xref: /openbmc/linux/net/ipv4/tcp_cubic.c (revision 22246614)
1 /*
2  * TCP CUBIC: Binary Increase Congestion control for TCP v2.2
3  * Home page:
4  *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
5  * This is from the implementation of CUBIC TCP in
6  * Injong Rhee, Lisong Xu.
7  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant
8  *  in PFLDnet 2005
9  * Available from:
10  *  http://netsrv.csc.ncsu.edu/export/cubic-paper.pdf
11  *
12  * Unless CUBIC is enabled and congestion window is large
13  * this behaves the same as the original Reno.
14  */
15 
16 #include <linux/mm.h>
17 #include <linux/module.h>
18 #include <linux/math64.h>
19 #include <net/tcp.h>
20 
21 #define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
22 					 * max_cwnd = snd_cwnd * beta
23 					 */
24 #define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
25 
26 static int fast_convergence __read_mostly = 1;
27 static int beta __read_mostly = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
28 static int initial_ssthresh __read_mostly;
29 static int bic_scale __read_mostly = 41;
30 static int tcp_friendliness __read_mostly = 1;
31 
32 static u32 cube_rtt_scale __read_mostly;
33 static u32 beta_scale __read_mostly;
34 static u64 cube_factor __read_mostly;
35 
36 /* Note parameters that are used for precomputing scale factors are read-only */
37 module_param(fast_convergence, int, 0644);
38 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
39 module_param(beta, int, 0644);
40 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
41 module_param(initial_ssthresh, int, 0644);
42 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
43 module_param(bic_scale, int, 0444);
44 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
45 module_param(tcp_friendliness, int, 0644);
46 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
47 
48 /* BIC TCP Parameters */
49 struct bictcp {
50 	u32	cnt;		/* increase cwnd by 1 after ACKs */
51 	u32 	last_max_cwnd;	/* last maximum snd_cwnd */
52 	u32	loss_cwnd;	/* congestion window at last loss */
53 	u32	last_cwnd;	/* the last snd_cwnd */
54 	u32	last_time;	/* time when updated last_cwnd */
55 	u32	bic_origin_point;/* origin point of bic function */
56 	u32	bic_K;		/* time to origin point from the beginning of the current epoch */
57 	u32	delay_min;	/* min delay */
58 	u32	epoch_start;	/* beginning of an epoch */
59 	u32	ack_cnt;	/* number of acks */
60 	u32	tcp_cwnd;	/* estimated tcp cwnd */
61 #define ACK_RATIO_SHIFT	4
62 	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
63 };
64 
65 static inline void bictcp_reset(struct bictcp *ca)
66 {
67 	ca->cnt = 0;
68 	ca->last_max_cwnd = 0;
69 	ca->loss_cwnd = 0;
70 	ca->last_cwnd = 0;
71 	ca->last_time = 0;
72 	ca->bic_origin_point = 0;
73 	ca->bic_K = 0;
74 	ca->delay_min = 0;
75 	ca->epoch_start = 0;
76 	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
77 	ca->ack_cnt = 0;
78 	ca->tcp_cwnd = 0;
79 }
80 
81 static void bictcp_init(struct sock *sk)
82 {
83 	bictcp_reset(inet_csk_ca(sk));
84 	if (initial_ssthresh)
85 		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
86 }
87 
88 /* calculate the cubic root of x using a table lookup followed by one
89  * Newton-Raphson iteration.
90  * Avg err ~= 0.195%
91  */
92 static u32 cubic_root(u64 a)
93 {
94 	u32 x, b, shift;
95 	/*
96 	 * cbrt(x) MSB values for x MSB values in [0..63].
97 	 * Precomputed then refined by hand - Willy Tarreau
98 	 *
99 	 * For x in [0..63],
100 	 *   v = cbrt(x << 18) - 1
101 	 *   cbrt(x) = (v[x] + 10) >> 6
102 	 */
103 	static const u8 v[] = {
104 		/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
105 		/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
106 		/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
107 		/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
108 		/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
109 		/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
110 		/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
111 		/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
112 	};
113 
114 	b = fls64(a);
115 	if (b < 7) {
116 		/* a in [0..63] */
117 		return ((u32)v[(u32)a] + 35) >> 6;
118 	}
119 
120 	b = ((b * 84) >> 8) - 1;
121 	shift = (a >> (b * 3));
122 
123 	x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
124 
125 	/*
126 	 * Newton-Raphson iteration
127 	 *                         2
128 	 * x    = ( 2 * x  +  a / x  ) / 3
129 	 *  k+1          k         k
130 	 */
131 	x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
132 	x = ((x * 341) >> 10);
133 	return x;
134 }
135 
136 /*
137  * Compute congestion window to use.
138  */
139 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
140 {
141 	u64 offs;
142 	u32 delta, t, bic_target, max_cnt;
143 
144 	ca->ack_cnt++;	/* count the number of ACKs */
145 
146 	if (ca->last_cwnd == cwnd &&
147 	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
148 		return;
149 
150 	ca->last_cwnd = cwnd;
151 	ca->last_time = tcp_time_stamp;
152 
153 	if (ca->epoch_start == 0) {
154 		ca->epoch_start = tcp_time_stamp;	/* record the beginning of an epoch */
155 		ca->ack_cnt = 1;			/* start counting */
156 		ca->tcp_cwnd = cwnd;			/* syn with cubic */
157 
158 		if (ca->last_max_cwnd <= cwnd) {
159 			ca->bic_K = 0;
160 			ca->bic_origin_point = cwnd;
161 		} else {
162 			/* Compute new K based on
163 			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
164 			 */
165 			ca->bic_K = cubic_root(cube_factor
166 					       * (ca->last_max_cwnd - cwnd));
167 			ca->bic_origin_point = ca->last_max_cwnd;
168 		}
169 	}
170 
171 	/* cubic function - calc*/
172 	/* calculate c * time^3 / rtt,
173 	 *  while considering overflow in calculation of time^3
174 	 * (so time^3 is done by using 64 bit)
175 	 * and without the support of division of 64bit numbers
176 	 * (so all divisions are done by using 32 bit)
177 	 *  also NOTE the unit of those veriables
178 	 *	  time  = (t - K) / 2^bictcp_HZ
179 	 *	  c = bic_scale >> 10
180 	 * rtt  = (srtt >> 3) / HZ
181 	 * !!! The following code does not have overflow problems,
182 	 * if the cwnd < 1 million packets !!!
183 	 */
184 
185 	/* change the unit from HZ to bictcp_HZ */
186 	t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
187 	     << BICTCP_HZ) / HZ;
188 
189 	if (t < ca->bic_K)		/* t - K */
190 		offs = ca->bic_K - t;
191 	else
192 		offs = t - ca->bic_K;
193 
194 	/* c/rtt * (t-K)^3 */
195 	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
196 	if (t < ca->bic_K)                                	/* below origin*/
197 		bic_target = ca->bic_origin_point - delta;
198 	else                                                	/* above origin*/
199 		bic_target = ca->bic_origin_point + delta;
200 
201 	/* cubic function - calc bictcp_cnt*/
202 	if (bic_target > cwnd) {
203 		ca->cnt = cwnd / (bic_target - cwnd);
204 	} else {
205 		ca->cnt = 100 * cwnd;              /* very small increment*/
206 	}
207 
208 	/* TCP Friendly */
209 	if (tcp_friendliness) {
210 		u32 scale = beta_scale;
211 		delta = (cwnd * scale) >> 3;
212 		while (ca->ack_cnt > delta) {		/* update tcp cwnd */
213 			ca->ack_cnt -= delta;
214 			ca->tcp_cwnd++;
215 		}
216 
217 		if (ca->tcp_cwnd > cwnd){	/* if bic is slower than tcp */
218 			delta = ca->tcp_cwnd - cwnd;
219 			max_cnt = cwnd / delta;
220 			if (ca->cnt > max_cnt)
221 				ca->cnt = max_cnt;
222 		}
223 	}
224 
225 	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
226 	if (ca->cnt == 0)			/* cannot be zero */
227 		ca->cnt = 1;
228 }
229 
230 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
231 {
232 	struct tcp_sock *tp = tcp_sk(sk);
233 	struct bictcp *ca = inet_csk_ca(sk);
234 
235 	if (!tcp_is_cwnd_limited(sk, in_flight))
236 		return;
237 
238 	if (tp->snd_cwnd <= tp->snd_ssthresh)
239 		tcp_slow_start(tp);
240 	else {
241 		bictcp_update(ca, tp->snd_cwnd);
242 
243 		/* In dangerous area, increase slowly.
244 		 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
245 		 */
246 		if (tp->snd_cwnd_cnt >= ca->cnt) {
247 			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
248 				tp->snd_cwnd++;
249 			tp->snd_cwnd_cnt = 0;
250 		} else
251 			tp->snd_cwnd_cnt++;
252 	}
253 
254 }
255 
256 static u32 bictcp_recalc_ssthresh(struct sock *sk)
257 {
258 	const struct tcp_sock *tp = tcp_sk(sk);
259 	struct bictcp *ca = inet_csk_ca(sk);
260 
261 	ca->epoch_start = 0;	/* end of epoch */
262 
263 	/* Wmax and fast convergence */
264 	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
265 		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
266 			/ (2 * BICTCP_BETA_SCALE);
267 	else
268 		ca->last_max_cwnd = tp->snd_cwnd;
269 
270 	ca->loss_cwnd = tp->snd_cwnd;
271 
272 	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
273 }
274 
275 static u32 bictcp_undo_cwnd(struct sock *sk)
276 {
277 	struct bictcp *ca = inet_csk_ca(sk);
278 
279 	return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
280 }
281 
282 static void bictcp_state(struct sock *sk, u8 new_state)
283 {
284 	if (new_state == TCP_CA_Loss)
285 		bictcp_reset(inet_csk_ca(sk));
286 }
287 
288 /* Track delayed acknowledgment ratio using sliding window
289  * ratio = (15*ratio + sample) / 16
290  */
291 static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
292 {
293 	const struct inet_connection_sock *icsk = inet_csk(sk);
294 	struct bictcp *ca = inet_csk_ca(sk);
295 	u32 delay;
296 
297 	if (icsk->icsk_ca_state == TCP_CA_Open) {
298 		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
299 		ca->delayed_ack += cnt;
300 	}
301 
302 	/* Some calls are for duplicates without timetamps */
303 	if (rtt_us < 0)
304 		return;
305 
306 	/* Discard delay samples right after fast recovery */
307 	if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
308 		return;
309 
310 	delay = usecs_to_jiffies(rtt_us) << 3;
311 	if (delay == 0)
312 		delay = 1;
313 
314 	/* first time call or link delay decreases */
315 	if (ca->delay_min == 0 || ca->delay_min > delay)
316 		ca->delay_min = delay;
317 }
318 
319 static struct tcp_congestion_ops cubictcp = {
320 	.init		= bictcp_init,
321 	.ssthresh	= bictcp_recalc_ssthresh,
322 	.cong_avoid	= bictcp_cong_avoid,
323 	.set_state	= bictcp_state,
324 	.undo_cwnd	= bictcp_undo_cwnd,
325 	.pkts_acked     = bictcp_acked,
326 	.owner		= THIS_MODULE,
327 	.name		= "cubic",
328 };
329 
330 static int __init cubictcp_register(void)
331 {
332 	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
333 
334 	/* Precompute a bunch of the scaling factors that are used per-packet
335 	 * based on SRTT of 100ms
336 	 */
337 
338 	beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
339 
340 	cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
341 
342 	/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
343 	 *  so K = cubic_root( (wmax-cwnd)*rtt/c )
344 	 * the unit of K is bictcp_HZ=2^10, not HZ
345 	 *
346 	 *  c = bic_scale >> 10
347 	 *  rtt = 100ms
348 	 *
349 	 * the following code has been designed and tested for
350 	 * cwnd < 1 million packets
351 	 * RTT < 100 seconds
352 	 * HZ < 1,000,00  (corresponding to 10 nano-second)
353 	 */
354 
355 	/* 1/c * 2^2*bictcp_HZ * srtt */
356 	cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
357 
358 	/* divide by bic_scale and by constant Srtt (100ms) */
359 	do_div(cube_factor, bic_scale * 10);
360 
361 	return tcp_register_congestion_control(&cubictcp);
362 }
363 
364 static void __exit cubictcp_unregister(void)
365 {
366 	tcp_unregister_congestion_control(&cubictcp);
367 }
368 
369 module_init(cubictcp_register);
370 module_exit(cubictcp_unregister);
371 
372 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
373 MODULE_LICENSE("GPL");
374 MODULE_DESCRIPTION("CUBIC TCP");
375 MODULE_VERSION("2.2");
376