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/module.h> 40 #include <linux/mm.h> 41 #include <net/tcp.h> 42 #include <linux/inet_diag.h> 43 #include "tcp_dctcp.h" 44 45 #define DCTCP_MAX_ALPHA 1024U 46 47 struct dctcp { 48 u32 old_delivered; 49 u32 old_delivered_ce; 50 u32 prior_rcv_nxt; 51 u32 dctcp_alpha; 52 u32 next_seq; 53 u32 ce_state; 54 u32 loss_cwnd; 55 }; 56 57 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */ 58 module_param(dctcp_shift_g, uint, 0644); 59 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha"); 60 61 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA; 62 module_param(dctcp_alpha_on_init, uint, 0644); 63 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value"); 64 65 static struct tcp_congestion_ops dctcp_reno; 66 67 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca) 68 { 69 ca->next_seq = tp->snd_nxt; 70 71 ca->old_delivered = tp->delivered; 72 ca->old_delivered_ce = tp->delivered_ce; 73 } 74 75 static void dctcp_init(struct sock *sk) 76 { 77 const struct tcp_sock *tp = tcp_sk(sk); 78 79 if ((tp->ecn_flags & TCP_ECN_OK) || 80 (sk->sk_state == TCP_LISTEN || 81 sk->sk_state == TCP_CLOSE)) { 82 struct dctcp *ca = inet_csk_ca(sk); 83 84 ca->prior_rcv_nxt = tp->rcv_nxt; 85 86 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA); 87 88 ca->loss_cwnd = 0; 89 ca->ce_state = 0; 90 91 dctcp_reset(tp, ca); 92 return; 93 } 94 95 /* No ECN support? Fall back to Reno. Also need to clear 96 * ECT from sk since it is set during 3WHS for DCTCP. 97 */ 98 inet_csk(sk)->icsk_ca_ops = &dctcp_reno; 99 INET_ECN_dontxmit(sk); 100 } 101 102 static u32 dctcp_ssthresh(struct sock *sk) 103 { 104 struct dctcp *ca = inet_csk_ca(sk); 105 struct tcp_sock *tp = tcp_sk(sk); 106 107 ca->loss_cwnd = tp->snd_cwnd; 108 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U); 109 } 110 111 static void dctcp_update_alpha(struct sock *sk, u32 flags) 112 { 113 const struct tcp_sock *tp = tcp_sk(sk); 114 struct dctcp *ca = inet_csk_ca(sk); 115 116 /* Expired RTT */ 117 if (!before(tp->snd_una, ca->next_seq)) { 118 u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce; 119 u32 alpha = ca->dctcp_alpha; 120 121 /* alpha = (1 - g) * alpha + g * F */ 122 123 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); 124 if (delivered_ce) { 125 u32 delivered = tp->delivered - ca->old_delivered; 126 127 /* If dctcp_shift_g == 1, a 32bit value would overflow 128 * after 8 M packets. 129 */ 130 delivered_ce <<= (10 - dctcp_shift_g); 131 delivered_ce /= max(1U, delivered); 132 133 alpha = min(alpha + delivered_ce, DCTCP_MAX_ALPHA); 134 } 135 /* dctcp_alpha can be read from dctcp_get_info() without 136 * synchro, so we ask compiler to not use dctcp_alpha 137 * as a temporary variable in prior operations. 138 */ 139 WRITE_ONCE(ca->dctcp_alpha, alpha); 140 dctcp_reset(tp, ca); 141 } 142 } 143 144 static void dctcp_react_to_loss(struct sock *sk) 145 { 146 struct dctcp *ca = inet_csk_ca(sk); 147 struct tcp_sock *tp = tcp_sk(sk); 148 149 ca->loss_cwnd = tp->snd_cwnd; 150 tp->snd_ssthresh = max(tp->snd_cwnd >> 1U, 2U); 151 } 152 153 static void dctcp_state(struct sock *sk, u8 new_state) 154 { 155 if (new_state == TCP_CA_Recovery && 156 new_state != inet_csk(sk)->icsk_ca_state) 157 dctcp_react_to_loss(sk); 158 /* We handle RTO in dctcp_cwnd_event to ensure that we perform only 159 * one loss-adjustment per RTT. 160 */ 161 } 162 163 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 164 { 165 struct dctcp *ca = inet_csk_ca(sk); 166 167 switch (ev) { 168 case CA_EVENT_ECN_IS_CE: 169 case CA_EVENT_ECN_NO_CE: 170 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state); 171 break; 172 case CA_EVENT_LOSS: 173 dctcp_react_to_loss(sk); 174 break; 175 default: 176 /* Don't care for the rest. */ 177 break; 178 } 179 } 180 181 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr, 182 union tcp_cc_info *info) 183 { 184 const struct dctcp *ca = inet_csk_ca(sk); 185 const struct tcp_sock *tp = tcp_sk(sk); 186 187 /* Fill it also in case of VEGASINFO due to req struct limits. 188 * We can still correctly retrieve it later. 189 */ 190 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 191 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 192 memset(&info->dctcp, 0, sizeof(info->dctcp)); 193 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 194 info->dctcp.dctcp_enabled = 1; 195 info->dctcp.dctcp_ce_state = (u16) ca->ce_state; 196 info->dctcp.dctcp_alpha = ca->dctcp_alpha; 197 info->dctcp.dctcp_ab_ecn = tp->mss_cache * 198 (tp->delivered_ce - ca->old_delivered_ce); 199 info->dctcp.dctcp_ab_tot = tp->mss_cache * 200 (tp->delivered - ca->old_delivered); 201 } 202 203 *attr = INET_DIAG_DCTCPINFO; 204 return sizeof(info->dctcp); 205 } 206 return 0; 207 } 208 209 static u32 dctcp_cwnd_undo(struct sock *sk) 210 { 211 const struct dctcp *ca = inet_csk_ca(sk); 212 213 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd); 214 } 215 216 static struct tcp_congestion_ops dctcp __read_mostly = { 217 .init = dctcp_init, 218 .in_ack_event = dctcp_update_alpha, 219 .cwnd_event = dctcp_cwnd_event, 220 .ssthresh = dctcp_ssthresh, 221 .cong_avoid = tcp_reno_cong_avoid, 222 .undo_cwnd = dctcp_cwnd_undo, 223 .set_state = dctcp_state, 224 .get_info = dctcp_get_info, 225 .flags = TCP_CONG_NEEDS_ECN, 226 .owner = THIS_MODULE, 227 .name = "dctcp", 228 }; 229 230 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 231 .ssthresh = tcp_reno_ssthresh, 232 .cong_avoid = tcp_reno_cong_avoid, 233 .undo_cwnd = tcp_reno_undo_cwnd, 234 .get_info = dctcp_get_info, 235 .owner = THIS_MODULE, 236 .name = "dctcp-reno", 237 }; 238 239 static int __init dctcp_register(void) 240 { 241 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 242 return tcp_register_congestion_control(&dctcp); 243 } 244 245 static void __exit dctcp_unregister(void) 246 { 247 tcp_unregister_congestion_control(&dctcp); 248 } 249 250 module_init(dctcp_register); 251 module_exit(dctcp_unregister); 252 253 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 254 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 255 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 256 257 MODULE_LICENSE("GPL v2"); 258 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 259