xref: /openbmc/linux/net/ipv4/tcp_cdg.c (revision 160b8e75)
1 /*
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 <linux/sched/clock.h>
31 
32 #include <net/tcp.h>
33 
34 #define HYSTART_ACK_TRAIN	1
35 #define HYSTART_DELAY		2
36 
37 static int window __read_mostly = 8;
38 static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
39 static unsigned int backoff_factor __read_mostly = 42;
40 static unsigned int hystart_detect __read_mostly = 3;
41 static unsigned int use_ineff __read_mostly = 5;
42 static bool use_shadow __read_mostly = true;
43 static bool use_tolerance __read_mostly;
44 
45 module_param(window, int, 0444);
46 MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
47 module_param(backoff_beta, uint, 0644);
48 MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
49 module_param(backoff_factor, uint, 0644);
50 MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
51 module_param(hystart_detect, uint, 0644);
52 MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
53 		 "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
54 module_param(use_ineff, uint, 0644);
55 MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
56 module_param(use_shadow, bool, 0644);
57 MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
58 module_param(use_tolerance, bool, 0644);
59 MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
60 
61 struct cdg_minmax {
62 	union {
63 		struct {
64 			s32 min;
65 			s32 max;
66 		};
67 		u64 v64;
68 	};
69 };
70 
71 enum cdg_state {
72 	CDG_UNKNOWN = 0,
73 	CDG_NONFULL = 1,
74 	CDG_FULL    = 2,
75 	CDG_BACKOFF = 3,
76 };
77 
78 struct cdg {
79 	struct cdg_minmax rtt;
80 	struct cdg_minmax rtt_prev;
81 	struct cdg_minmax *gradients;
82 	struct cdg_minmax gsum;
83 	bool gfilled;
84 	u8  tail;
85 	u8  state;
86 	u8  delack;
87 	u32 rtt_seq;
88 	u32 shadow_wnd;
89 	u16 backoff_cnt;
90 	u16 sample_cnt;
91 	s32 delay_min;
92 	u32 last_ack;
93 	u32 round_start;
94 };
95 
96 /**
97  * nexp_u32 - negative base-e exponential
98  * @ux: x in units of micro
99  *
100  * Returns exp(ux * -1e-6) * U32_MAX.
101  */
102 static u32 __pure nexp_u32(u32 ux)
103 {
104 	static const u16 v[] = {
105 		/* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
106 		65535,
107 		65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
108 		61378, 57484, 50423, 38795, 22965, 8047,  987,   14,
109 	};
110 	u32 msb = ux >> 8;
111 	u32 res;
112 	int i;
113 
114 	/* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
115 	if (msb > U16_MAX)
116 		return 0;
117 
118 	/* Scale first eight bits linearly: */
119 	res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);
120 
121 	/* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
122 	for (i = 1; msb; i++, msb >>= 1) {
123 		u32 y = v[i & -(msb & 1)] + U32_C(1);
124 
125 		res = ((u64)res * y) >> 16;
126 	}
127 
128 	return res;
129 }
130 
131 /* Based on the HyStart algorithm (by Ha et al.) that is implemented in
132  * tcp_cubic. Differences/experimental changes:
133  *   o Using Hayes' delayed ACK filter.
134  *   o Using a usec clock for the ACK train.
135  *   o Reset ACK train when application limited.
136  *   o Invoked at any cwnd (i.e. also when cwnd < 16).
137  *   o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
138  */
139 static void tcp_cdg_hystart_update(struct sock *sk)
140 {
141 	struct cdg *ca = inet_csk_ca(sk);
142 	struct tcp_sock *tp = tcp_sk(sk);
143 
144 	ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
145 	if (ca->delay_min == 0)
146 		return;
147 
148 	if (hystart_detect & HYSTART_ACK_TRAIN) {
149 		u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);
150 
151 		if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) {
152 			ca->last_ack = now_us;
153 			ca->round_start = now_us;
154 		} else if (before(now_us, ca->last_ack + 3000)) {
155 			u32 base_owd = max(ca->delay_min / 2U, 125U);
156 
157 			ca->last_ack = now_us;
158 			if (after(now_us, ca->round_start + base_owd)) {
159 				NET_INC_STATS(sock_net(sk),
160 					      LINUX_MIB_TCPHYSTARTTRAINDETECT);
161 				NET_ADD_STATS(sock_net(sk),
162 					      LINUX_MIB_TCPHYSTARTTRAINCWND,
163 					      tp->snd_cwnd);
164 				tp->snd_ssthresh = tp->snd_cwnd;
165 				return;
166 			}
167 		}
168 	}
169 
170 	if (hystart_detect & HYSTART_DELAY) {
171 		if (ca->sample_cnt < 8) {
172 			ca->sample_cnt++;
173 		} else {
174 			s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
175 					 125U);
176 
177 			if (ca->rtt.min > thresh) {
178 				NET_INC_STATS(sock_net(sk),
179 					      LINUX_MIB_TCPHYSTARTDELAYDETECT);
180 				NET_ADD_STATS(sock_net(sk),
181 					      LINUX_MIB_TCPHYSTARTDELAYCWND,
182 					      tp->snd_cwnd);
183 				tp->snd_ssthresh = tp->snd_cwnd;
184 			}
185 		}
186 	}
187 }
188 
189 static s32 tcp_cdg_grad(struct cdg *ca)
190 {
191 	s32 gmin = ca->rtt.min - ca->rtt_prev.min;
192 	s32 gmax = ca->rtt.max - ca->rtt_prev.max;
193 	s32 grad;
194 
195 	if (ca->gradients) {
196 		ca->gsum.min += gmin - ca->gradients[ca->tail].min;
197 		ca->gsum.max += gmax - ca->gradients[ca->tail].max;
198 		ca->gradients[ca->tail].min = gmin;
199 		ca->gradients[ca->tail].max = gmax;
200 		ca->tail = (ca->tail + 1) & (window - 1);
201 		gmin = ca->gsum.min;
202 		gmax = ca->gsum.max;
203 	}
204 
205 	/* We keep sums to ignore gradients during cwnd reductions;
206 	 * the paper's smoothed gradients otherwise simplify to:
207 	 * (rtt_latest - rtt_oldest) / window.
208 	 *
209 	 * We also drop division by window here.
210 	 */
211 	grad = gmin > 0 ? gmin : gmax;
212 
213 	/* Extrapolate missing values in gradient window: */
214 	if (!ca->gfilled) {
215 		if (!ca->gradients && window > 1)
216 			grad *= window; /* Memory allocation failed. */
217 		else if (ca->tail == 0)
218 			ca->gfilled = true;
219 		else
220 			grad = (grad * window) / (int)ca->tail;
221 	}
222 
223 	/* Backoff was effectual: */
224 	if (gmin <= -32 || gmax <= -32)
225 		ca->backoff_cnt = 0;
226 
227 	if (use_tolerance) {
228 		/* Reduce small variations to zero: */
229 		gmin = DIV_ROUND_CLOSEST(gmin, 64);
230 		gmax = DIV_ROUND_CLOSEST(gmax, 64);
231 
232 		if (gmin > 0 && gmax <= 0)
233 			ca->state = CDG_FULL;
234 		else if ((gmin > 0 && gmax > 0) || gmax < 0)
235 			ca->state = CDG_NONFULL;
236 	}
237 	return grad;
238 }
239 
240 static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
241 {
242 	struct cdg *ca = inet_csk_ca(sk);
243 	struct tcp_sock *tp = tcp_sk(sk);
244 
245 	if (prandom_u32() <= nexp_u32(grad * backoff_factor))
246 		return false;
247 
248 	if (use_ineff) {
249 		ca->backoff_cnt++;
250 		if (ca->backoff_cnt > use_ineff)
251 			return false;
252 	}
253 
254 	ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
255 	ca->state = CDG_BACKOFF;
256 	tcp_enter_cwr(sk);
257 	return true;
258 }
259 
260 /* Not called in CWR or Recovery state. */
261 static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
262 {
263 	struct cdg *ca = inet_csk_ca(sk);
264 	struct tcp_sock *tp = tcp_sk(sk);
265 	u32 prior_snd_cwnd;
266 	u32 incr;
267 
268 	if (tcp_in_slow_start(tp) && hystart_detect)
269 		tcp_cdg_hystart_update(sk);
270 
271 	if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
272 		s32 grad = 0;
273 
274 		if (ca->rtt_prev.v64)
275 			grad = tcp_cdg_grad(ca);
276 		ca->rtt_seq = tp->snd_nxt;
277 		ca->rtt_prev = ca->rtt;
278 		ca->rtt.v64 = 0;
279 		ca->last_ack = 0;
280 		ca->sample_cnt = 0;
281 
282 		if (grad > 0 && tcp_cdg_backoff(sk, grad))
283 			return;
284 	}
285 
286 	if (!tcp_is_cwnd_limited(sk)) {
287 		ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
288 		return;
289 	}
290 
291 	prior_snd_cwnd = tp->snd_cwnd;
292 	tcp_reno_cong_avoid(sk, ack, acked);
293 
294 	incr = tp->snd_cwnd - prior_snd_cwnd;
295 	ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
296 }
297 
298 static void tcp_cdg_acked(struct sock *sk, const struct ack_sample *sample)
299 {
300 	struct cdg *ca = inet_csk_ca(sk);
301 	struct tcp_sock *tp = tcp_sk(sk);
302 
303 	if (sample->rtt_us <= 0)
304 		return;
305 
306 	/* A heuristic for filtering delayed ACKs, adapted from:
307 	 * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
308 	 * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
309 	 */
310 	if (tp->sacked_out == 0) {
311 		if (sample->pkts_acked == 1 && ca->delack) {
312 			/* A delayed ACK is only used for the minimum if it is
313 			 * provenly lower than an existing non-zero minimum.
314 			 */
315 			ca->rtt.min = min(ca->rtt.min, sample->rtt_us);
316 			ca->delack--;
317 			return;
318 		} else if (sample->pkts_acked > 1 && ca->delack < 5) {
319 			ca->delack++;
320 		}
321 	}
322 
323 	ca->rtt.min = min_not_zero(ca->rtt.min, sample->rtt_us);
324 	ca->rtt.max = max(ca->rtt.max, sample->rtt_us);
325 }
326 
327 static u32 tcp_cdg_ssthresh(struct sock *sk)
328 {
329 	struct cdg *ca = inet_csk_ca(sk);
330 	struct tcp_sock *tp = tcp_sk(sk);
331 
332 	if (ca->state == CDG_BACKOFF)
333 		return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
334 
335 	if (ca->state == CDG_NONFULL && use_tolerance)
336 		return tp->snd_cwnd;
337 
338 	ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
339 	if (use_shadow)
340 		return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
341 	return max(2U, tp->snd_cwnd >> 1);
342 }
343 
344 static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
345 {
346 	struct cdg *ca = inet_csk_ca(sk);
347 	struct tcp_sock *tp = tcp_sk(sk);
348 	struct cdg_minmax *gradients;
349 
350 	switch (ev) {
351 	case CA_EVENT_CWND_RESTART:
352 		gradients = ca->gradients;
353 		if (gradients)
354 			memset(gradients, 0, window * sizeof(gradients[0]));
355 		memset(ca, 0, sizeof(*ca));
356 
357 		ca->gradients = gradients;
358 		ca->rtt_seq = tp->snd_nxt;
359 		ca->shadow_wnd = tp->snd_cwnd;
360 		break;
361 	case CA_EVENT_COMPLETE_CWR:
362 		ca->state = CDG_UNKNOWN;
363 		ca->rtt_seq = tp->snd_nxt;
364 		ca->rtt_prev = ca->rtt;
365 		ca->rtt.v64 = 0;
366 		break;
367 	default:
368 		break;
369 	}
370 }
371 
372 static void tcp_cdg_init(struct sock *sk)
373 {
374 	struct cdg *ca = inet_csk_ca(sk);
375 	struct tcp_sock *tp = tcp_sk(sk);
376 
377 	/* We silently fall back to window = 1 if allocation fails. */
378 	if (window > 1)
379 		ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
380 					GFP_NOWAIT | __GFP_NOWARN);
381 	ca->rtt_seq = tp->snd_nxt;
382 	ca->shadow_wnd = tp->snd_cwnd;
383 }
384 
385 static void tcp_cdg_release(struct sock *sk)
386 {
387 	struct cdg *ca = inet_csk_ca(sk);
388 
389 	kfree(ca->gradients);
390 }
391 
392 static struct tcp_congestion_ops tcp_cdg __read_mostly = {
393 	.cong_avoid = tcp_cdg_cong_avoid,
394 	.cwnd_event = tcp_cdg_cwnd_event,
395 	.pkts_acked = tcp_cdg_acked,
396 	.undo_cwnd = tcp_reno_undo_cwnd,
397 	.ssthresh = tcp_cdg_ssthresh,
398 	.release = tcp_cdg_release,
399 	.init = tcp_cdg_init,
400 	.owner = THIS_MODULE,
401 	.name = "cdg",
402 };
403 
404 static int __init tcp_cdg_register(void)
405 {
406 	if (backoff_beta > 1024 || window < 1 || window > 256)
407 		return -ERANGE;
408 	if (!is_power_of_2(window))
409 		return -EINVAL;
410 
411 	BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
412 	tcp_register_congestion_control(&tcp_cdg);
413 	return 0;
414 }
415 
416 static void __exit tcp_cdg_unregister(void)
417 {
418 	tcp_unregister_congestion_control(&tcp_cdg);
419 }
420 
421 module_init(tcp_cdg_register);
422 module_exit(tcp_cdg_unregister);
423 MODULE_AUTHOR("Kenneth Klette Jonassen");
424 MODULE_LICENSE("GPL");
425 MODULE_DESCRIPTION("TCP CDG");
426