_minmax gsum;
  83        bool gfilled;
  84        u8  tail;
  85        u8  state;
  86        u8  delack;
  87        u32 rtt_seq;
  88        u32 shadow_wnd;
  89        u16 backoff_cnt;
  90        u16 sample_cnt;
  91        s32 delay_min;
  92        u32 last_ack;
  93        u32 round_start;
  94};
  95
  96/**
  97 * nexp_u32 - negative base-e exponential
  98 * @ux: x in units of micro
  99 *
 100 * Returns exp(ux * -1e-6) * U32_MAX.
 101 */
 102static u32 __pure nexp_u32(u32 ux)
 103{
 104        static const u16 v[] = {
 105                /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
 106                65535,
 107                65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
 108                61378, 57484, 50423, 38795, 22965, 8047,  987,   14,
 109        };
 110        u32 msb = ux >> 8;
 111        u32 res;
 112        int i;
 113
 114        /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
 115        if (msb > U16_MAX)
 116                return 0;
 117
 118        /* Scale first eight bits linearly: */
 119        res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);
 120
 121        /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
 122        for (i = 1; msb; i++, msb >>= 1) {
 123                u32 y = v[i & -(msb & 1)] + U32_C(1);
 124
 125                res = ((u64)res * y) >> 16;
 126        }
 127
 128        return res;
 129}
 130
 131/* Based on the HyStart algorithm (by Ha et al.) that is implemented in
 132 * tcp_cubic. Differences/experimental changes:
 133 *   o Using Hayes' delayed ACK filter.
 134 *   o Using a usec clock for the ACK train.
 135 *   o Reset ACK train when application limited.
 136 *   o Invoked at any cwnd (i.e. also when cwnd < 16).
 137 *   o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
 138 */
 139static void tcp_cdg_hystart_update(struct sock *sk)
 140{
 141        struct cdg *ca = inet_csk_ca(sk);
 142        struct tcp_sock *tp = tcp_sk(sk);
 143
 144        ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
 145        if (ca->delay_min == 0)
 146                return;
 147
 148        if (hystart_detect & HYSTART_ACK_TRAIN) {
 149                u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);
 150
 151                if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) {
 152                        ca->last_ack = now_us;
 153                        ca->round_start = now_us;
 154                } else if (before(now_us, ca->last_ack + 3000)) {
 155                        u32 base_owd = max(ca->delay_min / 2U, 125U);
 156
 157                        ca->last_ack = now_us;
 158                        if (after(now_us, ca->round_start + base_owd)) {
 159                                NET_INC_STATS(sock_net(sk),
 160                                              LINUX_MIB_TCPHYSTARTTRAINDETECT);
 161                                NET_ADD_STATS(sock_net(sk),
 162                                              LINUX_MIB_TCPHYSTARTTRAINCWND,
 163                                              tp->snd_cwnd);
 164                                tp->snd_ssthresh = tp->snd_cwnd;
 165                                return;
 166                        }
 167                }
 168        }
 169
 170        if (hystart_detect & HYSTART_DELAY) {
 171                if (ca->sample_cnt < 8) {
 172                        ca->sample_cnt++;
 173                } else {
 174                        s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
 175                                         125U);
 176
 177                        if (ca->rtt.min > thresh) {
 178                                NET_INC_STATS(sock_net(sk),
 179                                              LINUX_MIB_TCPHYSTARTDELAYDETECT);
 180                                NET_ADD_STATS(sock_net(sk),
 181                                              LINUX_MIB_TCPHYSTARTDELAYCWND,
 182                                              tp->snd_cwnd);
 183                                tp->snd_ssthresh = tp->snd_cwnd;
 184                        }
 185                }
 186        }
 187}
 188
 189static s32 tcp_cdg_grad(struct cdg *ca)
 190{
 191        s32 gmin = ca->rtt.min - ca->rtt_prev.min;
 192        s32 gmax = ca->rtt.max - ca->rtt_prev.max;
 193        s32 grad;
 194
 195        if (ca->gradients) {
 196                ca->gsum.min += gmin - ca->gradients[ca->tail].min;
 197                ca->gsum.max += gmax - ca->gradients[ca->tail].max;
 198                ca->gradients[ca->tail].min = gmin;
 199                ca->gradients[ca->tail].max = gmax;
 200                ca->tail = (ca->tail + 1) & (window - 1);
 201                gmin = ca->gsum.min;
 202                gmax = ca->gsum.max;
 203        }
 204
 205        /* We keep sums to ignore gradients during cwnd reductions;
 206         * the paper's smoothed gradients otherwise simplify to:
 207         * (rtt_latest - rtt_oldest) / window.
 208         *
 209         * We also drop division by window here.
 210         */
 211        grad = gmin > 0 ? gmin : gmax;
 212
 213        /* Extrapolate missing values in gradient window: */
 214        if (!ca->gfilled) {
 215                if (!ca->gradients && window > 1)
 216                        grad *= window; /* Memory allocation failed. */
 217                else if (ca->tail == 0)
 218                        ca->gfilled = true;
 219                else
 220                        grad = (grad * window) / (int)ca->tail;
 221        }
 222
 223        /* Backoff was effectual: */
 224        if (gmin <= -32 || gmax <= -32)
 225                ca->backoff_cnt = 0;
 226
 227        if (use_tolerance) {
 228                /* Reduce small variations to zero: */
 229                gmin = DIV_ROUND_CLOSEST(gmin, 64);
 230                gmax = DIV_ROUND_CLOSEST(gmax, 64);
 231
 232                if (gmin > 0 && gmax <= 0)
 233                        ca->state = CDG_FULL;
 234                else if ((gmin > 0 && gmax > 0) || gmax < 0)
 235                        ca->state = CDG_NONFULL;
 236        }
 237        return grad;
 238}
 239
 240static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
 241{
 242        struct cdg *ca = inet_csk_ca(sk);
 243        struct tcp_sock *tp = tcp_sk(sk);
 244
 245        if (prandom_u32() <= nexp_u32(grad * backoff_factor))
 246                return false;
 247
 248        if (use_ineff) {
 249                ca->backoff_cnt++;
 250                if (ca->backoff_cnt > use_ineff)
 251                        return false;
 252        }
 253
 254        ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
 255        ca->state = CDG_BACKOFF;
 256        tcp_enter_cwr(sk);
 257        return true;
 258}
 259
 260/* Not called in CWR or Recovery state. */
 261static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 262{
 263        struct cdg *ca = inet_csk_ca(sk);
 264        struct tcp_sock *tp = tcp_sk(sk);
 265        u32 prior_snd_cwnd;
 266        u32 incr;
 267
 268        if (tcp_in_slow_start(tp) && hystart_detect)
 269                tcp_cdg_hystart_update(sk);
 270
 271        if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
 272                s32 grad = 0;
 273
 274                if (ca->rtt_prev.v64)
 275                        grad = tcp_cdg_grad(ca);
 276                ca->rtt_seq = tp->snd_nxt;
 277                ca->rtt_prev = ca->rtt;
 278                ca->rtt.v64 = 0;
 279                ca->last_ack = 0;
 280                ca->sample_cnt = 0;
 281
 282                if (grad > 0 && tcp_cdg_backoff(sk, grad))
 283                        return;
 284        }
 285
 286        if (!tcp_is_cwnd_limited(sk)) {
 287                ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
 288                return;
 289        }
 290
 291        prior_snd_cwnd = tp->snd_cwnd;
 292        tcp_reno_cong_avoid(sk, ack, acked);
 293
 294        incr = tp->snd_cwnd - prior_snd_cwnd;
 295        ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
 296}
 297
 298static void tcp_cdg_acked(struct sock *sk, const struct ack_sample *sample)
 299{
 300        struct cdg *ca = inet_csk_ca(sk);
 301        struct tcp_sock *tp = tcp_sk(sk);
 302
 303        if (sample->rtt_us <= 0)
 304                return;
 305
 306        /* A heuristic for filtering delayed ACKs, adapted from:
 307         * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
 308         * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
 309         */
 310        if (tp->sacked_out == 0) {
 311                if (sample->pkts_acked == 1 && ca->delack) {
 312                        /* A delayed ACK is only used for the minimum if it is
 313                         * provenly lower than an existing non-zero minimum.
 314                         */
 315                        ca->rtt.min = min(ca->rtt.min, sample->rtt_us);
 316                        ca->delack--;
 317                        return;
 318                } else if (sample->pkts_acked > 1 && ca->delack < 5) {
 319                        ca->delack++;
 320                }
 321        }
 322
 323        ca->rtt.min = min_not_zero(ca->rtt.min, sample->rtt_us);
 324        ca->rtt.max = max(ca->rtt.max, sample->rtt_us);
 325}
 326
 327static u32 tcp_cdg_ssthresh(struct sock *sk)
 328{
 329        struct cdg *ca = inet_csk_ca(sk);
 330        struct tcp_sock *tp = tcp_sk(sk);
 331
 332        if (ca->state == CDG_BACKOFF)
 333                return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
 334
 335        if (ca->state == CDG_NONFULL && use_tolerance)
 336                return tp->snd_cwnd;
 337
 338        ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
 339        if (use_shadow)
 340                return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
 341        return max(2U, tp->snd_cwnd >> 1);
 342}
 343
 344static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
 345{
 346        struct cdg *ca = inet_csk_ca(sk);
 347        struct tcp_sock *tp = tcp_sk(sk);
 348        struct cdg_minmax *gradients;
 349
 350        switch (ev) {
 351        case CA_EVENT_CWND_RESTART:
 352                gradients = ca->gradients;
 353                if (gradients)
 354                        memset(gradients, 0, window * sizeof(gradients[0]));
 355                memset(ca, 0, sizeof(*ca));
 356
 357                ca->gradients = gradients;
 358                ca->rtt_seq = tp->snd_nxt;
 359                ca->shadow_wnd = tp->snd_cwnd;
 360                break;
 361        case CA_EVENT_COMPLETE_CWR:
 362                ca->state = CDG_UNKNOWN;
 363                ca->rtt_seq = tp->snd_nxt;
 364                ca->rtt_prev = ca->rtt;
 365                ca->rtt.v64 = 0;
 366                break;
 367        default:
 368                break;
 369        }
 370}
 371
 372static void tcp_cdg_init(struct sock *sk)
 373{
 374        struct cdg *ca = inet_csk_ca(sk);
 375        struct tcp_sock *tp = tcp_sk(sk);
 376
 377        /* We silently fall back to window = 1 if allocation fails. */
 378        if (window > 1)
 379                ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
 380                                        GFP_NOWAIT | __GFP_NOWARN);
 381        ca->rtt_seq = tp->snd_nxt;
 382        ca->shadow_wnd = tp->snd_cwnd;
 383}
 384
 385static void tcp_cdg_release(struct sock *sk)
 386{
 387        struct cdg *ca = inet_csk_ca(sk);
 388
 389        kfree(ca->gradients);
 390}
 391
 392static struct tcp_congestion_ops tcp_cdg __read_mostly = {
 393        .cong_avoid = tcp_cdg_cong_avoid,
 394        .cwnd_event = tcp_cdg_cwnd_event,
 395        .pkts_acked = tcp_cdg_acked,
 396        .undo_cwnd = tcp_reno_undo_cwnd,
 397        .ssthresh = tcp_cdg_ssthresh,
 398        .release = tcp_cdg_release,
 399        .init = tcp_cdg_init,
 400        .owner = THIS_MODULE,
 401        .name = "cdg",
 402};
 403
 404static int __init tcp_cdg_register(void)
 405{
 406        if (backoff_beta > 1024 || window < 1 || window > 256)
 407                return -ERANGE;
 408        if (!is_power_of_2(window))
 409                return -EINVAL;
 410
 411        BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
 412        tcp_register_congestion_control(&tcp_cdg);
 413        return 0;
 414}
 415
 416static void __exit tcp_cdg_unregister(void)
 417{
 418        tcp_unregister_congestion_control(&tcp_cdg);
 419}
 420
 421module_init(tcp_cdg_register);
 422module_exit(tcp_cdg_unregister);
 423MODULE_AUTHOR("Kenneth Klette Jonassen");
 424MODULE_LICENSE("GPL");
 425MODULE_DESCRIPTION("TCP CDG");
 426