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 module_param(dctcp_shift_g, uint, 0644); 62 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha"); 63 64 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA; 65 module_param(dctcp_alpha_on_init, uint, 0644); 66 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value"); 67 68 static struct tcp_congestion_ops dctcp_reno; 69 70 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca) 71 { 72 ca->next_seq = tp->snd_nxt; 73 74 ca->old_delivered = tp->delivered; 75 ca->old_delivered_ce = tp->delivered_ce; 76 } 77 78 static void dctcp_init(struct sock *sk) 79 { 80 const struct tcp_sock *tp = tcp_sk(sk); 81 82 if ((tp->ecn_flags & TCP_ECN_OK) || 83 (sk->sk_state == TCP_LISTEN || 84 sk->sk_state == TCP_CLOSE)) { 85 struct dctcp *ca = inet_csk_ca(sk); 86 87 ca->prior_rcv_nxt = tp->rcv_nxt; 88 89 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA); 90 91 ca->loss_cwnd = 0; 92 ca->ce_state = 0; 93 94 dctcp_reset(tp, ca); 95 tcp_plb_init(sk, &ca->plb); 96 97 return; 98 } 99 100 /* No ECN support? Fall back to Reno. Also need to clear 101 * ECT from sk since it is set during 3WHS for DCTCP. 102 */ 103 inet_csk(sk)->icsk_ca_ops = &dctcp_reno; 104 INET_ECN_dontxmit(sk); 105 } 106 107 static u32 dctcp_ssthresh(struct sock *sk) 108 { 109 struct dctcp *ca = inet_csk_ca(sk); 110 struct tcp_sock *tp = tcp_sk(sk); 111 112 ca->loss_cwnd = tcp_snd_cwnd(tp); 113 return max(tcp_snd_cwnd(tp) - ((tcp_snd_cwnd(tp) * ca->dctcp_alpha) >> 11U), 2U); 114 } 115 116 static void dctcp_update_alpha(struct sock *sk, u32 flags) 117 { 118 const struct tcp_sock *tp = tcp_sk(sk); 119 struct dctcp *ca = inet_csk_ca(sk); 120 121 /* Expired RTT */ 122 if (!before(tp->snd_una, ca->next_seq)) { 123 u32 delivered = tp->delivered - ca->old_delivered; 124 u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce; 125 u32 alpha = ca->dctcp_alpha; 126 u32 ce_ratio = 0; 127 128 if (delivered > 0) { 129 /* dctcp_alpha keeps EWMA of fraction of ECN marked 130 * packets. Because of EWMA smoothing, PLB reaction can 131 * be slow so we use ce_ratio which is an instantaneous 132 * measure of congestion. ce_ratio is the fraction of 133 * ECN marked packets in the previous RTT. 134 */ 135 if (delivered_ce > 0) 136 ce_ratio = (delivered_ce << TCP_PLB_SCALE) / delivered; 137 tcp_plb_update_state(sk, &ca->plb, (int)ce_ratio); 138 tcp_plb_check_rehash(sk, &ca->plb); 139 } 140 141 /* alpha = (1 - g) * alpha + g * F */ 142 143 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); 144 if (delivered_ce) { 145 146 /* If dctcp_shift_g == 1, a 32bit value would overflow 147 * after 8 M packets. 148 */ 149 delivered_ce <<= (10 - dctcp_shift_g); 150 delivered_ce /= max(1U, delivered); 151 152 alpha = min(alpha + delivered_ce, DCTCP_MAX_ALPHA); 153 } 154 /* dctcp_alpha can be read from dctcp_get_info() without 155 * synchro, so we ask compiler to not use dctcp_alpha 156 * as a temporary variable in prior operations. 157 */ 158 WRITE_ONCE(ca->dctcp_alpha, alpha); 159 dctcp_reset(tp, ca); 160 } 161 } 162 163 static void dctcp_react_to_loss(struct sock *sk) 164 { 165 struct dctcp *ca = inet_csk_ca(sk); 166 struct tcp_sock *tp = tcp_sk(sk); 167 168 ca->loss_cwnd = tcp_snd_cwnd(tp); 169 tp->snd_ssthresh = max(tcp_snd_cwnd(tp) >> 1U, 2U); 170 } 171 172 static void dctcp_state(struct sock *sk, u8 new_state) 173 { 174 if (new_state == TCP_CA_Recovery && 175 new_state != inet_csk(sk)->icsk_ca_state) 176 dctcp_react_to_loss(sk); 177 /* We handle RTO in dctcp_cwnd_event to ensure that we perform only 178 * one loss-adjustment per RTT. 179 */ 180 } 181 182 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 183 { 184 struct dctcp *ca = inet_csk_ca(sk); 185 186 switch (ev) { 187 case CA_EVENT_ECN_IS_CE: 188 case CA_EVENT_ECN_NO_CE: 189 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state); 190 break; 191 case CA_EVENT_LOSS: 192 tcp_plb_update_state_upon_rto(sk, &ca->plb); 193 dctcp_react_to_loss(sk); 194 break; 195 case CA_EVENT_TX_START: 196 tcp_plb_check_rehash(sk, &ca->plb); /* Maybe rehash when inflight is 0 */ 197 break; 198 default: 199 /* Don't care for the rest. */ 200 break; 201 } 202 } 203 204 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr, 205 union tcp_cc_info *info) 206 { 207 const struct dctcp *ca = inet_csk_ca(sk); 208 const struct tcp_sock *tp = tcp_sk(sk); 209 210 /* Fill it also in case of VEGASINFO due to req struct limits. 211 * We can still correctly retrieve it later. 212 */ 213 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 214 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 215 memset(&info->dctcp, 0, sizeof(info->dctcp)); 216 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 217 info->dctcp.dctcp_enabled = 1; 218 info->dctcp.dctcp_ce_state = (u16) ca->ce_state; 219 info->dctcp.dctcp_alpha = ca->dctcp_alpha; 220 info->dctcp.dctcp_ab_ecn = tp->mss_cache * 221 (tp->delivered_ce - ca->old_delivered_ce); 222 info->dctcp.dctcp_ab_tot = tp->mss_cache * 223 (tp->delivered - ca->old_delivered); 224 } 225 226 *attr = INET_DIAG_DCTCPINFO; 227 return sizeof(info->dctcp); 228 } 229 return 0; 230 } 231 232 static u32 dctcp_cwnd_undo(struct sock *sk) 233 { 234 const struct dctcp *ca = inet_csk_ca(sk); 235 struct tcp_sock *tp = tcp_sk(sk); 236 237 return max(tcp_snd_cwnd(tp), ca->loss_cwnd); 238 } 239 240 static struct tcp_congestion_ops dctcp __read_mostly = { 241 .init = dctcp_init, 242 .in_ack_event = dctcp_update_alpha, 243 .cwnd_event = dctcp_cwnd_event, 244 .ssthresh = dctcp_ssthresh, 245 .cong_avoid = tcp_reno_cong_avoid, 246 .undo_cwnd = dctcp_cwnd_undo, 247 .set_state = dctcp_state, 248 .get_info = dctcp_get_info, 249 .flags = TCP_CONG_NEEDS_ECN, 250 .owner = THIS_MODULE, 251 .name = "dctcp", 252 }; 253 254 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 255 .ssthresh = tcp_reno_ssthresh, 256 .cong_avoid = tcp_reno_cong_avoid, 257 .undo_cwnd = tcp_reno_undo_cwnd, 258 .get_info = dctcp_get_info, 259 .owner = THIS_MODULE, 260 .name = "dctcp-reno", 261 }; 262 263 BTF_SET8_START(tcp_dctcp_check_kfunc_ids) 264 #ifdef CONFIG_X86 265 #ifdef CONFIG_DYNAMIC_FTRACE 266 BTF_ID_FLAGS(func, dctcp_init) 267 BTF_ID_FLAGS(func, dctcp_update_alpha) 268 BTF_ID_FLAGS(func, dctcp_cwnd_event) 269 BTF_ID_FLAGS(func, dctcp_ssthresh) 270 BTF_ID_FLAGS(func, dctcp_cwnd_undo) 271 BTF_ID_FLAGS(func, dctcp_state) 272 #endif 273 #endif 274 BTF_SET8_END(tcp_dctcp_check_kfunc_ids) 275 276 static const struct btf_kfunc_id_set tcp_dctcp_kfunc_set = { 277 .owner = THIS_MODULE, 278 .set = &tcp_dctcp_check_kfunc_ids, 279 }; 280 281 static int __init dctcp_register(void) 282 { 283 int ret; 284 285 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 286 287 ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &tcp_dctcp_kfunc_set); 288 if (ret < 0) 289 return ret; 290 return tcp_register_congestion_control(&dctcp); 291 } 292 293 static void __exit dctcp_unregister(void) 294 { 295 tcp_unregister_congestion_control(&dctcp); 296 } 297 298 module_init(dctcp_register); 299 module_exit(dctcp_unregister); 300 301 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 302 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 303 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 304 305 MODULE_LICENSE("GPL v2"); 306 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 307