1 // SPDX-License-Identifier: GPL-2.0-only
2 
3 /* WARNING: This implemenation is not necessarily the same
4  * as the tcp_cubic.c.  The purpose is mainly for testing
5  * the kernel BPF logic.
6  *
7  * Highlights:
8  * 1. CONFIG_HZ .kconfig map is used.
9  * 2. In bictcp_update(), calculation is changed to use usec
10  *    resolution (i.e. USEC_PER_JIFFY) instead of using jiffies.
11  *    Thus, usecs_to_jiffies() is not used in the bpf_cubic.c.
12  * 3. In bitctcp_update() [under tcp_friendliness], the original
13  *    "while (ca->ack_cnt > delta)" loop is changed to the equivalent
14  *    "ca->ack_cnt / delta" operation.
15  */
16 
17 #include <linux/bpf.h>
18 #include "bpf_tcp_helpers.h"
19 
20 char _license[] SEC("license") = "GPL";
21 
22 #define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
23 
24 #define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
25 					 * max_cwnd = snd_cwnd * beta
26 					 */
27 #define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
28 
29 /* Two methods of hybrid slow start */
30 #define HYSTART_ACK_TRAIN	0x1
31 #define HYSTART_DELAY		0x2
32 
33 /* Number of delay samples for detecting the increase of delay */
34 #define HYSTART_MIN_SAMPLES	8
35 #define HYSTART_DELAY_MIN	(4000U)	/* 4ms */
36 #define HYSTART_DELAY_MAX	(16000U)	/* 16 ms */
37 #define HYSTART_DELAY_THRESH(x)	clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
38 
39 static int fast_convergence = 1;
40 static const int beta = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
41 static int initial_ssthresh;
42 static const int bic_scale = 41;
43 static int tcp_friendliness = 1;
44 
45 static int hystart = 1;
46 static int hystart_detect = HYSTART_ACK_TRAIN | HYSTART_DELAY;
47 static int hystart_low_window = 16;
48 static int hystart_ack_delta_us = 2000;
49 
50 static const __u32 cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
51 static const __u32 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
52 				/ (BICTCP_BETA_SCALE - beta);
53 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
54  *  so K = cubic_root( (wmax-cwnd)*rtt/c )
55  * the unit of K is bictcp_HZ=2^10, not HZ
56  *
57  *  c = bic_scale >> 10
58  *  rtt = 100ms
59  *
60  * the following code has been designed and tested for
61  * cwnd < 1 million packets
62  * RTT < 100 seconds
63  * HZ < 1,000,00  (corresponding to 10 nano-second)
64  */
65 
66 /* 1/c * 2^2*bictcp_HZ * srtt, 2^40 */
67 static const __u64 cube_factor = (__u64)(1ull << (10+3*BICTCP_HZ))
68 				/ (bic_scale * 10);
69 
70 /* BIC TCP Parameters */
71 struct bictcp {
72 	__u32	cnt;		/* increase cwnd by 1 after ACKs */
73 	__u32	last_max_cwnd;	/* last maximum snd_cwnd */
74 	__u32	last_cwnd;	/* the last snd_cwnd */
75 	__u32	last_time;	/* time when updated last_cwnd */
76 	__u32	bic_origin_point;/* origin point of bic function */
77 	__u32	bic_K;		/* time to origin point
78 				   from the beginning of the current epoch */
79 	__u32	delay_min;	/* min delay (usec) */
80 	__u32	epoch_start;	/* beginning of an epoch */
81 	__u32	ack_cnt;	/* number of acks */
82 	__u32	tcp_cwnd;	/* estimated tcp cwnd */
83 	__u16	unused;
84 	__u8	sample_cnt;	/* number of samples to decide curr_rtt */
85 	__u8	found;		/* the exit point is found? */
86 	__u32	round_start;	/* beginning of each round */
87 	__u32	end_seq;	/* end_seq of the round */
88 	__u32	last_ack;	/* last time when the ACK spacing is close */
89 	__u32	curr_rtt;	/* the minimum rtt of current round */
90 };
91 
92 static inline void bictcp_reset(struct bictcp *ca)
93 {
94 	ca->cnt = 0;
95 	ca->last_max_cwnd = 0;
96 	ca->last_cwnd = 0;
97 	ca->last_time = 0;
98 	ca->bic_origin_point = 0;
99 	ca->bic_K = 0;
100 	ca->delay_min = 0;
101 	ca->epoch_start = 0;
102 	ca->ack_cnt = 0;
103 	ca->tcp_cwnd = 0;
104 	ca->found = 0;
105 }
106 
107 extern unsigned long CONFIG_HZ __kconfig;
108 #define HZ CONFIG_HZ
109 #define USEC_PER_MSEC	1000UL
110 #define USEC_PER_SEC	1000000UL
111 #define USEC_PER_JIFFY	(USEC_PER_SEC / HZ)
112 
113 static __always_inline __u64 div64_u64(__u64 dividend, __u64 divisor)
114 {
115 	return dividend / divisor;
116 }
117 
118 #define div64_ul div64_u64
119 
120 #define BITS_PER_U64 (sizeof(__u64) * 8)
121 static __always_inline int fls64(__u64 x)
122 {
123 	int num = BITS_PER_U64 - 1;
124 
125 	if (x == 0)
126 		return 0;
127 
128 	if (!(x & (~0ull << (BITS_PER_U64-32)))) {
129 		num -= 32;
130 		x <<= 32;
131 	}
132 	if (!(x & (~0ull << (BITS_PER_U64-16)))) {
133 		num -= 16;
134 		x <<= 16;
135 	}
136 	if (!(x & (~0ull << (BITS_PER_U64-8)))) {
137 		num -= 8;
138 		x <<= 8;
139 	}
140 	if (!(x & (~0ull << (BITS_PER_U64-4)))) {
141 		num -= 4;
142 		x <<= 4;
143 	}
144 	if (!(x & (~0ull << (BITS_PER_U64-2)))) {
145 		num -= 2;
146 		x <<= 2;
147 	}
148 	if (!(x & (~0ull << (BITS_PER_U64-1))))
149 		num -= 1;
150 
151 	return num + 1;
152 }
153 
154 static __always_inline __u32 bictcp_clock_us(const struct sock *sk)
155 {
156 	return tcp_sk(sk)->tcp_mstamp;
157 }
158 
159 static __always_inline void bictcp_hystart_reset(struct sock *sk)
160 {
161 	struct tcp_sock *tp = tcp_sk(sk);
162 	struct bictcp *ca = inet_csk_ca(sk);
163 
164 	ca->round_start = ca->last_ack = bictcp_clock_us(sk);
165 	ca->end_seq = tp->snd_nxt;
166 	ca->curr_rtt = ~0U;
167 	ca->sample_cnt = 0;
168 }
169 
170 /* "struct_ops/" prefix is not a requirement
171  * It will be recognized as BPF_PROG_TYPE_STRUCT_OPS
172  * as long as it is used in one of the func ptr
173  * under SEC(".struct_ops").
174  */
175 SEC("struct_ops/bictcp_init")
176 void BPF_PROG(bictcp_init, struct sock *sk)
177 {
178 	struct bictcp *ca = inet_csk_ca(sk);
179 
180 	bictcp_reset(ca);
181 
182 	if (hystart)
183 		bictcp_hystart_reset(sk);
184 
185 	if (!hystart && initial_ssthresh)
186 		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
187 }
188 
189 /* No prefix in SEC will also work.
190  * The remaining tcp-cubic functions have an easier way.
191  */
192 SEC("no-sec-prefix-bictcp_cwnd_event")
193 void BPF_PROG(bictcp_cwnd_event, struct sock *sk, enum tcp_ca_event event)
194 {
195 	if (event == CA_EVENT_TX_START) {
196 		struct bictcp *ca = inet_csk_ca(sk);
197 		__u32 now = tcp_jiffies32;
198 		__s32 delta;
199 
200 		delta = now - tcp_sk(sk)->lsndtime;
201 
202 		/* We were application limited (idle) for a while.
203 		 * Shift epoch_start to keep cwnd growth to cubic curve.
204 		 */
205 		if (ca->epoch_start && delta > 0) {
206 			ca->epoch_start += delta;
207 			if (after(ca->epoch_start, now))
208 				ca->epoch_start = now;
209 		}
210 		return;
211 	}
212 }
213 
214 /*
215  * cbrt(x) MSB values for x MSB values in [0..63].
216  * Precomputed then refined by hand - Willy Tarreau
217  *
218  * For x in [0..63],
219  *   v = cbrt(x << 18) - 1
220  *   cbrt(x) = (v[x] + 10) >> 6
221  */
222 static const __u8 v[] = {
223 	/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
224 	/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
225 	/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
226 	/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
227 	/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
228 	/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
229 	/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
230 	/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
231 };
232 
233 /* calculate the cubic root of x using a table lookup followed by one
234  * Newton-Raphson iteration.
235  * Avg err ~= 0.195%
236  */
237 static __always_inline __u32 cubic_root(__u64 a)
238 {
239 	__u32 x, b, shift;
240 
241 	if (a < 64) {
242 		/* a in [0..63] */
243 		return ((__u32)v[(__u32)a] + 35) >> 6;
244 	}
245 
246 	b = fls64(a);
247 	b = ((b * 84) >> 8) - 1;
248 	shift = (a >> (b * 3));
249 
250 	/* it is needed for verifier's bound check on v */
251 	if (shift >= 64)
252 		return 0;
253 
254 	x = ((__u32)(((__u32)v[shift] + 10) << b)) >> 6;
255 
256 	/*
257 	 * Newton-Raphson iteration
258 	 *                         2
259 	 * x    = ( 2 * x  +  a / x  ) / 3
260 	 *  k+1          k         k
261 	 */
262 	x = (2 * x + (__u32)div64_u64(a, (__u64)x * (__u64)(x - 1)));
263 	x = ((x * 341) >> 10);
264 	return x;
265 }
266 
267 /*
268  * Compute congestion window to use.
269  */
270 static __always_inline void bictcp_update(struct bictcp *ca, __u32 cwnd,
271 					  __u32 acked)
272 {
273 	__u32 delta, bic_target, max_cnt;
274 	__u64 offs, t;
275 
276 	ca->ack_cnt += acked;	/* count the number of ACKed packets */
277 
278 	if (ca->last_cwnd == cwnd &&
279 	    (__s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
280 		return;
281 
282 	/* The CUBIC function can update ca->cnt at most once per jiffy.
283 	 * On all cwnd reduction events, ca->epoch_start is set to 0,
284 	 * which will force a recalculation of ca->cnt.
285 	 */
286 	if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
287 		goto tcp_friendliness;
288 
289 	ca->last_cwnd = cwnd;
290 	ca->last_time = tcp_jiffies32;
291 
292 	if (ca->epoch_start == 0) {
293 		ca->epoch_start = tcp_jiffies32;	/* record beginning */
294 		ca->ack_cnt = acked;			/* start counting */
295 		ca->tcp_cwnd = cwnd;			/* syn with cubic */
296 
297 		if (ca->last_max_cwnd <= cwnd) {
298 			ca->bic_K = 0;
299 			ca->bic_origin_point = cwnd;
300 		} else {
301 			/* Compute new K based on
302 			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
303 			 */
304 			ca->bic_K = cubic_root(cube_factor
305 					       * (ca->last_max_cwnd - cwnd));
306 			ca->bic_origin_point = ca->last_max_cwnd;
307 		}
308 	}
309 
310 	/* cubic function - calc*/
311 	/* calculate c * time^3 / rtt,
312 	 *  while considering overflow in calculation of time^3
313 	 * (so time^3 is done by using 64 bit)
314 	 * and without the support of division of 64bit numbers
315 	 * (so all divisions are done by using 32 bit)
316 	 *  also NOTE the unit of those veriables
317 	 *	  time  = (t - K) / 2^bictcp_HZ
318 	 *	  c = bic_scale >> 10
319 	 * rtt  = (srtt >> 3) / HZ
320 	 * !!! The following code does not have overflow problems,
321 	 * if the cwnd < 1 million packets !!!
322 	 */
323 
324 	t = (__s32)(tcp_jiffies32 - ca->epoch_start) * USEC_PER_JIFFY;
325 	t += ca->delay_min;
326 	/* change the unit from usec to bictcp_HZ */
327 	t <<= BICTCP_HZ;
328 	t /= USEC_PER_SEC;
329 
330 	if (t < ca->bic_K)		/* t - K */
331 		offs = ca->bic_K - t;
332 	else
333 		offs = t - ca->bic_K;
334 
335 	/* c/rtt * (t-K)^3 */
336 	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
337 	if (t < ca->bic_K)                            /* below origin*/
338 		bic_target = ca->bic_origin_point - delta;
339 	else                                          /* above origin*/
340 		bic_target = ca->bic_origin_point + delta;
341 
342 	/* cubic function - calc bictcp_cnt*/
343 	if (bic_target > cwnd) {
344 		ca->cnt = cwnd / (bic_target - cwnd);
345 	} else {
346 		ca->cnt = 100 * cwnd;              /* very small increment*/
347 	}
348 
349 	/*
350 	 * The initial growth of cubic function may be too conservative
351 	 * when the available bandwidth is still unknown.
352 	 */
353 	if (ca->last_max_cwnd == 0 && ca->cnt > 20)
354 		ca->cnt = 20;	/* increase cwnd 5% per RTT */
355 
356 tcp_friendliness:
357 	/* TCP Friendly */
358 	if (tcp_friendliness) {
359 		__u32 scale = beta_scale;
360 		__u32 n;
361 
362 		/* update tcp cwnd */
363 		delta = (cwnd * scale) >> 3;
364 		if (ca->ack_cnt > delta && delta) {
365 			n = ca->ack_cnt / delta;
366 			ca->ack_cnt -= n * delta;
367 			ca->tcp_cwnd += n;
368 		}
369 
370 		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
371 			delta = ca->tcp_cwnd - cwnd;
372 			max_cnt = cwnd / delta;
373 			if (ca->cnt > max_cnt)
374 				ca->cnt = max_cnt;
375 		}
376 	}
377 
378 	/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
379 	 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
380 	 */
381 	ca->cnt = max(ca->cnt, 2U);
382 }
383 
384 /* Or simply use the BPF_STRUCT_OPS to avoid the SEC boiler plate. */
385 void BPF_STRUCT_OPS(bictcp_cong_avoid, struct sock *sk, __u32 ack, __u32 acked)
386 {
387 	struct tcp_sock *tp = tcp_sk(sk);
388 	struct bictcp *ca = inet_csk_ca(sk);
389 
390 	if (!tcp_is_cwnd_limited(sk))
391 		return;
392 
393 	if (tcp_in_slow_start(tp)) {
394 		if (hystart && after(ack, ca->end_seq))
395 			bictcp_hystart_reset(sk);
396 		acked = tcp_slow_start(tp, acked);
397 		if (!acked)
398 			return;
399 	}
400 	bictcp_update(ca, tp->snd_cwnd, acked);
401 	tcp_cong_avoid_ai(tp, ca->cnt, acked);
402 }
403 
404 __u32 BPF_STRUCT_OPS(bictcp_recalc_ssthresh, struct sock *sk)
405 {
406 	const struct tcp_sock *tp = tcp_sk(sk);
407 	struct bictcp *ca = inet_csk_ca(sk);
408 
409 	ca->epoch_start = 0;	/* end of epoch */
410 
411 	/* Wmax and fast convergence */
412 	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
413 		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
414 			/ (2 * BICTCP_BETA_SCALE);
415 	else
416 		ca->last_max_cwnd = tp->snd_cwnd;
417 
418 	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
419 }
420 
421 void BPF_STRUCT_OPS(bictcp_state, struct sock *sk, __u8 new_state)
422 {
423 	if (new_state == TCP_CA_Loss) {
424 		bictcp_reset(inet_csk_ca(sk));
425 		bictcp_hystart_reset(sk);
426 	}
427 }
428 
429 #define GSO_MAX_SIZE		65536
430 
431 /* Account for TSO/GRO delays.
432  * Otherwise short RTT flows could get too small ssthresh, since during
433  * slow start we begin with small TSO packets and ca->delay_min would
434  * not account for long aggregation delay when TSO packets get bigger.
435  * Ideally even with a very small RTT we would like to have at least one
436  * TSO packet being sent and received by GRO, and another one in qdisc layer.
437  * We apply another 100% factor because @rate is doubled at this point.
438  * We cap the cushion to 1ms.
439  */
440 static __always_inline __u32 hystart_ack_delay(struct sock *sk)
441 {
442 	unsigned long rate;
443 
444 	rate = sk->sk_pacing_rate;
445 	if (!rate)
446 		return 0;
447 	return min((__u64)USEC_PER_MSEC,
448 		   div64_ul((__u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
449 }
450 
451 static __always_inline void hystart_update(struct sock *sk, __u32 delay)
452 {
453 	struct tcp_sock *tp = tcp_sk(sk);
454 	struct bictcp *ca = inet_csk_ca(sk);
455 	__u32 threshold;
456 
457 	if (hystart_detect & HYSTART_ACK_TRAIN) {
458 		__u32 now = bictcp_clock_us(sk);
459 
460 		/* first detection parameter - ack-train detection */
461 		if ((__s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
462 			ca->last_ack = now;
463 
464 			threshold = ca->delay_min + hystart_ack_delay(sk);
465 
466 			/* Hystart ack train triggers if we get ack past
467 			 * ca->delay_min/2.
468 			 * Pacing might have delayed packets up to RTT/2
469 			 * during slow start.
470 			 */
471 			if (sk->sk_pacing_status == SK_PACING_NONE)
472 				threshold >>= 1;
473 
474 			if ((__s32)(now - ca->round_start) > threshold) {
475 				ca->found = 1;
476 				tp->snd_ssthresh = tp->snd_cwnd;
477 			}
478 		}
479 	}
480 
481 	if (hystart_detect & HYSTART_DELAY) {
482 		/* obtain the minimum delay of more than sampling packets */
483 		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
484 			if (ca->curr_rtt > delay)
485 				ca->curr_rtt = delay;
486 
487 			ca->sample_cnt++;
488 		} else {
489 			if (ca->curr_rtt > ca->delay_min +
490 			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
491 				ca->found = 1;
492 				tp->snd_ssthresh = tp->snd_cwnd;
493 			}
494 		}
495 	}
496 }
497 
498 void BPF_STRUCT_OPS(bictcp_acked, struct sock *sk,
499 		    const struct ack_sample *sample)
500 {
501 	const struct tcp_sock *tp = tcp_sk(sk);
502 	struct bictcp *ca = inet_csk_ca(sk);
503 	__u32 delay;
504 
505 	/* Some calls are for duplicates without timetamps */
506 	if (sample->rtt_us < 0)
507 		return;
508 
509 	/* Discard delay samples right after fast recovery */
510 	if (ca->epoch_start && (__s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
511 		return;
512 
513 	delay = sample->rtt_us;
514 	if (delay == 0)
515 		delay = 1;
516 
517 	/* first time call or link delay decreases */
518 	if (ca->delay_min == 0 || ca->delay_min > delay)
519 		ca->delay_min = delay;
520 
521 	/* hystart triggers when cwnd is larger than some threshold */
522 	if (!ca->found && tcp_in_slow_start(tp) && hystart &&
523 	    tp->snd_cwnd >= hystart_low_window)
524 		hystart_update(sk, delay);
525 }
526 
527 __u32 BPF_STRUCT_OPS(tcp_reno_undo_cwnd, struct sock *sk)
528 {
529 	const struct tcp_sock *tp = tcp_sk(sk);
530 
531 	return max(tp->snd_cwnd, tp->prior_cwnd);
532 }
533 
534 SEC(".struct_ops")
535 struct tcp_congestion_ops cubic = {
536 	.init		= (void *)bictcp_init,
537 	.ssthresh	= (void *)bictcp_recalc_ssthresh,
538 	.cong_avoid	= (void *)bictcp_cong_avoid,
539 	.set_state	= (void *)bictcp_state,
540 	.undo_cwnd	= (void *)tcp_reno_undo_cwnd,
541 	.cwnd_event	= (void *)bictcp_cwnd_event,
542 	.pkts_acked     = (void *)bictcp_acked,
543 	.name		= "bpf_cubic",
544 };
545