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)");
314
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