linux/net/ipv4/tcp_cdg.c
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   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
  38static int window __read_mostly = 8;
  39static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
  40static unsigned int backoff_factor __read_mostly = 42;
  41static unsigned int hystart_detect __read_mostly = 3;
  42static unsigned int use_ineff __read_mostly = 5;
  43static bool use_shadow __read_mostly = true;
  44static bool use_tolerance __read_mostly;
  45
  46module_param(window, int, 0444);
  47MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
  48module_param(backoff_beta, uint, 0644);
  49MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
  50module_param(backoff_factor, uint, 0644);
  51MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
  52module_param(hystart_detect, uint, 0644);
  53MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
  54                 "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
  55module_param(use_ineff, uint, 0644);
  56MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
  57module_param(use_shadow, bool, 0644);
  58MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
  59module_param(use_tolerance, bool, 0644);
  60MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
  61
  62struct cdg_minmax {
  63        union {
  64                struct {
  65                        s32 min;
  66                        s32 max;
  67                };
  68                u64 v64;
  69        };
  70};
  71
  72enum cdg_state {
  73        CDG_UNKNOWN = 0,
  74        CDG_NONFULL = 1,
  75        CDG_FULL    = 2,
  76        CDG_BACKOFF = 3,
  77};
  78
  79struct 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 */
 103static 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 */
 140static 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
 190static 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
 241static 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. */
 262static 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
 299static 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
 328static 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
 345static 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
 373static 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
 386static void tcp_cdg_release(struct sock *sk)
 387{
 388        struct cdg *ca = inet_csk_ca(sk);
 389
 390        kfree(ca->gradients);
 391}
 392
 393static 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
 405static 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
 417static void __exit tcp_cdg_unregister(void)
 418{
 419        tcp_unregister_congestion_control(&tcp_cdg);
 420}
 421
 422module_init(tcp_cdg_register);
 423module_exit(tcp_cdg_unregister);
 424MODULE_AUTHOR("Kenneth Klette Jonassen");
 425MODULE_LICENSE("GPL");
 426MODULE_DESCRIPTION("TCP CDG");
 427
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