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 <linux/stddef.h>
19 #include <linux/tcp.h>
20 #include "bpf_tcp_helpers.h"
21 
22 char _license[] SEC("license") = "GPL";
23 
24 #define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
25 
26 #define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
27 					 * max_cwnd = snd_cwnd * beta
28 					 */
29 #define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
30 
31 /* Two methods of hybrid slow start */
32 #define HYSTART_ACK_TRAIN	0x1
33 #define HYSTART_DELAY		0x2
34 
35 /* Number of delay samples for detecting the increase of delay */
36 #define HYSTART_MIN_SAMPLES	8
37 #define HYSTART_DELAY_MIN	(4000U)	/* 4ms */
38 #define HYSTART_DELAY_MAX	(16000U)	/* 16 ms */
39 #define HYSTART_DELAY_THRESH(x)	clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
40 
41 static int fast_convergence = 1;
42 static const int beta = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
43 static int initial_ssthresh;
44 static const int bic_scale = 41;
45 static int tcp_friendliness = 1;
46 
47 static int hystart = 1;
48 static int hystart_detect = HYSTART_ACK_TRAIN | HYSTART_DELAY;
49 static int hystart_low_window = 16;
50 static int hystart_ack_delta_us = 2000;
51 
52 static const __u32 cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
53 static const __u32 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
54 				/ (BICTCP_BETA_SCALE - beta);
55 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
56  *  so K = cubic_root( (wmax-cwnd)*rtt/c )
57  * the unit of K is bictcp_HZ=2^10, not HZ
58  *
59  *  c = bic_scale >> 10
60  *  rtt = 100ms
61  *
62  * the following code has been designed and tested for
63  * cwnd < 1 million packets
64  * RTT < 100 seconds
65  * HZ < 1,000,00  (corresponding to 10 nano-second)
66  */
67 
68 /* 1/c * 2^2*bictcp_HZ * srtt, 2^40 */
69 static const __u64 cube_factor = (__u64)(1ull << (10+3*BICTCP_HZ))
70 				/ (bic_scale * 10);
71 
72 /* BIC TCP Parameters */
73 struct bictcp {
74 	__u32	cnt;		/* increase cwnd by 1 after ACKs */
75 	__u32	last_max_cwnd;	/* last maximum snd_cwnd */
76 	__u32	last_cwnd;	/* the last snd_cwnd */
77 	__u32	last_time;	/* time when updated last_cwnd */
78 	__u32	bic_origin_point;/* origin point of bic function */
79 	__u32	bic_K;		/* time to origin point
80 				   from the beginning of the current epoch */
81 	__u32	delay_min;	/* min delay (usec) */
82 	__u32	epoch_start;	/* beginning of an epoch */
83 	__u32	ack_cnt;	/* number of acks */
84 	__u32	tcp_cwnd;	/* estimated tcp cwnd */
85 	__u16	unused;
86 	__u8	sample_cnt;	/* number of samples to decide curr_rtt */
87 	__u8	found;		/* the exit point is found? */
88 	__u32	round_start;	/* beginning of each round */
89 	__u32	end_seq;	/* end_seq of the round */
90 	__u32	last_ack;	/* last time when the ACK spacing is close */
91 	__u32	curr_rtt;	/* the minimum rtt of current round */
92 };
93 
94 static inline void bictcp_reset(struct bictcp *ca)
95 {
96 	ca->cnt = 0;
97 	ca->last_max_cwnd = 0;
98 	ca->last_cwnd = 0;
99 	ca->last_time = 0;
100 	ca->bic_origin_point = 0;
101 	ca->bic_K = 0;
102 	ca->delay_min = 0;
103 	ca->epoch_start = 0;
104 	ca->ack_cnt = 0;
105 	ca->tcp_cwnd = 0;
106 	ca->found = 0;
107 }
108 
109 extern unsigned long CONFIG_HZ __kconfig;
110 #define HZ CONFIG_HZ
111 #define USEC_PER_MSEC	1000UL
112 #define USEC_PER_SEC	1000000UL
113 #define USEC_PER_JIFFY	(USEC_PER_SEC / HZ)
114 
115 static __always_inline __u64 div64_u64(__u64 dividend, __u64 divisor)
116 {
117 	return dividend / divisor;
118 }
119 
120 #define div64_ul div64_u64
121 
122 #define BITS_PER_U64 (sizeof(__u64) * 8)
123 static __always_inline int fls64(__u64 x)
124 {
125 	int num = BITS_PER_U64 - 1;
126 
127 	if (x == 0)
128 		return 0;
129 
130 	if (!(x & (~0ull << (BITS_PER_U64-32)))) {
131 		num -= 32;
132 		x <<= 32;
133 	}
134 	if (!(x & (~0ull << (BITS_PER_U64-16)))) {
135 		num -= 16;
136 		x <<= 16;
137 	}
138 	if (!(x & (~0ull << (BITS_PER_U64-8)))) {
139 		num -= 8;
140 		x <<= 8;
141 	}
142 	if (!(x & (~0ull << (BITS_PER_U64-4)))) {
143 		num -= 4;
144 		x <<= 4;
145 	}
146 	if (!(x & (~0ull << (BITS_PER_U64-2)))) {
147 		num -= 2;
148 		x <<= 2;
149 	}
150 	if (!(x & (~0ull << (BITS_PER_U64-1))))
151 		num -= 1;
152 
153 	return num + 1;
154 }
155 
156 static __always_inline __u32 bictcp_clock_us(const struct sock *sk)
157 {
158 	return tcp_sk(sk)->tcp_mstamp;
159 }
160 
161 static __always_inline void bictcp_hystart_reset(struct sock *sk)
162 {
163 	struct tcp_sock *tp = tcp_sk(sk);
164 	struct bictcp *ca = inet_csk_ca(sk);
165 
166 	ca->round_start = ca->last_ack = bictcp_clock_us(sk);
167 	ca->end_seq = tp->snd_nxt;
168 	ca->curr_rtt = ~0U;
169 	ca->sample_cnt = 0;
170 }
171 
172 /* "struct_ops/" prefix is not a requirement
173  * It will be recognized as BPF_PROG_TYPE_STRUCT_OPS
174  * as long as it is used in one of the func ptr
175  * under SEC(".struct_ops").
176  */
177 SEC("struct_ops/bictcp_init")
178 void BPF_PROG(bictcp_init, struct sock *sk)
179 {
180 	struct bictcp *ca = inet_csk_ca(sk);
181 
182 	bictcp_reset(ca);
183 
184 	if (hystart)
185 		bictcp_hystart_reset(sk);
186 
187 	if (!hystart && initial_ssthresh)
188 		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
189 }
190 
191 /* No prefix in SEC will also work.
192  * The remaining tcp-cubic functions have an easier way.
193  */
194 SEC("no-sec-prefix-bictcp_cwnd_event")
195 void BPF_PROG(bictcp_cwnd_event, struct sock *sk, enum tcp_ca_event event)
196 {
197 	if (event == CA_EVENT_TX_START) {
198 		struct bictcp *ca = inet_csk_ca(sk);
199 		__u32 now = tcp_jiffies32;
200 		__s32 delta;
201 
202 		delta = now - tcp_sk(sk)->lsndtime;
203 
204 		/* We were application limited (idle) for a while.
205 		 * Shift epoch_start to keep cwnd growth to cubic curve.
206 		 */
207 		if (ca->epoch_start && delta > 0) {
208 			ca->epoch_start += delta;
209 			if (after(ca->epoch_start, now))
210 				ca->epoch_start = now;
211 		}
212 		return;
213 	}
214 }
215 
216 /*
217  * cbrt(x) MSB values for x MSB values in [0..63].
218  * Precomputed then refined by hand - Willy Tarreau
219  *
220  * For x in [0..63],
221  *   v = cbrt(x << 18) - 1
222  *   cbrt(x) = (v[x] + 10) >> 6
223  */
224 static const __u8 v[] = {
225 	/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
226 	/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
227 	/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
228 	/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
229 	/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
230 	/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
231 	/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
232 	/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
233 };
234 
235 /* calculate the cubic root of x using a table lookup followed by one
236  * Newton-Raphson iteration.
237  * Avg err ~= 0.195%
238  */
239 static __always_inline __u32 cubic_root(__u64 a)
240 {
241 	__u32 x, b, shift;
242 
243 	if (a < 64) {
244 		/* a in [0..63] */
245 		return ((__u32)v[(__u32)a] + 35) >> 6;
246 	}
247 
248 	b = fls64(a);
249 	b = ((b * 84) >> 8) - 1;
250 	shift = (a >> (b * 3));
251 
252 	/* it is needed for verifier's bound check on v */
253 	if (shift >= 64)
254 		return 0;
255 
256 	x = ((__u32)(((__u32)v[shift] + 10) << b)) >> 6;
257 
258 	/*
259 	 * Newton-Raphson iteration
260 	 *                         2
261 	 * x    = ( 2 * x  +  a / x  ) / 3
262 	 *  k+1          k         k
263 	 */
264 	x = (2 * x + (__u32)div64_u64(a, (__u64)x * (__u64)(x - 1)));
265 	x = ((x * 341) >> 10);
266 	return x;
267 }
268 
269 /*
270  * Compute congestion window to use.
271  */
272 static __always_inline void bictcp_update(struct bictcp *ca, __u32 cwnd,
273 					  __u32 acked)
274 {
275 	__u32 delta, bic_target, max_cnt;
276 	__u64 offs, t;
277 
278 	ca->ack_cnt += acked;	/* count the number of ACKed packets */
279 
280 	if (ca->last_cwnd == cwnd &&
281 	    (__s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
282 		return;
283 
284 	/* The CUBIC function can update ca->cnt at most once per jiffy.
285 	 * On all cwnd reduction events, ca->epoch_start is set to 0,
286 	 * which will force a recalculation of ca->cnt.
287 	 */
288 	if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
289 		goto tcp_friendliness;
290 
291 	ca->last_cwnd = cwnd;
292 	ca->last_time = tcp_jiffies32;
293 
294 	if (ca->epoch_start == 0) {
295 		ca->epoch_start = tcp_jiffies32;	/* record beginning */
296 		ca->ack_cnt = acked;			/* start counting */
297 		ca->tcp_cwnd = cwnd;			/* syn with cubic */
298 
299 		if (ca->last_max_cwnd <= cwnd) {
300 			ca->bic_K = 0;
301 			ca->bic_origin_point = cwnd;
302 		} else {
303 			/* Compute new K based on
304 			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
305 			 */
306 			ca->bic_K = cubic_root(cube_factor
307 					       * (ca->last_max_cwnd - cwnd));
308 			ca->bic_origin_point = ca->last_max_cwnd;
309 		}
310 	}
311 
312 	/* cubic function - calc*/
313 	/* calculate c * time^3 / rtt,
314 	 *  while considering overflow in calculation of time^3
315 	 * (so time^3 is done by using 64 bit)
316 	 * and without the support of division of 64bit numbers
317 	 * (so all divisions are done by using 32 bit)
318 	 *  also NOTE the unit of those veriables
319 	 *	  time  = (t - K) / 2^bictcp_HZ
320 	 *	  c = bic_scale >> 10
321 	 * rtt  = (srtt >> 3) / HZ
322 	 * !!! The following code does not have overflow problems,
323 	 * if the cwnd < 1 million packets !!!
324 	 */
325 
326 	t = (__s32)(tcp_jiffies32 - ca->epoch_start) * USEC_PER_JIFFY;
327 	t += ca->delay_min;
328 	/* change the unit from usec to bictcp_HZ */
329 	t <<= BICTCP_HZ;
330 	t /= USEC_PER_SEC;
331 
332 	if (t < ca->bic_K)		/* t - K */
333 		offs = ca->bic_K - t;
334 	else
335 		offs = t - ca->bic_K;
336 
337 	/* c/rtt * (t-K)^3 */
338 	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
339 	if (t < ca->bic_K)                            /* below origin*/
340 		bic_target = ca->bic_origin_point - delta;
341 	else                                          /* above origin*/
342 		bic_target = ca->bic_origin_point + delta;
343 
344 	/* cubic function - calc bictcp_cnt*/
345 	if (bic_target > cwnd) {
346 		ca->cnt = cwnd / (bic_target - cwnd);
347 	} else {
348 		ca->cnt = 100 * cwnd;              /* very small increment*/
349 	}
350 
351 	/*
352 	 * The initial growth of cubic function may be too conservative
353 	 * when the available bandwidth is still unknown.
354 	 */
355 	if (ca->last_max_cwnd == 0 && ca->cnt > 20)
356 		ca->cnt = 20;	/* increase cwnd 5% per RTT */
357 
358 tcp_friendliness:
359 	/* TCP Friendly */
360 	if (tcp_friendliness) {
361 		__u32 scale = beta_scale;
362 		__u32 n;
363 
364 		/* update tcp cwnd */
365 		delta = (cwnd * scale) >> 3;
366 		if (ca->ack_cnt > delta && delta) {
367 			n = ca->ack_cnt / delta;
368 			ca->ack_cnt -= n * delta;
369 			ca->tcp_cwnd += n;
370 		}
371 
372 		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
373 			delta = ca->tcp_cwnd - cwnd;
374 			max_cnt = cwnd / delta;
375 			if (ca->cnt > max_cnt)
376 				ca->cnt = max_cnt;
377 		}
378 	}
379 
380 	/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
381 	 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
382 	 */
383 	ca->cnt = max(ca->cnt, 2U);
384 }
385 
386 /* Or simply use the BPF_STRUCT_OPS to avoid the SEC boiler plate. */
387 void BPF_STRUCT_OPS(bictcp_cong_avoid, struct sock *sk, __u32 ack, __u32 acked)
388 {
389 	struct tcp_sock *tp = tcp_sk(sk);
390 	struct bictcp *ca = inet_csk_ca(sk);
391 
392 	if (!tcp_is_cwnd_limited(sk))
393 		return;
394 
395 	if (tcp_in_slow_start(tp)) {
396 		if (hystart && after(ack, ca->end_seq))
397 			bictcp_hystart_reset(sk);
398 		acked = tcp_slow_start(tp, acked);
399 		if (!acked)
400 			return;
401 	}
402 	bictcp_update(ca, tp->snd_cwnd, acked);
403 	tcp_cong_avoid_ai(tp, ca->cnt, acked);
404 }
405 
406 __u32 BPF_STRUCT_OPS(bictcp_recalc_ssthresh, struct sock *sk)
407 {
408 	const struct tcp_sock *tp = tcp_sk(sk);
409 	struct bictcp *ca = inet_csk_ca(sk);
410 
411 	ca->epoch_start = 0;	/* end of epoch */
412 
413 	/* Wmax and fast convergence */
414 	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
415 		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
416 			/ (2 * BICTCP_BETA_SCALE);
417 	else
418 		ca->last_max_cwnd = tp->snd_cwnd;
419 
420 	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
421 }
422 
423 void BPF_STRUCT_OPS(bictcp_state, struct sock *sk, __u8 new_state)
424 {
425 	if (new_state == TCP_CA_Loss) {
426 		bictcp_reset(inet_csk_ca(sk));
427 		bictcp_hystart_reset(sk);
428 	}
429 }
430 
431 #define GSO_MAX_SIZE		65536
432 
433 /* Account for TSO/GRO delays.
434  * Otherwise short RTT flows could get too small ssthresh, since during
435  * slow start we begin with small TSO packets and ca->delay_min would
436  * not account for long aggregation delay when TSO packets get bigger.
437  * Ideally even with a very small RTT we would like to have at least one
438  * TSO packet being sent and received by GRO, and another one in qdisc layer.
439  * We apply another 100% factor because @rate is doubled at this point.
440  * We cap the cushion to 1ms.
441  */
442 static __always_inline __u32 hystart_ack_delay(struct sock *sk)
443 {
444 	unsigned long rate;
445 
446 	rate = sk->sk_pacing_rate;
447 	if (!rate)
448 		return 0;
449 	return min((__u64)USEC_PER_MSEC,
450 		   div64_ul((__u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
451 }
452 
453 static __always_inline void hystart_update(struct sock *sk, __u32 delay)
454 {
455 	struct tcp_sock *tp = tcp_sk(sk);
456 	struct bictcp *ca = inet_csk_ca(sk);
457 	__u32 threshold;
458 
459 	if (hystart_detect & HYSTART_ACK_TRAIN) {
460 		__u32 now = bictcp_clock_us(sk);
461 
462 		/* first detection parameter - ack-train detection */
463 		if ((__s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
464 			ca->last_ack = now;
465 
466 			threshold = ca->delay_min + hystart_ack_delay(sk);
467 
468 			/* Hystart ack train triggers if we get ack past
469 			 * ca->delay_min/2.
470 			 * Pacing might have delayed packets up to RTT/2
471 			 * during slow start.
472 			 */
473 			if (sk->sk_pacing_status == SK_PACING_NONE)
474 				threshold >>= 1;
475 
476 			if ((__s32)(now - ca->round_start) > threshold) {
477 				ca->found = 1;
478 				tp->snd_ssthresh = tp->snd_cwnd;
479 			}
480 		}
481 	}
482 
483 	if (hystart_detect & HYSTART_DELAY) {
484 		/* obtain the minimum delay of more than sampling packets */
485 		if (ca->curr_rtt > delay)
486 			ca->curr_rtt = delay;
487 		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
488 			ca->sample_cnt++;
489 		} else {
490 			if (ca->curr_rtt > ca->delay_min +
491 			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
492 				ca->found = 1;
493 				tp->snd_ssthresh = tp->snd_cwnd;
494 			}
495 		}
496 	}
497 }
498 
499 void BPF_STRUCT_OPS(bictcp_acked, struct sock *sk,
500 		    const struct ack_sample *sample)
501 {
502 	const struct tcp_sock *tp = tcp_sk(sk);
503 	struct bictcp *ca = inet_csk_ca(sk);
504 	__u32 delay;
505 
506 	/* Some calls are for duplicates without timetamps */
507 	if (sample->rtt_us < 0)
508 		return;
509 
510 	/* Discard delay samples right after fast recovery */
511 	if (ca->epoch_start && (__s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
512 		return;
513 
514 	delay = sample->rtt_us;
515 	if (delay == 0)
516 		delay = 1;
517 
518 	/* first time call or link delay decreases */
519 	if (ca->delay_min == 0 || ca->delay_min > delay)
520 		ca->delay_min = delay;
521 
522 	/* hystart triggers when cwnd is larger than some threshold */
523 	if (!ca->found && tcp_in_slow_start(tp) && hystart &&
524 	    tp->snd_cwnd >= hystart_low_window)
525 		hystart_update(sk, delay);
526 }
527 
528 __u32 BPF_STRUCT_OPS(tcp_reno_undo_cwnd, struct sock *sk)
529 {
530 	const struct tcp_sock *tp = tcp_sk(sk);
531 
532 	return max(tp->snd_cwnd, tp->prior_cwnd);
533 }
534 
535 SEC(".struct_ops")
536 struct tcp_congestion_ops cubic = {
537 	.init		= (void *)bictcp_init,
538 	.ssthresh	= (void *)bictcp_recalc_ssthresh,
539 	.cong_avoid	= (void *)bictcp_cong_avoid,
540 	.set_state	= (void *)bictcp_state,
541 	.undo_cwnd	= (void *)tcp_reno_undo_cwnd,
542 	.cwnd_event	= (void *)bictcp_cwnd_event,
543 	.pkts_acked     = (void *)bictcp_acked,
544 	.name		= "bpf_cubic",
545 };
546