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Kenneth Klette Jonassen2b0a8c92015-06-10 19:08:17 +02001/*
2 * CAIA Delay-Gradient (CDG) congestion control
3 *
4 * This implementation is based on the paper:
5 * D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
6 * delay gradients." In IFIP Networking, pages 328-341. Springer, 2011.
7 *
8 * Scavenger traffic (Less-than-Best-Effort) should disable coexistence
9 * heuristics using parameters use_shadow=0 and use_ineff=0.
10 *
11 * Parameters window, backoff_beta, and backoff_factor are crucial for
12 * throughput and delay. Future work is needed to determine better defaults,
13 * and to provide guidelines for use in different environments/contexts.
14 *
15 * Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/.
16 * Parameter window is only configurable when loading tcp_cdg as a module.
17 *
18 * Notable differences from paper/FreeBSD:
19 * o Using Hybrid Slow start and Proportional Rate Reduction.
20 * o Add toggle for shadow window mechanism. Suggested by David Hayes.
21 * o Add toggle for non-congestion loss tolerance.
22 * o Scaling parameter G is changed to a backoff factor;
23 * conversion is given by: backoff_factor = 1000/(G * window).
24 * o Limit shadow window to 2 * cwnd, or to cwnd when application limited.
25 * o More accurate e^-x.
26 */
27#include <linux/kernel.h>
28#include <linux/random.h>
29#include <linux/module.h>
30#include <net/tcp.h>
31
32#define HYSTART_ACK_TRAIN 1
33#define HYSTART_DELAY 2
34
35static int window __read_mostly = 8;
36static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
37static unsigned int backoff_factor __read_mostly = 42;
38static unsigned int hystart_detect __read_mostly = 3;
39static unsigned int use_ineff __read_mostly = 5;
40static bool use_shadow __read_mostly = true;
41static bool use_tolerance __read_mostly;
42
43module_param(window, int, 0444);
44MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
45module_param(backoff_beta, uint, 0644);
46MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
47module_param(backoff_factor, uint, 0644);
48MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
49module_param(hystart_detect, uint, 0644);
50MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
51 "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
52module_param(use_ineff, uint, 0644);
53MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
54module_param(use_shadow, bool, 0644);
55MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
56module_param(use_tolerance, bool, 0644);
57MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
58
59struct minmax {
60 union {
61 struct {
62 s32 min;
63 s32 max;
64 };
65 u64 v64;
66 };
67};
68
69enum cdg_state {
70 CDG_UNKNOWN = 0,
71 CDG_NONFULL = 1,
72 CDG_FULL = 2,
73 CDG_BACKOFF = 3,
74};
75
76struct cdg {
77 struct minmax rtt;
78 struct minmax rtt_prev;
79 struct minmax *gradients;
80 struct minmax gsum;
81 bool gfilled;
82 u8 tail;
83 u8 state;
84 u8 delack;
85 u32 rtt_seq;
86 u32 undo_cwnd;
87 u32 shadow_wnd;
88 u16 backoff_cnt;
89 u16 sample_cnt;
90 s32 delay_min;
91 u32 last_ack;
92 u32 round_start;
93};
94
95/**
96 * nexp_u32 - negative base-e exponential
97 * @ux: x in units of micro
98 *
99 * Returns exp(ux * -1e-6) * U32_MAX.
100 */
101static u32 __pure nexp_u32(u32 ux)
102{
103 static const u16 v[] = {
104 /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
105 65535,
106 65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
107 61378, 57484, 50423, 38795, 22965, 8047, 987, 14,
108 };
109 u32 msb = ux >> 8;
110 u32 res;
111 int i;
112
113 /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
114 if (msb > U16_MAX)
115 return 0;
116
117 /* Scale first eight bits linearly: */
118 res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);
119
120 /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
121 for (i = 1; msb; i++, msb >>= 1) {
122 u32 y = v[i & -(msb & 1)] + U32_C(1);
123
124 res = ((u64)res * y) >> 16;
125 }
126
127 return res;
128}
129
130/* Based on the HyStart algorithm (by Ha et al.) that is implemented in
131 * tcp_cubic. Differences/experimental changes:
132 * o Using Hayes' delayed ACK filter.
133 * o Using a usec clock for the ACK train.
134 * o Reset ACK train when application limited.
135 * o Invoked at any cwnd (i.e. also when cwnd < 16).
136 * o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
137 */
138static void tcp_cdg_hystart_update(struct sock *sk)
139{
140 struct cdg *ca = inet_csk_ca(sk);
141 struct tcp_sock *tp = tcp_sk(sk);
142
143 ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
144 if (ca->delay_min == 0)
145 return;
146
147 if (hystart_detect & HYSTART_ACK_TRAIN) {
Kenneth Klette Jonassen758f0d42015-06-12 17:24:03 +0200148 u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);
Kenneth Klette Jonassen2b0a8c92015-06-10 19:08:17 +0200149
150 if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) {
151 ca->last_ack = now_us;
152 ca->round_start = now_us;
153 } else if (before(now_us, ca->last_ack + 3000)) {
154 u32 base_owd = max(ca->delay_min / 2U, 125U);
155
156 ca->last_ack = now_us;
157 if (after(now_us, ca->round_start + base_owd)) {
158 NET_INC_STATS_BH(sock_net(sk),
159 LINUX_MIB_TCPHYSTARTTRAINDETECT);
160 NET_ADD_STATS_BH(sock_net(sk),
161 LINUX_MIB_TCPHYSTARTTRAINCWND,
162 tp->snd_cwnd);
163 tp->snd_ssthresh = tp->snd_cwnd;
164 return;
165 }
166 }
167 }
168
169 if (hystart_detect & HYSTART_DELAY) {
170 if (ca->sample_cnt < 8) {
171 ca->sample_cnt++;
172 } else {
173 s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
174 125U);
175
176 if (ca->rtt.min > thresh) {
177 NET_INC_STATS_BH(sock_net(sk),
178 LINUX_MIB_TCPHYSTARTDELAYDETECT);
179 NET_ADD_STATS_BH(sock_net(sk),
180 LINUX_MIB_TCPHYSTARTDELAYCWND,
181 tp->snd_cwnd);
182 tp->snd_ssthresh = tp->snd_cwnd;
183 }
184 }
185 }
186}
187
188static s32 tcp_cdg_grad(struct cdg *ca)
189{
190 s32 gmin = ca->rtt.min - ca->rtt_prev.min;
191 s32 gmax = ca->rtt.max - ca->rtt_prev.max;
192 s32 grad;
193
194 if (ca->gradients) {
195 ca->gsum.min += gmin - ca->gradients[ca->tail].min;
196 ca->gsum.max += gmax - ca->gradients[ca->tail].max;
197 ca->gradients[ca->tail].min = gmin;
198 ca->gradients[ca->tail].max = gmax;
199 ca->tail = (ca->tail + 1) & (window - 1);
200 gmin = ca->gsum.min;
201 gmax = ca->gsum.max;
202 }
203
204 /* We keep sums to ignore gradients during cwnd reductions;
205 * the paper's smoothed gradients otherwise simplify to:
206 * (rtt_latest - rtt_oldest) / window.
207 *
208 * We also drop division by window here.
209 */
210 grad = gmin > 0 ? gmin : gmax;
211
212 /* Extrapolate missing values in gradient window: */
213 if (!ca->gfilled) {
214 if (!ca->gradients && window > 1)
215 grad *= window; /* Memory allocation failed. */
216 else if (ca->tail == 0)
217 ca->gfilled = true;
218 else
219 grad = (grad * window) / (int)ca->tail;
220 }
221
222 /* Backoff was effectual: */
223 if (gmin <= -32 || gmax <= -32)
224 ca->backoff_cnt = 0;
225
226 if (use_tolerance) {
227 /* Reduce small variations to zero: */
228 gmin = DIV_ROUND_CLOSEST(gmin, 64);
229 gmax = DIV_ROUND_CLOSEST(gmax, 64);
230
231 if (gmin > 0 && gmax <= 0)
232 ca->state = CDG_FULL;
233 else if ((gmin > 0 && gmax > 0) || gmax < 0)
234 ca->state = CDG_NONFULL;
235 }
236 return grad;
237}
238
239static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
240{
241 struct cdg *ca = inet_csk_ca(sk);
242 struct tcp_sock *tp = tcp_sk(sk);
243
244 if (prandom_u32() <= nexp_u32(grad * backoff_factor))
245 return false;
246
247 if (use_ineff) {
248 ca->backoff_cnt++;
249 if (ca->backoff_cnt > use_ineff)
250 return false;
251 }
252
253 ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
254 ca->state = CDG_BACKOFF;
255 tcp_enter_cwr(sk);
256 return true;
257}
258
259/* Not called in CWR or Recovery state. */
260static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
261{
262 struct cdg *ca = inet_csk_ca(sk);
263 struct tcp_sock *tp = tcp_sk(sk);
264 u32 prior_snd_cwnd;
265 u32 incr;
266
Yuchung Cheng76174002015-07-09 13:16:30 -0700267 if (tcp_in_slow_start(tp) && hystart_detect)
Kenneth Klette Jonassen2b0a8c92015-06-10 19:08:17 +0200268 tcp_cdg_hystart_update(sk);
269
270 if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
271 s32 grad = 0;
272
273 if (ca->rtt_prev.v64)
274 grad = tcp_cdg_grad(ca);
275 ca->rtt_seq = tp->snd_nxt;
276 ca->rtt_prev = ca->rtt;
277 ca->rtt.v64 = 0;
278 ca->last_ack = 0;
279 ca->sample_cnt = 0;
280
281 if (grad > 0 && tcp_cdg_backoff(sk, grad))
282 return;
283 }
284
285 if (!tcp_is_cwnd_limited(sk)) {
286 ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
287 return;
288 }
289
290 prior_snd_cwnd = tp->snd_cwnd;
291 tcp_reno_cong_avoid(sk, ack, acked);
292
293 incr = tp->snd_cwnd - prior_snd_cwnd;
294 ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
295}
296
297static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
298{
299 struct cdg *ca = inet_csk_ca(sk);
300 struct tcp_sock *tp = tcp_sk(sk);
301
302 if (rtt_us <= 0)
303 return;
304
305 /* A heuristic for filtering delayed ACKs, adapted from:
306 * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
307 * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
308 */
309 if (tp->sacked_out == 0) {
310 if (num_acked == 1 && ca->delack) {
311 /* A delayed ACK is only used for the minimum if it is
312 * provenly lower than an existing non-zero minimum.
313 */
314 ca->rtt.min = min(ca->rtt.min, rtt_us);
315 ca->delack--;
316 return;
317 } else if (num_acked > 1 && ca->delack < 5) {
318 ca->delack++;
319 }
320 }
321
322 ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
323 ca->rtt.max = max(ca->rtt.max, rtt_us);
324}
325
326static u32 tcp_cdg_ssthresh(struct sock *sk)
327{
328 struct cdg *ca = inet_csk_ca(sk);
329 struct tcp_sock *tp = tcp_sk(sk);
330
331 ca->undo_cwnd = tp->snd_cwnd;
332
333 if (ca->state == CDG_BACKOFF)
334 return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
335
336 if (ca->state == CDG_NONFULL && use_tolerance)
337 return tp->snd_cwnd;
338
339 ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
340 if (use_shadow)
341 return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
342 return max(2U, tp->snd_cwnd >> 1);
343}
344
345static u32 tcp_cdg_undo_cwnd(struct sock *sk)
346{
347 struct cdg *ca = inet_csk_ca(sk);
348
349 return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd);
350}
351
352static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
353{
354 struct cdg *ca = inet_csk_ca(sk);
355 struct tcp_sock *tp = tcp_sk(sk);
356 struct minmax *gradients;
357
358 switch (ev) {
359 case CA_EVENT_CWND_RESTART:
360 gradients = ca->gradients;
361 if (gradients)
362 memset(gradients, 0, window * sizeof(gradients[0]));
363 memset(ca, 0, sizeof(*ca));
364
365 ca->gradients = gradients;
366 ca->rtt_seq = tp->snd_nxt;
367 ca->shadow_wnd = tp->snd_cwnd;
368 break;
369 case CA_EVENT_COMPLETE_CWR:
370 ca->state = CDG_UNKNOWN;
371 ca->rtt_seq = tp->snd_nxt;
372 ca->rtt_prev = ca->rtt;
373 ca->rtt.v64 = 0;
374 break;
375 default:
376 break;
377 }
378}
379
380static void tcp_cdg_init(struct sock *sk)
381{
382 struct cdg *ca = inet_csk_ca(sk);
383 struct tcp_sock *tp = tcp_sk(sk);
384
385 /* We silently fall back to window = 1 if allocation fails. */
386 if (window > 1)
387 ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
388 GFP_NOWAIT | __GFP_NOWARN);
389 ca->rtt_seq = tp->snd_nxt;
390 ca->shadow_wnd = tp->snd_cwnd;
391}
392
393static void tcp_cdg_release(struct sock *sk)
394{
395 struct cdg *ca = inet_csk_ca(sk);
396
397 kfree(ca->gradients);
398}
399
400struct tcp_congestion_ops tcp_cdg __read_mostly = {
401 .cong_avoid = tcp_cdg_cong_avoid,
402 .cwnd_event = tcp_cdg_cwnd_event,
403 .pkts_acked = tcp_cdg_acked,
404 .undo_cwnd = tcp_cdg_undo_cwnd,
405 .ssthresh = tcp_cdg_ssthresh,
406 .release = tcp_cdg_release,
407 .init = tcp_cdg_init,
408 .owner = THIS_MODULE,
409 .name = "cdg",
410};
411
412static int __init tcp_cdg_register(void)
413{
414 if (backoff_beta > 1024 || window < 1 || window > 256)
415 return -ERANGE;
416 if (!is_power_of_2(window))
417 return -EINVAL;
418
419 BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
420 tcp_register_congestion_control(&tcp_cdg);
421 return 0;
422}
423
424static void __exit tcp_cdg_unregister(void)
425{
426 tcp_unregister_congestion_control(&tcp_cdg);
427}
428
429module_init(tcp_cdg_register);
430module_exit(tcp_cdg_unregister);
431MODULE_AUTHOR("Kenneth Klette Jonassen");
432MODULE_LICENSE("GPL");
433MODULE_DESCRIPTION("TCP CDG");