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
22char _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
41static int fast_convergence = 1;
42static const int beta = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
43static int initial_ssthresh;
44static const int bic_scale = 41;
45static int tcp_friendliness = 1;
46
47static int hystart = 1;
48static int hystart_detect = HYSTART_ACK_TRAIN | HYSTART_DELAY;
49static int hystart_low_window = 16;
50static int hystart_ack_delta_us = 2000;
51
52static const __u32 cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
53static 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 */
69static const __u64 cube_factor = (__u64)(1ull << (10+3*BICTCP_HZ))
70				/ (bic_scale * 10);
71
72/* BIC TCP Parameters */
73struct 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
94static 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
109extern 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
115static __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)
123static __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
156static __always_inline __u32 bictcp_clock_us(const struct sock *sk)
157{
158	return tcp_sk(sk)->tcp_mstamp;
159}
160
161static __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 a requirement */
173SEC("struct_ops/bpf_cubic_init")
174void BPF_PROG(bpf_cubic_init, struct sock *sk)
175{
176	struct bictcp *ca = inet_csk_ca(sk);
177
178	bictcp_reset(ca);
179
180	if (hystart)
181		bictcp_hystart_reset(sk);
182
183	if (!hystart && initial_ssthresh)
184		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
185}
186
187/* "struct_ops" prefix is a requirement */
188SEC("struct_ops/bpf_cubic_cwnd_event")
189void BPF_PROG(bpf_cubic_cwnd_event, struct sock *sk, enum tcp_ca_event event)
190{
191	if (event == CA_EVENT_TX_START) {
192		struct bictcp *ca = inet_csk_ca(sk);
193		__u32 now = tcp_jiffies32;
194		__s32 delta;
195
196		delta = now - tcp_sk(sk)->lsndtime;
197
198		/* We were application limited (idle) for a while.
199		 * Shift epoch_start to keep cwnd growth to cubic curve.
200		 */
201		if (ca->epoch_start && delta > 0) {
202			ca->epoch_start += delta;
203			if (after(ca->epoch_start, now))
204				ca->epoch_start = now;
205		}
206		return;
207	}
208}
209
210/*
211 * cbrt(x) MSB values for x MSB values in [0..63].
212 * Precomputed then refined by hand - Willy Tarreau
213 *
214 * For x in [0..63],
215 *   v = cbrt(x << 18) - 1
216 *   cbrt(x) = (v[x] + 10) >> 6
217 */
218static const __u8 v[] = {
219	/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
220	/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
221	/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
222	/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
223	/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
224	/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
225	/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
226	/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
227};
228
229/* calculate the cubic root of x using a table lookup followed by one
230 * Newton-Raphson iteration.
231 * Avg err ~= 0.195%
232 */
233static __always_inline __u32 cubic_root(__u64 a)
234{
235	__u32 x, b, shift;
236
237	if (a < 64) {
238		/* a in [0..63] */
239		return ((__u32)v[(__u32)a] + 35) >> 6;
240	}
241
242	b = fls64(a);
243	b = ((b * 84) >> 8) - 1;
244	shift = (a >> (b * 3));
245
246	/* it is needed for verifier's bound check on v */
247	if (shift >= 64)
248		return 0;
249
250	x = ((__u32)(((__u32)v[shift] + 10) << b)) >> 6;
251
252	/*
253	 * Newton-Raphson iteration
254	 *                         2
255	 * x    = ( 2 * x  +  a / x  ) / 3
256	 *  k+1          k         k
257	 */
258	x = (2 * x + (__u32)div64_u64(a, (__u64)x * (__u64)(x - 1)));
259	x = ((x * 341) >> 10);
260	return x;
261}
262
263/*
264 * Compute congestion window to use.
265 */
266static __always_inline void bictcp_update(struct bictcp *ca, __u32 cwnd,
267					  __u32 acked)
268{
269	__u32 delta, bic_target, max_cnt;
270	__u64 offs, t;
271
272	ca->ack_cnt += acked;	/* count the number of ACKed packets */
273
274	if (ca->last_cwnd == cwnd &&
275	    (__s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
276		return;
277
278	/* The CUBIC function can update ca->cnt at most once per jiffy.
279	 * On all cwnd reduction events, ca->epoch_start is set to 0,
280	 * which will force a recalculation of ca->cnt.
281	 */
282	if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
283		goto tcp_friendliness;
284
285	ca->last_cwnd = cwnd;
286	ca->last_time = tcp_jiffies32;
287
288	if (ca->epoch_start == 0) {
289		ca->epoch_start = tcp_jiffies32;	/* record beginning */
290		ca->ack_cnt = acked;			/* start counting */
291		ca->tcp_cwnd = cwnd;			/* syn with cubic */
292
293		if (ca->last_max_cwnd <= cwnd) {
294			ca->bic_K = 0;
295			ca->bic_origin_point = cwnd;
296		} else {
297			/* Compute new K based on
298			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
299			 */
300			ca->bic_K = cubic_root(cube_factor
301					       * (ca->last_max_cwnd - cwnd));
302			ca->bic_origin_point = ca->last_max_cwnd;
303		}
304	}
305
306	/* cubic function - calc*/
307	/* calculate c * time^3 / rtt,
308	 *  while considering overflow in calculation of time^3
309	 * (so time^3 is done by using 64 bit)
310	 * and without the support of division of 64bit numbers
311	 * (so all divisions are done by using 32 bit)
312	 *  also NOTE the unit of those veriables
313	 *	  time  = (t - K) / 2^bictcp_HZ
314	 *	  c = bic_scale >> 10
315	 * rtt  = (srtt >> 3) / HZ
316	 * !!! The following code does not have overflow problems,
317	 * if the cwnd < 1 million packets !!!
318	 */
319
320	t = (__s32)(tcp_jiffies32 - ca->epoch_start) * USEC_PER_JIFFY;
321	t += ca->delay_min;
322	/* change the unit from usec to bictcp_HZ */
323	t <<= BICTCP_HZ;
324	t /= USEC_PER_SEC;
325
326	if (t < ca->bic_K)		/* t - K */
327		offs = ca->bic_K - t;
328	else
329		offs = t - ca->bic_K;
330
331	/* c/rtt * (t-K)^3 */
332	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
333	if (t < ca->bic_K)                            /* below origin*/
334		bic_target = ca->bic_origin_point - delta;
335	else                                          /* above origin*/
336		bic_target = ca->bic_origin_point + delta;
337
338	/* cubic function - calc bictcp_cnt*/
339	if (bic_target > cwnd) {
340		ca->cnt = cwnd / (bic_target - cwnd);
341	} else {
342		ca->cnt = 100 * cwnd;              /* very small increment*/
343	}
344
345	/*
346	 * The initial growth of cubic function may be too conservative
347	 * when the available bandwidth is still unknown.
348	 */
349	if (ca->last_max_cwnd == 0 && ca->cnt > 20)
350		ca->cnt = 20;	/* increase cwnd 5% per RTT */
351
352tcp_friendliness:
353	/* TCP Friendly */
354	if (tcp_friendliness) {
355		__u32 scale = beta_scale;
356		__u32 n;
357
358		/* update tcp cwnd */
359		delta = (cwnd * scale) >> 3;
360		if (ca->ack_cnt > delta && delta) {
361			n = ca->ack_cnt / delta;
362			ca->ack_cnt -= n * delta;
363			ca->tcp_cwnd += n;
364		}
365
366		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
367			delta = ca->tcp_cwnd - cwnd;
368			max_cnt = cwnd / delta;
369			if (ca->cnt > max_cnt)
370				ca->cnt = max_cnt;
371		}
372	}
373
374	/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
375	 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
376	 */
377	ca->cnt = max(ca->cnt, 2U);
378}
379
380/* Or simply use the BPF_STRUCT_OPS to avoid the SEC boiler plate. */
381void BPF_STRUCT_OPS(bpf_cubic_cong_avoid, struct sock *sk, __u32 ack, __u32 acked)
382{
383	struct tcp_sock *tp = tcp_sk(sk);
384	struct bictcp *ca = inet_csk_ca(sk);
385
386	if (!tcp_is_cwnd_limited(sk))
387		return;
388
389	if (tcp_in_slow_start(tp)) {
390		if (hystart && after(ack, ca->end_seq))
391			bictcp_hystart_reset(sk);
392		acked = tcp_slow_start(tp, acked);
393		if (!acked)
394			return;
395	}
396	bictcp_update(ca, tp->snd_cwnd, acked);
397	tcp_cong_avoid_ai(tp, ca->cnt, acked);
398}
399
400__u32 BPF_STRUCT_OPS(bpf_cubic_recalc_ssthresh, struct sock *sk)
401{
402	const struct tcp_sock *tp = tcp_sk(sk);
403	struct bictcp *ca = inet_csk_ca(sk);
404
405	ca->epoch_start = 0;	/* end of epoch */
406
407	/* Wmax and fast convergence */
408	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
409		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
410			/ (2 * BICTCP_BETA_SCALE);
411	else
412		ca->last_max_cwnd = tp->snd_cwnd;
413
414	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
415}
416
417void BPF_STRUCT_OPS(bpf_cubic_state, struct sock *sk, __u8 new_state)
418{
419	if (new_state == TCP_CA_Loss) {
420		bictcp_reset(inet_csk_ca(sk));
421		bictcp_hystart_reset(sk);
422	}
423}
424
425#define GSO_MAX_SIZE		65536
426
427/* Account for TSO/GRO delays.
428 * Otherwise short RTT flows could get too small ssthresh, since during
429 * slow start we begin with small TSO packets and ca->delay_min would
430 * not account for long aggregation delay when TSO packets get bigger.
431 * Ideally even with a very small RTT we would like to have at least one
432 * TSO packet being sent and received by GRO, and another one in qdisc layer.
433 * We apply another 100% factor because @rate is doubled at this point.
434 * We cap the cushion to 1ms.
435 */
436static __always_inline __u32 hystart_ack_delay(struct sock *sk)
437{
438	unsigned long rate;
439
440	rate = sk->sk_pacing_rate;
441	if (!rate)
442		return 0;
443	return min((__u64)USEC_PER_MSEC,
444		   div64_ul((__u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
445}
446
447static __always_inline void hystart_update(struct sock *sk, __u32 delay)
448{
449	struct tcp_sock *tp = tcp_sk(sk);
450	struct bictcp *ca = inet_csk_ca(sk);
451	__u32 threshold;
452
453	if (hystart_detect & HYSTART_ACK_TRAIN) {
454		__u32 now = bictcp_clock_us(sk);
455
456		/* first detection parameter - ack-train detection */
457		if ((__s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
458			ca->last_ack = now;
459
460			threshold = ca->delay_min + hystart_ack_delay(sk);
461
462			/* Hystart ack train triggers if we get ack past
463			 * ca->delay_min/2.
464			 * Pacing might have delayed packets up to RTT/2
465			 * during slow start.
466			 */
467			if (sk->sk_pacing_status == SK_PACING_NONE)
468				threshold >>= 1;
469
470			if ((__s32)(now - ca->round_start) > threshold) {
471				ca->found = 1;
472				tp->snd_ssthresh = tp->snd_cwnd;
473			}
474		}
475	}
476
477	if (hystart_detect & HYSTART_DELAY) {
478		/* obtain the minimum delay of more than sampling packets */
479		if (ca->curr_rtt > delay)
480			ca->curr_rtt = delay;
481		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
482			ca->sample_cnt++;
483		} else {
484			if (ca->curr_rtt > ca->delay_min +
485			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
486				ca->found = 1;
487				tp->snd_ssthresh = tp->snd_cwnd;
488			}
489		}
490	}
491}
492
493int bpf_cubic_acked_called = 0;
494
495void BPF_STRUCT_OPS(bpf_cubic_acked, struct sock *sk,
496		    const struct ack_sample *sample)
497{
498	const struct tcp_sock *tp = tcp_sk(sk);
499	struct bictcp *ca = inet_csk_ca(sk);
500	__u32 delay;
501
502	bpf_cubic_acked_called = 1;
503	/* Some calls are for duplicates without timetamps */
504	if (sample->rtt_us < 0)
505		return;
506
507	/* Discard delay samples right after fast recovery */
508	if (ca->epoch_start && (__s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
509		return;
510
511	delay = sample->rtt_us;
512	if (delay == 0)
513		delay = 1;
514
515	/* first time call or link delay decreases */
516	if (ca->delay_min == 0 || ca->delay_min > delay)
517		ca->delay_min = delay;
518
519	/* hystart triggers when cwnd is larger than some threshold */
520	if (!ca->found && tcp_in_slow_start(tp) && hystart &&
521	    tp->snd_cwnd >= hystart_low_window)
522		hystart_update(sk, delay);
523}
524
525extern __u32 tcp_reno_undo_cwnd(struct sock *sk) __ksym;
526
527__u32 BPF_STRUCT_OPS(bpf_cubic_undo_cwnd, struct sock *sk)
528{
529	return tcp_reno_undo_cwnd(sk);
530}
531
532SEC(".struct_ops")
533struct tcp_congestion_ops cubic = {
534	.init		= (void *)bpf_cubic_init,
535	.ssthresh	= (void *)bpf_cubic_recalc_ssthresh,
536	.cong_avoid	= (void *)bpf_cubic_cong_avoid,
537	.set_state	= (void *)bpf_cubic_state,
538	.undo_cwnd	= (void *)bpf_cubic_undo_cwnd,
539	.cwnd_event	= (void *)bpf_cubic_cwnd_event,
540	.pkts_acked     = (void *)bpf_cubic_acked,
541	.name		= "bpf_cubic",
542};
543