net/ipv4/tcp_dctcp.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* DataCenter TCP (DCTCP) congestion control.
   3  *
   4  * http://simula.stanford.edu/~alizade/Site/DCTCP.html
   5  *
   6  * This is an implementation of DCTCP over Reno, an enhancement to the
   7  * TCP congestion control algorithm designed for data centers. DCTCP
   8  * leverages Explicit Congestion Notification (ECN) in the network to
   9  * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
  10  * the following three data center transport requirements:
  11  *
  12  *  - High burst tolerance (incast due to partition/aggregate)
  13  *  - Low latency (short flows, queries)
  14  *  - High throughput (continuous data updates, large file transfers)
  15  *    with commodity shallow buffered switches
  16  *
  17  * The algorithm is described in detail in the following two papers:
  18  *
  19  * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
  20  *    Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
  21  *      "Data Center TCP (DCTCP)", Data Center Networks session
  22  *      Proc. ACM SIGCOMM, New Delhi, 2010.
  23  *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
  24  *
  25  * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
  26  *      "Analysis of DCTCP: Stability, Convergence, and Fairness"
  27  *      Proc. ACM SIGMETRICS, San Jose, 2011.
  28  *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
  29  *
  30  * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
  31  *
  32  * Authors:
  33  *
  34  *      Daniel Borkmann <dborkman@redhat.com>
  35  *      Florian Westphal <fw@strlen.de>
  36  *      Glenn Judd <glenn.judd@morganstanley.com>
  37  */
  38
  39 #include <linux/btf.h>
  40 #include <linux/btf_ids.h>
  41 #include <linux/module.h>
  42 #include <linux/mm.h>
  43 #include <net/tcp.h>
  44 #include <linux/inet_diag.h>
  45 #include "tcp_dctcp.h"
  46
  47 #define DCTCP_MAX_ALPHA 1024U
  48
  49 struct dctcp {
  50         u32 old_delivered;
  51         u32 old_delivered_ce;
  52         u32 prior_rcv_nxt;
  53         u32 dctcp_alpha;
  54         u32 next_seq;
  55         u32 ce_state;
  56         u32 loss_cwnd;
  57         struct tcp_plb_state plb;
  58 };
  59
  60 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
  61
  62 static int dctcp_shift_g_set(const char *val, const struct kernel_param *kp)
  63 {
  64         return param_set_uint_minmax(val, kp, 0, 10);
  65 }
  66
  67 static const struct kernel_param_ops dctcp_shift_g_ops = {
  68         .set = dctcp_shift_g_set,
  69         .get = param_get_uint,
  70 };
  71
  72 module_param_cb(dctcp_shift_g, &dctcp_shift_g_ops, &dctcp_shift_g, 0644);
  73 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
  74
  75 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
  76 module_param(dctcp_alpha_on_init, uint, 0644);
  77 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
  78
  79 static struct tcp_congestion_ops dctcp_reno;
  80
  81 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
  82 {
  83         ca->next_seq = tp->snd_nxt;
  84
  85         ca->old_delivered = tp->delivered;
  86         ca->old_delivered_ce = tp->delivered_ce;
  87 }
  88
  89 __bpf_kfunc static void dctcp_init(struct sock *sk)
  90 {
  91         const struct tcp_sock *tp = tcp_sk(sk);
  92
  93         if ((tp->ecn_flags & TCP_ECN_OK) ||
  94             (sk->sk_state == TCP_LISTEN ||
  95              sk->sk_state == TCP_CLOSE)) {
  96                 struct dctcp *ca = inet_csk_ca(sk);
  97
  98                 ca->prior_rcv_nxt = tp->rcv_nxt;
  99
 100                 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
 101
 102                 ca->loss_cwnd = 0;
 103                 ca->ce_state = 0;
 104
 105                 dctcp_reset(tp, ca);
 106                 tcp_plb_init(sk, &ca->plb);
 107
 108                 return;
 109         }
 110
 111         /* No ECN support? Fall back to Reno. Also need to clear
 112          * ECT from sk since it is set during 3WHS for DCTCP.
 113          */
 114         inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
 115         INET_ECN_dontxmit(sk);
 116 }
 117
 118 __bpf_kfunc static u32 dctcp_ssthresh(struct sock *sk)
 119 {
 120         struct dctcp *ca = inet_csk_ca(sk);
 121         struct tcp_sock *tp = tcp_sk(sk);
 122
 123         ca->loss_cwnd = tcp_snd_cwnd(tp);
 124         return max(tcp_snd_cwnd(tp) - ((tcp_snd_cwnd(tp) * ca->dctcp_alpha) >> 11U), 2U);
 125 }
 126
 127 __bpf_kfunc static void dctcp_update_alpha(struct sock *sk, u32 flags)
 128 {
 129         const struct tcp_sock *tp = tcp_sk(sk);
 130         struct dctcp *ca = inet_csk_ca(sk);
 131
 132         /* Expired RTT */
 133         if (!before(tp->snd_una, ca->next_seq)) {
 134                 u32 delivered = tp->delivered - ca->old_delivered;
 135                 u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce;
 136                 u32 alpha = ca->dctcp_alpha;
 137                 u32 ce_ratio = 0;
 138
 139                 if (delivered > 0) {
 140                         /* dctcp_alpha keeps EWMA of fraction of ECN marked
 141                          * packets. Because of EWMA smoothing, PLB reaction can
 142                          * be slow so we use ce_ratio which is an instantaneous
 143                          * measure of congestion. ce_ratio is the fraction of
 144                          * ECN marked packets in the previous RTT.
 145                          */
 146                         if (delivered_ce > 0)
 147                                 ce_ratio = (delivered_ce << TCP_PLB_SCALE) / delivered;
 148                         tcp_plb_update_state(sk, &ca->plb, (int)ce_ratio);
 149                         tcp_plb_check_rehash(sk, &ca->plb);
 150                 }
 151
 152                 /* alpha = (1 - g) * alpha + g * F */
 153
 154                 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
 155                 if (delivered_ce) {
 156
 157                         /* If dctcp_shift_g == 1, a 32bit value would overflow
 158                          * after 8 M packets.
 159                          */
 160                         delivered_ce <<= (10 - dctcp_shift_g);
 161                         delivered_ce /= max(1U, delivered);
 162
 163                         alpha = min(alpha + delivered_ce, DCTCP_MAX_ALPHA);
 164                 }
 165                 /* dctcp_alpha can be read from dctcp_get_info() without
 166                  * synchro, so we ask compiler to not use dctcp_alpha
 167                  * as a temporary variable in prior operations.
 168                  */
 169                 WRITE_ONCE(ca->dctcp_alpha, alpha);
 170                 dctcp_reset(tp, ca);
 171         }
 172 }
 173
 174 static void dctcp_react_to_loss(struct sock *sk)
 175 {
 176         struct dctcp *ca = inet_csk_ca(sk);
 177         struct tcp_sock *tp = tcp_sk(sk);
 178
 179         ca->loss_cwnd = tcp_snd_cwnd(tp);
 180         tp->snd_ssthresh = max(tcp_snd_cwnd(tp) >> 1U, 2U);
 181 }
 182
 183 __bpf_kfunc static void dctcp_state(struct sock *sk, u8 new_state)
 184 {
 185         if (new_state == TCP_CA_Recovery &&
 186             new_state != inet_csk(sk)->icsk_ca_state)
 187                 dctcp_react_to_loss(sk);
 188         /* We handle RTO in dctcp_cwnd_event to ensure that we perform only
 189          * one loss-adjustment per RTT.
 190          */
 191 }
 192
 193 __bpf_kfunc static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
 194 {
 195         struct dctcp *ca = inet_csk_ca(sk);
 196
 197         switch (ev) {
 198         case CA_EVENT_ECN_IS_CE:
 199         case CA_EVENT_ECN_NO_CE:
 200                 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state);
 201                 break;
 202         case CA_EVENT_LOSS:
 203                 tcp_plb_update_state_upon_rto(sk, &ca->plb);
 204                 dctcp_react_to_loss(sk);
 205                 break;
 206         case CA_EVENT_TX_START:
 207                 tcp_plb_check_rehash(sk, &ca->plb); /* Maybe rehash when inflight is 0 */
 208                 break;
 209         default:
 210                 /* Don't care for the rest. */
 211                 break;
 212         }
 213 }
 214
 215 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 216                              union tcp_cc_info *info)
 217 {
 218         const struct dctcp *ca = inet_csk_ca(sk);
 219         const struct tcp_sock *tp = tcp_sk(sk);
 220
 221         /* Fill it also in case of VEGASINFO due to req struct limits.
 222          * We can still correctly retrieve it later.
 223          */
 224         if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
 225             ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 226                 memset(&info->dctcp, 0, sizeof(info->dctcp));
 227                 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
 228                         info->dctcp.dctcp_enabled = 1;
 229                         info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
 230                         info->dctcp.dctcp_alpha = ca->dctcp_alpha;
 231                         info->dctcp.dctcp_ab_ecn = tp->mss_cache *
 232                                                    (tp->delivered_ce - ca->old_delivered_ce);
 233                         info->dctcp.dctcp_ab_tot = tp->mss_cache *
 234                                                    (tp->delivered - ca->old_delivered);
 235                 }
 236
 237                 *attr = INET_DIAG_DCTCPINFO;
 238                 return sizeof(info->dctcp);
 239         }
 240         return 0;
 241 }
 242
 243 __bpf_kfunc static u32 dctcp_cwnd_undo(struct sock *sk)
 244 {
 245         const struct dctcp *ca = inet_csk_ca(sk);
 246         struct tcp_sock *tp = tcp_sk(sk);
 247
 248         return max(tcp_snd_cwnd(tp), ca->loss_cwnd);
 249 }
 250
 251 static struct tcp_congestion_ops dctcp __read_mostly = {
 252         .init           = dctcp_init,
 253         .in_ack_event   = dctcp_update_alpha,
 254         .cwnd_event     = dctcp_cwnd_event,
 255         .ssthresh       = dctcp_ssthresh,
 256         .cong_avoid     = tcp_reno_cong_avoid,
 257         .undo_cwnd      = dctcp_cwnd_undo,
 258         .set_state      = dctcp_state,
 259         .get_info       = dctcp_get_info,
 260         .flags          = TCP_CONG_NEEDS_ECN,
 261         .owner          = THIS_MODULE,
 262         .name           = "dctcp",
 263 };
 264
 265 static struct tcp_congestion_ops dctcp_reno __read_mostly = {
 266         .ssthresh       = tcp_reno_ssthresh,
 267         .cong_avoid     = tcp_reno_cong_avoid,
 268         .undo_cwnd      = tcp_reno_undo_cwnd,
 269         .get_info       = dctcp_get_info,
 270         .owner          = THIS_MODULE,
 271         .name           = "dctcp-reno",
 272 };
 273
 274 BTF_KFUNCS_START(tcp_dctcp_check_kfunc_ids)
 275 BTF_ID_FLAGS(func, dctcp_init)
 276 BTF_ID_FLAGS(func, dctcp_update_alpha)
 277 BTF_ID_FLAGS(func, dctcp_cwnd_event)
 278 BTF_ID_FLAGS(func, dctcp_ssthresh)
 279 BTF_ID_FLAGS(func, dctcp_cwnd_undo)
 280 BTF_ID_FLAGS(func, dctcp_state)
 281 BTF_KFUNCS_END(tcp_dctcp_check_kfunc_ids)
 282
 283 static const struct btf_kfunc_id_set tcp_dctcp_kfunc_set = {
 284         .owner = THIS_MODULE,
 285         .set   = &tcp_dctcp_check_kfunc_ids,
 286 };
 287
 288 static int __init dctcp_register(void)
 289 {
 290         int ret;
 291
 292         BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
 293
 294         ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &tcp_dctcp_kfunc_set);
 295         if (ret < 0)
 296                 return ret;
 297         return tcp_register_congestion_control(&dctcp);
 298 }
 299
 300 static void __exit dctcp_unregister(void)
 301 {
 302         tcp_unregister_congestion_control(&dctcp);
 303 }
 304
 305 module_init(dctcp_register);
 306 module_exit(dctcp_unregister);
 307
 308 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
 309 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
 310 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
 311
 312 MODULE_LICENSE("GPL v2");
 313 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");