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 #include "tcp_dctcp.h"
  48
  49 #define DCTCP_MAX_ALPHA 1024U
  50
  51 struct dctcp {
  52         u32 acked_bytes_ecn;
  53         u32 acked_bytes_total;
  54         u32 prior_snd_una;
  55         u32 prior_rcv_nxt;
  56         u32 dctcp_alpha;
  57         u32 next_seq;
  58         u32 ce_state;
  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->loss_cwnd = 0;
 100                 ca->ce_state = 0;
 101
 102                 dctcp_reset(tp, ca);
 103                 return;
 104         }
 105
 106         /* No ECN support? Fall back to Reno. Also need to clear
 107          * ECT from sk since it is set during 3WHS for DCTCP.
 108          */
 109         inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
 110         INET_ECN_dontxmit(sk);
 111 }
 112
 113 static u32 dctcp_ssthresh(struct sock *sk)
 114 {
 115         struct dctcp *ca = inet_csk_ca(sk);
 116         struct tcp_sock *tp = tcp_sk(sk);
 117
 118         ca->loss_cwnd = tp->snd_cwnd;
 119         return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
 120 }
 121
 122 static void dctcp_update_alpha(struct sock *sk, u32 flags)
 123 {
 124         const struct tcp_sock *tp = tcp_sk(sk);
 125         struct dctcp *ca = inet_csk_ca(sk);
 126         u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
 127
 128         /* If ack did not advance snd_una, count dupack as MSS size.
 129          * If ack did update window, do not count it at all.
 130          */
 131         if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
 132                 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
 133         if (acked_bytes) {
 134                 ca->acked_bytes_total += acked_bytes;
 135                 ca->prior_snd_una = tp->snd_una;
 136
 137                 if (flags & CA_ACK_ECE)
 138                         ca->acked_bytes_ecn += acked_bytes;
 139         }
 140
 141         /* Expired RTT */
 142         if (!before(tp->snd_una, ca->next_seq)) {
 143                 u64 bytes_ecn = ca->acked_bytes_ecn;
 144                 u32 alpha = ca->dctcp_alpha;
 145
 146                 /* alpha = (1 - g) * alpha + g * F */
 147
 148                 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
 149                 if (bytes_ecn) {
 150                         /* If dctcp_shift_g == 1, a 32bit value would overflow
 151                          * after 8 Mbytes.
 152                          */
 153                         bytes_ecn <<= (10 - dctcp_shift_g);
 154                         do_div(bytes_ecn, max(1U, ca->acked_bytes_total));
 155
 156                         alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
 157                 }
 158                 /* dctcp_alpha can be read from dctcp_get_info() without
 159                  * synchro, so we ask compiler to not use dctcp_alpha
 160                  * as a temporary variable in prior operations.
 161                  */
 162                 WRITE_ONCE(ca->dctcp_alpha, alpha);
 163                 dctcp_reset(tp, ca);
 164         }
 165 }
 166
 167 static void dctcp_state(struct sock *sk, u8 new_state)
 168 {
 169         if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
 170                 struct dctcp *ca = inet_csk_ca(sk);
 171
 172                 /* If this extension is enabled, we clamp dctcp_alpha to
 173                  * max on packet loss; the motivation is that dctcp_alpha
 174                  * is an indicator to the extend of congestion and packet
 175                  * loss is an indicator of extreme congestion; setting
 176                  * this in practice turned out to be beneficial, and
 177                  * effectively assumes total congestion which reduces the
 178                  * window by half.
 179                  */
 180                 ca->dctcp_alpha = DCTCP_MAX_ALPHA;
 181         }
 182 }
 183
 184 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 185 {
 186         struct dctcp *ca = inet_csk_ca(sk);
 187
 188         switch (ev) {
 189         case CA_EVENT_ECN_IS_CE:
 190         case CA_EVENT_ECN_NO_CE:
 191                 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state);
 192                 break;
 193         default:
 194                 /* Don't care for the rest. */
 195                 break;
 196         }
 197 }
 198
 199 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 200                              union tcp_cc_info *info)
 201 {
 202         const struct dctcp *ca = inet_csk_ca(sk);
 203
 204         /* Fill it also in case of VEGASINFO due to req struct limits.
 205          * We can still correctly retrieve it later.
 206          */
 207         if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
 208             ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 209                 memset(&info->dctcp, 0, sizeof(info->dctcp));
 210                 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
 211                         info->dctcp.dctcp_enabled = 1;
 212                         info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
 213                         info->dctcp.dctcp_alpha = ca->dctcp_alpha;
 214                         info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn;
 215                         info->dctcp.dctcp_ab_tot = ca->acked_bytes_total;
 216                 }
 217
 218                 *attr = INET_DIAG_DCTCPINFO;
 219                 return sizeof(info->dctcp);
 220         }
 221         return 0;
 222 }
 223
 224 static u32 dctcp_cwnd_undo(struct sock *sk)
 225 {
 226         const struct dctcp *ca = inet_csk_ca(sk);
 227
 228         return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
 229 }
 230
 231 static struct tcp_congestion_ops dctcp __read_mostly = {
 232         .init           = dctcp_init,
 233         .in_ack_event   = dctcp_update_alpha,
 234         .cwnd_event     = dctcp_cwnd_event,
 235         .ssthresh       = dctcp_ssthresh,
 236         .cong_avoid     = tcp_reno_cong_avoid,
 237         .undo_cwnd      = dctcp_cwnd_undo,
 238         .set_state      = dctcp_state,
 239         .get_info       = dctcp_get_info,
 240         .flags          = TCP_CONG_NEEDS_ECN,
 241         .owner          = THIS_MODULE,
 242         .name           = "dctcp",
 243 };
 244
 245 static struct tcp_congestion_ops dctcp_reno __read_mostly = {
 246         .ssthresh       = tcp_reno_ssthresh,
 247         .cong_avoid     = tcp_reno_cong_avoid,
 248         .undo_cwnd      = tcp_reno_undo_cwnd,
 249         .get_info       = dctcp_get_info,
 250         .owner          = THIS_MODULE,
 251         .name           = "dctcp-reno",
 252 };
 253
 254 static int __init dctcp_register(void)
 255 {
 256         BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
 257         return tcp_register_congestion_control(&dctcp);
 258 }
 259
 260 static void __exit dctcp_unregister(void)
 261 {
 262         tcp_unregister_congestion_control(&dctcp);
 263 }
 264
 265 module_init(dctcp_register);
 266 module_exit(dctcp_unregister);
 267
 268 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
 269 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
 270 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
 271
 272 MODULE_LICENSE("GPL v2");
 273 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");