net/ipv4/tcp_nv.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * TCP NV: TCP with Congestion Avoidance
   4  *
   5  * TCP-NV is a successor of TCP-Vegas that has been developed to
   6  * deal with the issues that occur in modern networks.
   7  * Like TCP-Vegas, TCP-NV supports true congestion avoidance,
   8  * the ability to detect congestion before packet losses occur.
   9  * When congestion (queue buildup) starts to occur, TCP-NV
  10  * predicts what the cwnd size should be for the current
  11  * throughput and it reduces the cwnd proportionally to
  12  * the difference between the current cwnd and the predicted cwnd.
  13  *
  14  * NV is only recommeneded for traffic within a data center, and when
  15  * all the flows are NV (at least those within the data center). This
  16  * is due to the inherent unfairness between flows using losses to
  17  * detect congestion (congestion control) and those that use queue
  18  * buildup to detect congestion (congestion avoidance).
  19  *
  20  * Note: High NIC coalescence values may lower the performance of NV
  21  * due to the increased noise in RTT values. In particular, we have
  22  * seen issues with rx-frames values greater than 8.
  23  *
  24  * TODO:
  25  * 1) Add mechanism to deal with reverse congestion.
  26  */
  27
  28 #include <linux/mm.h>
  29 #include <linux/module.h>
  30 #include <linux/math64.h>
  31 #include <net/tcp.h>
  32 #include <linux/inet_diag.h>
  33
  34 /* TCP NV parameters
  35  *
  36  * nv_pad               Max number of queued packets allowed in network
  37  * nv_pad_buffer        Do not grow cwnd if this closed to nv_pad
  38  * nv_reset_period      How often (in) seconds)to reset min_rtt
  39  * nv_min_cwnd          Don't decrease cwnd below this if there are no losses
  40  * nv_cong_dec_mult     Decrease cwnd by X% (30%) of congestion when detected
  41  * nv_ssthresh_factor   On congestion set ssthresh to this * <desired cwnd> / 8
  42  * nv_rtt_factor        RTT averaging factor
  43  * nv_loss_dec_factor   Decrease cwnd to this (80%) when losses occur
  44  * nv_dec_eval_min_calls        Wait this many RTT measurements before dec cwnd
  45  * nv_inc_eval_min_calls        Wait this many RTT measurements before inc cwnd
  46  * nv_ssthresh_eval_min_calls   Wait this many RTT measurements before stopping
  47  *                              slow-start due to congestion
  48  * nv_stop_rtt_cnt      Only grow cwnd for this many RTTs after non-congestion
  49  * nv_rtt_min_cnt       Wait these many RTTs before making congesion decision
  50  * nv_cwnd_growth_rate_neg
  51  * nv_cwnd_growth_rate_pos
  52  *      How quickly to double growth rate (not rate) of cwnd when not
  53  *      congested. One value (nv_cwnd_growth_rate_neg) for when
  54  *      rate < 1 pkt/RTT (after losses). The other (nv_cwnd_growth_rate_pos)
  55  *      otherwise.
  56  */
  57
  58 static int nv_pad __read_mostly = 10;
  59 static int nv_pad_buffer __read_mostly = 2;
  60 static int nv_reset_period __read_mostly = 5; /* in seconds */
  61 static int nv_min_cwnd __read_mostly = 2;
  62 static int nv_cong_dec_mult __read_mostly = 30 * 128 / 100; /* = 30% */
  63 static int nv_ssthresh_factor __read_mostly = 8; /* = 1 */
  64 static int nv_rtt_factor __read_mostly = 128; /* = 1/2*old + 1/2*new */
  65 static int nv_loss_dec_factor __read_mostly = 819; /* => 80% */
  66 static int nv_cwnd_growth_rate_neg __read_mostly = 8;
  67 static int nv_cwnd_growth_rate_pos __read_mostly; /* 0 => fixed like Reno */
  68 static int nv_dec_eval_min_calls __read_mostly = 60;
  69 static int nv_inc_eval_min_calls __read_mostly = 20;
  70 static int nv_ssthresh_eval_min_calls __read_mostly = 30;
  71 static int nv_stop_rtt_cnt __read_mostly = 10;
  72 static int nv_rtt_min_cnt __read_mostly = 2;
  73
  74 module_param(nv_pad, int, 0644);
  75 MODULE_PARM_DESC(nv_pad, "max queued packets allowed in network");
  76 module_param(nv_reset_period, int, 0644);
  77 MODULE_PARM_DESC(nv_reset_period, "nv_min_rtt reset period (secs)");
  78 module_param(nv_min_cwnd, int, 0644);
  79 MODULE_PARM_DESC(nv_min_cwnd, "NV will not decrease cwnd below this value"
  80                  " without losses");
  81
  82 /* TCP NV Parameters */
  83 struct tcpnv {
  84         unsigned long nv_min_rtt_reset_jiffies;  /* when to switch to
  85                                                   * nv_min_rtt_new */
  86         s8  cwnd_growth_factor; /* Current cwnd growth factor,
  87                                  * < 0 => less than 1 packet/RTT */
  88         u8  available8;
  89         u16 available16;
  90         u8  nv_allow_cwnd_growth:1, /* whether cwnd can grow */
  91                 nv_reset:1,         /* whether to reset values */
  92                 nv_catchup:1;       /* whether we are growing because
  93                                      * of temporary cwnd decrease */
  94         u8  nv_eval_call_cnt;   /* call count since last eval */
  95         u8  nv_min_cwnd;        /* nv won't make a ca decision if cwnd is
  96                                  * smaller than this. It may grow to handle
  97                                  * TSO, LRO and interrupt coalescence because
  98                                  * with these a small cwnd cannot saturate
  99                                  * the link. Note that this is different from
 100                                  * the file local nv_min_cwnd */
 101         u8  nv_rtt_cnt;         /* RTTs without making ca decision */;
 102         u32 nv_last_rtt;        /* last rtt */
 103         u32 nv_min_rtt;         /* active min rtt. Used to determine slope */
 104         u32 nv_min_rtt_new;     /* min rtt for future use */
 105         u32 nv_base_rtt;        /* If non-zero it represents the threshold for
 106                                  * congestion */
 107         u32 nv_lower_bound_rtt; /* Used in conjunction with nv_base_rtt. It is
 108                                  * set to 80% of nv_base_rtt. It helps reduce
 109                                  * unfairness between flows */
 110         u32 nv_rtt_max_rate;    /* max rate seen during current RTT */
 111         u32 nv_rtt_start_seq;   /* current RTT ends when packet arrives
 112                                  * acking beyond nv_rtt_start_seq */
 113         u32 nv_last_snd_una;    /* Previous value of tp->snd_una. It is
 114                                  * used to determine bytes acked since last
 115                                  * call to bictcp_acked */
 116         u32 nv_no_cong_cnt;     /* Consecutive no congestion decisions */
 117 };
 118
 119 #define NV_INIT_RTT       U32_MAX
 120 #define NV_MIN_CWND       4
 121 #define NV_MIN_CWND_GROW  2
 122 #define NV_TSO_CWND_BOUND 80
 123
 124 static inline void tcpnv_reset(struct tcpnv *ca, struct sock *sk)
 125 {
 126         struct tcp_sock *tp = tcp_sk(sk);
 127
 128         ca->nv_reset = 0;
 129         ca->nv_no_cong_cnt = 0;
 130         ca->nv_rtt_cnt = 0;
 131         ca->nv_last_rtt = 0;
 132         ca->nv_rtt_max_rate = 0;
 133         ca->nv_rtt_start_seq = tp->snd_una;
 134         ca->nv_eval_call_cnt = 0;
 135         ca->nv_last_snd_una = tp->snd_una;
 136 }
 137
 138 static void tcpnv_init(struct sock *sk)
 139 {
 140         struct tcpnv *ca = inet_csk_ca(sk);
 141         int base_rtt;
 142
 143         tcpnv_reset(ca, sk);
 144
 145         /* See if base_rtt is available from socket_ops bpf program.
 146          * It is meant to be used in environments, such as communication
 147          * within a datacenter, where we have reasonable estimates of
 148          * RTTs
 149          */
 150         base_rtt = tcp_call_bpf(sk, BPF_SOCK_OPS_BASE_RTT, 0, NULL);
 151         if (base_rtt > 0) {
 152                 ca->nv_base_rtt = base_rtt;
 153                 ca->nv_lower_bound_rtt = (base_rtt * 205) >> 8; /* 80% */
 154         } else {
 155                 ca->nv_base_rtt = 0;
 156                 ca->nv_lower_bound_rtt = 0;
 157         }
 158
 159         ca->nv_allow_cwnd_growth = 1;
 160         ca->nv_min_rtt_reset_jiffies = jiffies + 2 * HZ;
 161         ca->nv_min_rtt = NV_INIT_RTT;
 162         ca->nv_min_rtt_new = NV_INIT_RTT;
 163         ca->nv_min_cwnd = NV_MIN_CWND;
 164         ca->nv_catchup = 0;
 165         ca->cwnd_growth_factor = 0;
 166 }
 167
 168 /* If provided, apply upper (base_rtt) and lower (lower_bound_rtt)
 169  * bounds to RTT.
 170  */
 171 inline u32 nv_get_bounded_rtt(struct tcpnv *ca, u32 val)
 172 {
 173         if (ca->nv_lower_bound_rtt > 0 && val < ca->nv_lower_bound_rtt)
 174                 return ca->nv_lower_bound_rtt;
 175         else if (ca->nv_base_rtt > 0 && val > ca->nv_base_rtt)
 176                 return ca->nv_base_rtt;
 177         else
 178                 return val;
 179 }
 180
 181 static void tcpnv_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 182 {
 183         struct tcp_sock *tp = tcp_sk(sk);
 184         struct tcpnv *ca = inet_csk_ca(sk);
 185         u32 cnt;
 186
 187         if (!tcp_is_cwnd_limited(sk))
 188                 return;
 189
 190         /* Only grow cwnd if NV has not detected congestion */
 191         if (!ca->nv_allow_cwnd_growth)
 192                 return;
 193
 194         if (tcp_in_slow_start(tp)) {
 195                 acked = tcp_slow_start(tp, acked);
 196                 if (!acked)
 197                         return;
 198         }
 199
 200         if (ca->cwnd_growth_factor < 0) {
 201                 cnt = tp->snd_cwnd << -ca->cwnd_growth_factor;
 202                 tcp_cong_avoid_ai(tp, cnt, acked);
 203         } else {
 204                 cnt = max(4U, tp->snd_cwnd >> ca->cwnd_growth_factor);
 205                 tcp_cong_avoid_ai(tp, cnt, acked);
 206         }
 207 }
 208
 209 static u32 tcpnv_recalc_ssthresh(struct sock *sk)
 210 {
 211         const struct tcp_sock *tp = tcp_sk(sk);
 212
 213         return max((tp->snd_cwnd * nv_loss_dec_factor) >> 10, 2U);
 214 }
 215
 216 static void tcpnv_state(struct sock *sk, u8 new_state)
 217 {
 218         struct tcpnv *ca = inet_csk_ca(sk);
 219
 220         if (new_state == TCP_CA_Open && ca->nv_reset) {
 221                 tcpnv_reset(ca, sk);
 222         } else if (new_state == TCP_CA_Loss || new_state == TCP_CA_CWR ||
 223                 new_state == TCP_CA_Recovery) {
 224                 ca->nv_reset = 1;
 225                 ca->nv_allow_cwnd_growth = 0;
 226                 if (new_state == TCP_CA_Loss) {
 227                         /* Reset cwnd growth factor to Reno value */
 228                         if (ca->cwnd_growth_factor > 0)
 229                                 ca->cwnd_growth_factor = 0;
 230                         /* Decrease growth rate if allowed */
 231                         if (nv_cwnd_growth_rate_neg > 0 &&
 232                             ca->cwnd_growth_factor > -8)
 233                                 ca->cwnd_growth_factor--;
 234                 }
 235         }
 236 }
 237
 238 /* Do congestion avoidance calculations for TCP-NV
 239  */
 240 static void tcpnv_acked(struct sock *sk, const struct ack_sample *sample)
 241 {
 242         const struct inet_connection_sock *icsk = inet_csk(sk);
 243         struct tcp_sock *tp = tcp_sk(sk);
 244         struct tcpnv *ca = inet_csk_ca(sk);
 245         unsigned long now = jiffies;
 246         u64 rate64;
 247         u32 rate, max_win, cwnd_by_slope;
 248         u32 avg_rtt;
 249         u32 bytes_acked = 0;
 250
 251         /* Some calls are for duplicates without timetamps */
 252         if (sample->rtt_us < 0)
 253                 return;
 254
 255         /* If not in TCP_CA_Open or TCP_CA_Disorder states, skip. */
 256         if (icsk->icsk_ca_state != TCP_CA_Open &&
 257             icsk->icsk_ca_state != TCP_CA_Disorder)
 258                 return;
 259
 260         /* Stop cwnd growth if we were in catch up mode */
 261         if (ca->nv_catchup && tp->snd_cwnd >= nv_min_cwnd) {
 262                 ca->nv_catchup = 0;
 263                 ca->nv_allow_cwnd_growth = 0;
 264         }
 265
 266         bytes_acked = tp->snd_una - ca->nv_last_snd_una;
 267         ca->nv_last_snd_una = tp->snd_una;
 268
 269         if (sample->in_flight == 0)
 270                 return;
 271
 272         /* Calculate moving average of RTT */
 273         if (nv_rtt_factor > 0) {
 274                 if (ca->nv_last_rtt > 0) {
 275                         avg_rtt = (((u64)sample->rtt_us) * nv_rtt_factor +
 276                                    ((u64)ca->nv_last_rtt)
 277                                    * (256 - nv_rtt_factor)) >> 8;
 278                 } else {
 279                         avg_rtt = sample->rtt_us;
 280                         ca->nv_min_rtt = avg_rtt << 1;
 281                 }
 282                 ca->nv_last_rtt = avg_rtt;
 283         } else {
 284                 avg_rtt = sample->rtt_us;
 285         }
 286
 287         /* rate in 100's bits per second */
 288         rate64 = ((u64)sample->in_flight) * 80000;
 289         do_div(rate64, avg_rtt ?: 1);
 290         rate = (u32)rate64;
 291
 292         /* Remember the maximum rate seen during this RTT
 293          * Note: It may be more than one RTT. This function should be
 294          *       called at least nv_dec_eval_min_calls times.
 295          */
 296         if (ca->nv_rtt_max_rate < rate)
 297                 ca->nv_rtt_max_rate = rate;
 298
 299         /* We have valid information, increment counter */
 300         if (ca->nv_eval_call_cnt < 255)
 301                 ca->nv_eval_call_cnt++;
 302
 303         /* Apply bounds to rtt. Only used to update min_rtt */
 304         avg_rtt = nv_get_bounded_rtt(ca, avg_rtt);
 305
 306         /* update min rtt if necessary */
 307         if (avg_rtt < ca->nv_min_rtt)
 308                 ca->nv_min_rtt = avg_rtt;
 309
 310         /* update future min_rtt if necessary */
 311         if (avg_rtt < ca->nv_min_rtt_new)
 312                 ca->nv_min_rtt_new = avg_rtt;
 313
 314         /* nv_min_rtt is updated with the minimum (possibley averaged) rtt
 315          * seen in the last sysctl_tcp_nv_reset_period seconds (i.e. a
 316          * warm reset). This new nv_min_rtt will be continued to be updated
 317          * and be used for another sysctl_tcp_nv_reset_period seconds,
 318          * when it will be updated again.
 319          * In practice we introduce some randomness, so the actual period used
 320          * is chosen randomly from the range:
 321          *   [sysctl_tcp_nv_reset_period*3/4, sysctl_tcp_nv_reset_period*5/4)
 322          */
 323         if (time_after_eq(now, ca->nv_min_rtt_reset_jiffies)) {
 324                 unsigned char rand;
 325
 326                 ca->nv_min_rtt = ca->nv_min_rtt_new;
 327                 ca->nv_min_rtt_new = NV_INIT_RTT;
 328                 get_random_bytes(&rand, 1);
 329                 ca->nv_min_rtt_reset_jiffies =
 330                         now + ((nv_reset_period * (384 + rand) * HZ) >> 9);
 331                 /* Every so often we decrease ca->nv_min_cwnd in case previous
 332                  *  value is no longer accurate.
 333                  */
 334                 ca->nv_min_cwnd = max(ca->nv_min_cwnd / 2, NV_MIN_CWND);
 335         }
 336
 337         /* Once per RTT check if we need to do congestion avoidance */
 338         if (before(ca->nv_rtt_start_seq, tp->snd_una)) {
 339                 ca->nv_rtt_start_seq = tp->snd_nxt;
 340                 if (ca->nv_rtt_cnt < 0xff)
 341                         /* Increase counter for RTTs without CA decision */
 342                         ca->nv_rtt_cnt++;
 343
 344                 /* If this function is only called once within an RTT
 345                  * the cwnd is probably too small (in some cases due to
 346                  * tso, lro or interrupt coalescence), so we increase
 347                  * ca->nv_min_cwnd.
 348                  */
 349                 if (ca->nv_eval_call_cnt == 1 &&
 350                     bytes_acked >= (ca->nv_min_cwnd - 1) * tp->mss_cache &&
 351                     ca->nv_min_cwnd < (NV_TSO_CWND_BOUND + 1)) {
 352                         ca->nv_min_cwnd = min(ca->nv_min_cwnd
 353                                               + NV_MIN_CWND_GROW,
 354                                               NV_TSO_CWND_BOUND + 1);
 355                         ca->nv_rtt_start_seq = tp->snd_nxt +
 356                                 ca->nv_min_cwnd * tp->mss_cache;
 357                         ca->nv_eval_call_cnt = 0;
 358                         ca->nv_allow_cwnd_growth = 1;
 359                         return;
 360                 }
 361
 362                 /* Find the ideal cwnd for current rate from slope
 363                  * slope = 80000.0 * mss / nv_min_rtt
 364                  * cwnd_by_slope = nv_rtt_max_rate / slope
 365                  */
 366                 cwnd_by_slope = (u32)
 367                         div64_u64(((u64)ca->nv_rtt_max_rate) * ca->nv_min_rtt,
 368                                   80000ULL * tp->mss_cache);
 369                 max_win = cwnd_by_slope + nv_pad;
 370
 371                 /* If cwnd > max_win, decrease cwnd
 372                  * if cwnd < max_win, grow cwnd
 373                  * else leave the same
 374                  */
 375                 if (tp->snd_cwnd > max_win) {
 376                         /* there is congestion, check that it is ok
 377                          * to make a CA decision
 378                          * 1. We should have at least nv_dec_eval_min_calls
 379                          *    data points before making a CA  decision
 380                          * 2. We only make a congesion decision after
 381                          *    nv_rtt_min_cnt RTTs
 382                          */
 383                         if (ca->nv_rtt_cnt < nv_rtt_min_cnt) {
 384                                 return;
 385                         } else if (tp->snd_ssthresh == TCP_INFINITE_SSTHRESH) {
 386                                 if (ca->nv_eval_call_cnt <
 387                                     nv_ssthresh_eval_min_calls)
 388                                         return;
 389                                 /* otherwise we will decrease cwnd */
 390                         } else if (ca->nv_eval_call_cnt <
 391                                    nv_dec_eval_min_calls) {
 392                                 if (ca->nv_allow_cwnd_growth &&
 393                                     ca->nv_rtt_cnt > nv_stop_rtt_cnt)
 394                                         ca->nv_allow_cwnd_growth = 0;
 395                                 return;
 396                         }
 397
 398                         /* We have enough data to determine we are congested */
 399                         ca->nv_allow_cwnd_growth = 0;
 400                         tp->snd_ssthresh =
 401                                 (nv_ssthresh_factor * max_win) >> 3;
 402                         if (tp->snd_cwnd - max_win > 2) {
 403                                 /* gap > 2, we do exponential cwnd decrease */
 404                                 int dec;
 405
 406                                 dec = max(2U, ((tp->snd_cwnd - max_win) *
 407                                                nv_cong_dec_mult) >> 7);
 408                                 tp->snd_cwnd -= dec;
 409                         } else if (nv_cong_dec_mult > 0) {
 410                                 tp->snd_cwnd = max_win;
 411                         }
 412                         if (ca->cwnd_growth_factor > 0)
 413                                 ca->cwnd_growth_factor = 0;
 414                         ca->nv_no_cong_cnt = 0;
 415                 } else if (tp->snd_cwnd <= max_win - nv_pad_buffer) {
 416                         /* There is no congestion, grow cwnd if allowed*/
 417                         if (ca->nv_eval_call_cnt < nv_inc_eval_min_calls)
 418                                 return;
 419
 420                         ca->nv_allow_cwnd_growth = 1;
 421                         ca->nv_no_cong_cnt++;
 422                         if (ca->cwnd_growth_factor < 0 &&
 423                             nv_cwnd_growth_rate_neg > 0 &&
 424                             ca->nv_no_cong_cnt > nv_cwnd_growth_rate_neg) {
 425                                 ca->cwnd_growth_factor++;
 426                                 ca->nv_no_cong_cnt = 0;
 427                         } else if (ca->cwnd_growth_factor >= 0 &&
 428                                    nv_cwnd_growth_rate_pos > 0 &&
 429                                    ca->nv_no_cong_cnt >
 430                                    nv_cwnd_growth_rate_pos) {
 431                                 ca->cwnd_growth_factor++;
 432                                 ca->nv_no_cong_cnt = 0;
 433                         }
 434                 } else {
 435                         /* cwnd is in-between, so do nothing */
 436                         return;
 437                 }
 438
 439                 /* update state */
 440                 ca->nv_eval_call_cnt = 0;
 441                 ca->nv_rtt_cnt = 0;
 442                 ca->nv_rtt_max_rate = 0;
 443
 444                 /* Don't want to make cwnd < nv_min_cwnd
 445                  * (it wasn't before, if it is now is because nv
 446                  *  decreased it).
 447                  */
 448                 if (tp->snd_cwnd < nv_min_cwnd)
 449                         tp->snd_cwnd = nv_min_cwnd;
 450         }
 451 }
 452
 453 /* Extract info for Tcp socket info provided via netlink */
 454 static size_t tcpnv_get_info(struct sock *sk, u32 ext, int *attr,
 455                              union tcp_cc_info *info)
 456 {
 457         const struct tcpnv *ca = inet_csk_ca(sk);
 458
 459         if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 460                 info->vegas.tcpv_enabled = 1;
 461                 info->vegas.tcpv_rttcnt = ca->nv_rtt_cnt;
 462                 info->vegas.tcpv_rtt = ca->nv_last_rtt;
 463                 info->vegas.tcpv_minrtt = ca->nv_min_rtt;
 464
 465                 *attr = INET_DIAG_VEGASINFO;
 466                 return sizeof(struct tcpvegas_info);
 467         }
 468         return 0;
 469 }
 470
 471 static struct tcp_congestion_ops tcpnv __read_mostly = {
 472         .init           = tcpnv_init,
 473         .ssthresh       = tcpnv_recalc_ssthresh,
 474         .cong_avoid     = tcpnv_cong_avoid,
 475         .set_state      = tcpnv_state,
 476         .undo_cwnd      = tcp_reno_undo_cwnd,
 477         .pkts_acked     = tcpnv_acked,
 478         .get_info       = tcpnv_get_info,
 479
 480         .owner          = THIS_MODULE,
 481         .name           = "nv",
 482 };
 483
 484 static int __init tcpnv_register(void)
 485 {
 486         BUILD_BUG_ON(sizeof(struct tcpnv) > ICSK_CA_PRIV_SIZE);
 487
 488         return tcp_register_congestion_control(&tcpnv);
 489 }
 490
 491 static void __exit tcpnv_unregister(void)
 492 {
 493         tcp_unregister_congestion_control(&tcpnv);
 494 }
 495
 496 module_init(tcpnv_register);
 497 module_exit(tcpnv_unregister);
 498
 499 MODULE_AUTHOR("Lawrence Brakmo");
 500 MODULE_LICENSE("GPL");
 501 MODULE_DESCRIPTION("TCP NV");
 502 MODULE_VERSION("1.0");