net/ipv4/tcp_dctcp.c

   1 /* DataCenter TCP (DCTCP) congestion control.
   2  *
   3  * http://simula.stanford.edu/~alizade/Site/DCTCP.html
   4  *
   5  * This is an implementation of DCTCP over Reno, an enhancement to the
   6  * TCP congestion control algorithm designed for data centers. DCTCP
   7  * leverages Explicit Congestion Notification (ECN) in the network to
   8  * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
   9  * the following three data center transport requirements:
  10  *
  11  *  - High burst tolerance (incast due to partition/aggregate)
  12  *  - Low latency (short flows, queries)
  13  *  - High throughput (continuous data updates, large file transfers)
  14  *    with commodity shallow buffered switches
  15  *
  16  * The algorithm is described in detail in the following two papers:
  17  *
  18  * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
  19  *    Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
  20  *      "Data Center TCP (DCTCP)", Data Center Networks session
  21  *      Proc. ACM SIGCOMM, New Delhi, 2010.
  22  *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
  23  *
  24  * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
  25  *      "Analysis of DCTCP: Stability, Convergence, and Fairness"
  26  *      Proc. ACM SIGMETRICS, San Jose, 2011.
  27  *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
  28  *
  29  * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
  30  *
  31  * Authors:
  32  *
  33  *      Daniel Borkmann <dborkman@redhat.com>
  34  *      Florian Westphal <fw@strlen.de>
  35  *      Glenn Judd <glenn.judd@morganstanley.com>
  36  *
  37  * This program is free software; you can redistribute it and/or modify
  38  * it under the terms of the GNU General Public License as published by
  39  * the Free Software Foundation; either version 2 of the License, or (at
  40  * your option) any later version.
  41  */
  42
  43 #include <linux/module.h>
  44 #include <linux/mm.h>
  45 #include <net/tcp.h>
  46 #include <linux/inet_diag.h>
  47
  48 #define DCTCP_MAX_ALPHA 1024U
  49
  50 struct dctcp {
  51         u32 acked_bytes_ecn;
  52         u32 acked_bytes_total;
  53         u32 prior_snd_una;
  54         u32 prior_rcv_nxt;
  55         u32 dctcp_alpha;
  56         u32 next_seq;
  57         u32 ce_state;
  58         u32 delayed_ack_reserved;
  59         u32 loss_cwnd;
  60 };
  61
  62 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
  63 module_param(dctcp_shift_g, uint, 0644);
  64 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
  65
  66 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
  67 module_param(dctcp_alpha_on_init, uint, 0644);
  68 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
  69
  70 static unsigned int dctcp_clamp_alpha_on_loss __read_mostly;
  71 module_param(dctcp_clamp_alpha_on_loss, uint, 0644);
  72 MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss,
  73                  "parameter for clamping alpha on loss");
  74
  75 static struct tcp_congestion_ops dctcp_reno;
  76
  77 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
  78 {
  79         ca->next_seq = tp->snd_nxt;
  80
  81         ca->acked_bytes_ecn = 0;
  82         ca->acked_bytes_total = 0;
  83 }
  84
  85 static void dctcp_init(struct sock *sk)
  86 {
  87         const struct tcp_sock *tp = tcp_sk(sk);
  88
  89         if ((tp->ecn_flags & TCP_ECN_OK) ||
  90             (sk->sk_state == TCP_LISTEN ||
  91              sk->sk_state == TCP_CLOSE)) {
  92                 struct dctcp *ca = inet_csk_ca(sk);
  93
  94                 ca->prior_snd_una = tp->snd_una;
  95                 ca->prior_rcv_nxt = tp->rcv_nxt;
  96
  97                 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
  98
  99                 ca->delayed_ack_reserved = 0;
 100                 ca->loss_cwnd = 0;
 101                 ca->ce_state = 0;
 102
 103                 dctcp_reset(tp, ca);
 104                 return;
 105         }
 106
 107         /* No ECN support? Fall back to Reno. Also need to clear
 108          * ECT from sk since it is set during 3WHS for DCTCP.
 109          */
 110         inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
 111         INET_ECN_dontxmit(sk);
 112 }
 113
 114 static u32 dctcp_ssthresh(struct sock *sk)
 115 {
 116         struct dctcp *ca = inet_csk_ca(sk);
 117         struct tcp_sock *tp = tcp_sk(sk);
 118
 119         ca->loss_cwnd = tp->snd_cwnd;
 120         return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
 121 }
 122
 123 /* Minimal DCTP CE state machine:
 124  *
 125  * S:   0 <- last pkt was non-CE
 126  *      1 <- last pkt was CE
 127  */
 128
 129 static void dctcp_ce_state_0_to_1(struct sock *sk)
 130 {
 131         struct dctcp *ca = inet_csk_ca(sk);
 132         struct tcp_sock *tp = tcp_sk(sk);
 133
 134         /* State has changed from CE=0 to CE=1 and delayed
 135          * ACK has not sent yet.
 136          */
 137         if (!ca->ce_state && ca->delayed_ack_reserved) {
 138                 u32 tmp_rcv_nxt;
 139
 140                 /* Save current rcv_nxt. */
 141                 tmp_rcv_nxt = tp->rcv_nxt;
 142
 143                 /* Generate previous ack with CE=0. */
 144                 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
 145                 tp->rcv_nxt = ca->prior_rcv_nxt;
 146
 147                 tcp_send_ack(sk);
 148
 149                 /* Recover current rcv_nxt. */
 150                 tp->rcv_nxt = tmp_rcv_nxt;
 151         }
 152
 153         ca->prior_rcv_nxt = tp->rcv_nxt;
 154         ca->ce_state = 1;
 155
 156         tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
 157 }
 158
 159 static void dctcp_ce_state_1_to_0(struct sock *sk)
 160 {
 161         struct dctcp *ca = inet_csk_ca(sk);
 162         struct tcp_sock *tp = tcp_sk(sk);
 163
 164         /* State has changed from CE=1 to CE=0 and delayed
 165          * ACK has not sent yet.
 166          */
 167         if (ca->ce_state && ca->delayed_ack_reserved) {
 168                 u32 tmp_rcv_nxt;
 169
 170                 /* Save current rcv_nxt. */
 171                 tmp_rcv_nxt = tp->rcv_nxt;
 172
 173                 /* Generate previous ack with CE=1. */
 174                 tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
 175                 tp->rcv_nxt = ca->prior_rcv_nxt;
 176
 177                 tcp_send_ack(sk);
 178
 179                 /* Recover current rcv_nxt. */
 180                 tp->rcv_nxt = tmp_rcv_nxt;
 181         }
 182
 183         ca->prior_rcv_nxt = tp->rcv_nxt;
 184         ca->ce_state = 0;
 185
 186         tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
 187 }
 188
 189 static void dctcp_update_alpha(struct sock *sk, u32 flags)
 190 {
 191         const struct tcp_sock *tp = tcp_sk(sk);
 192         struct dctcp *ca = inet_csk_ca(sk);
 193         u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
 194
 195         /* If ack did not advance snd_una, count dupack as MSS size.
 196          * If ack did update window, do not count it at all.
 197          */
 198         if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
 199                 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
 200         if (acked_bytes) {
 201                 ca->acked_bytes_total += acked_bytes;
 202                 ca->prior_snd_una = tp->snd_una;
 203
 204                 if (flags & CA_ACK_ECE)
 205                         ca->acked_bytes_ecn += acked_bytes;
 206         }
 207
 208         /* Expired RTT */
 209         if (!before(tp->snd_una, ca->next_seq)) {
 210                 u64 bytes_ecn = ca->acked_bytes_ecn;
 211                 u32 alpha = ca->dctcp_alpha;
 212
 213                 /* alpha = (1 - g) * alpha + g * F */
 214
 215                 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
 216                 if (bytes_ecn) {
 217                         /* If dctcp_shift_g == 1, a 32bit value would overflow
 218                          * after 8 Mbytes.
 219                          */
 220                         bytes_ecn <<= (10 - dctcp_shift_g);
 221                         do_div(bytes_ecn, max(1U, ca->acked_bytes_total));
 222
 223                         alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
 224                 }
 225                 /* dctcp_alpha can be read from dctcp_get_info() without
 226                  * synchro, so we ask compiler to not use dctcp_alpha
 227                  * as a temporary variable in prior operations.
 228                  */
 229                 WRITE_ONCE(ca->dctcp_alpha, alpha);
 230                 dctcp_reset(tp, ca);
 231         }
 232 }
 233
 234 static void dctcp_state(struct sock *sk, u8 new_state)
 235 {
 236         if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
 237                 struct dctcp *ca = inet_csk_ca(sk);
 238
 239                 /* If this extension is enabled, we clamp dctcp_alpha to
 240                  * max on packet loss; the motivation is that dctcp_alpha
 241                  * is an indicator to the extend of congestion and packet
 242                  * loss is an indicator of extreme congestion; setting
 243                  * this in practice turned out to be beneficial, and
 244                  * effectively assumes total congestion which reduces the
 245                  * window by half.
 246                  */
 247                 ca->dctcp_alpha = DCTCP_MAX_ALPHA;
 248         }
 249 }
 250
 251 static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev)
 252 {
 253         struct dctcp *ca = inet_csk_ca(sk);
 254
 255         switch (ev) {
 256         case CA_EVENT_DELAYED_ACK:
 257                 if (!ca->delayed_ack_reserved)
 258                         ca->delayed_ack_reserved = 1;
 259                 break;
 260         case CA_EVENT_NON_DELAYED_ACK:
 261                 if (ca->delayed_ack_reserved)
 262                         ca->delayed_ack_reserved = 0;
 263                 break;
 264         default:
 265                 /* Don't care for the rest. */
 266                 break;
 267         }
 268 }
 269
 270 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 271 {
 272         switch (ev) {
 273         case CA_EVENT_ECN_IS_CE:
 274                 dctcp_ce_state_0_to_1(sk);
 275                 break;
 276         case CA_EVENT_ECN_NO_CE:
 277                 dctcp_ce_state_1_to_0(sk);
 278                 break;
 279         case CA_EVENT_DELAYED_ACK:
 280         case CA_EVENT_NON_DELAYED_ACK:
 281                 dctcp_update_ack_reserved(sk, ev);
 282                 break;
 283         default:
 284                 /* Don't care for the rest. */
 285                 break;
 286         }
 287 }
 288
 289 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 290                              union tcp_cc_info *info)
 291 {
 292         const struct dctcp *ca = inet_csk_ca(sk);
 293
 294         /* Fill it also in case of VEGASINFO due to req struct limits.
 295          * We can still correctly retrieve it later.
 296          */
 297         if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
 298             ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 299                 memset(&info->dctcp, 0, sizeof(info->dctcp));
 300                 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
 301                         info->dctcp.dctcp_enabled = 1;
 302                         info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
 303                         info->dctcp.dctcp_alpha = ca->dctcp_alpha;
 304                         info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn;
 305                         info->dctcp.dctcp_ab_tot = ca->acked_bytes_total;
 306                 }
 307
 308                 *attr = INET_DIAG_DCTCPINFO;
 309                 return sizeof(info->dctcp);
 310         }
 311         return 0;
 312 }
 313
 314 static u32 dctcp_cwnd_undo(struct sock *sk)
 315 {
 316         const struct dctcp *ca = inet_csk_ca(sk);
 317
 318         return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
 319 }
 320
 321 static struct tcp_congestion_ops dctcp __read_mostly = {
 322         .init           = dctcp_init,
 323         .in_ack_event   = dctcp_update_alpha,
 324         .cwnd_event     = dctcp_cwnd_event,
 325         .ssthresh       = dctcp_ssthresh,
 326         .cong_avoid     = tcp_reno_cong_avoid,
 327         .undo_cwnd      = dctcp_cwnd_undo,
 328         .set_state      = dctcp_state,
 329         .get_info       = dctcp_get_info,
 330         .flags          = TCP_CONG_NEEDS_ECN,
 331         .owner          = THIS_MODULE,
 332         .name           = "dctcp",
 333 };
 334
 335 static struct tcp_congestion_ops dctcp_reno __read_mostly = {
 336         .ssthresh       = tcp_reno_ssthresh,
 337         .cong_avoid     = tcp_reno_cong_avoid,
 338         .undo_cwnd      = tcp_reno_undo_cwnd,
 339         .get_info       = dctcp_get_info,
 340         .owner          = THIS_MODULE,
 341         .name           = "dctcp-reno",
 342 };
 343
 344 static int __init dctcp_register(void)
 345 {
 346         BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
 347         return tcp_register_congestion_control(&dctcp);
 348 }
 349
 350 static void __exit dctcp_unregister(void)
 351 {
 352         tcp_unregister_congestion_control(&dctcp);
 353 }
 354
 355 module_init(dctcp_register);
 356 module_exit(dctcp_unregister);
 357
 358 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
 359 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
 360 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
 361
 362 MODULE_LICENSE("GPL v2");
 363 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");