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 "bpf_tracing_net.h"
18#include <bpf/bpf_tracing.h>
19
20char _license[] SEC("license") = "GPL";
21
22#define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
23#define min(a, b) ((a) < (b) ? (a) : (b))
24#define max(a, b) ((a) > (b) ? (a) : (b))
25static bool before(__u32 seq1, __u32 seq2)
26{
27	return (__s32)(seq1-seq2) < 0;
28}
29#define after(seq2, seq1) 	before(seq1, seq2)
30
31extern __u32 tcp_slow_start(struct tcp_sock *tp, __u32 acked) __ksym;
32extern void tcp_cong_avoid_ai(struct tcp_sock *tp, __u32 w, __u32 acked) __ksym;
33
34#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
35					 * max_cwnd = snd_cwnd * beta
36					 */
37#define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
38
39/* Two methods of hybrid slow start */
40#define HYSTART_ACK_TRAIN	0x1
41#define HYSTART_DELAY		0x2
42
43/* Number of delay samples for detecting the increase of delay */
44#define HYSTART_MIN_SAMPLES	8
45#define HYSTART_DELAY_MIN	(4000U)	/* 4ms */
46#define HYSTART_DELAY_MAX	(16000U)	/* 16 ms */
47#define HYSTART_DELAY_THRESH(x)	clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
48
49static int fast_convergence = 1;
50static const int beta = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
51static int initial_ssthresh;
52static const int bic_scale = 41;
53static int tcp_friendliness = 1;
54
55static int hystart = 1;
56static int hystart_detect = HYSTART_ACK_TRAIN | HYSTART_DELAY;
57static int hystart_low_window = 16;
58static int hystart_ack_delta_us = 2000;
59
60static const __u32 cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
61static const __u32 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
62				/ (BICTCP_BETA_SCALE - beta);
63/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
64 *  so K = cubic_root( (wmax-cwnd)*rtt/c )
65 * the unit of K is bictcp_HZ=2^10, not HZ
66 *
67 *  c = bic_scale >> 10
68 *  rtt = 100ms
69 *
70 * the following code has been designed and tested for
71 * cwnd < 1 million packets
72 * RTT < 100 seconds
73 * HZ < 1,000,00  (corresponding to 10 nano-second)
74 */
75
76/* 1/c * 2^2*bictcp_HZ * srtt, 2^40 */
77static const __u64 cube_factor = (__u64)(1ull << (10+3*BICTCP_HZ))
78				/ (bic_scale * 10);
79
80/* BIC TCP Parameters */
81struct bpf_bictcp {
82	__u32	cnt;		/* increase cwnd by 1 after ACKs */
83	__u32	last_max_cwnd;	/* last maximum snd_cwnd */
84	__u32	last_cwnd;	/* the last snd_cwnd */
85	__u32	last_time;	/* time when updated last_cwnd */
86	__u32	bic_origin_point;/* origin point of bic function */
87	__u32	bic_K;		/* time to origin point
88				   from the beginning of the current epoch */
89	__u32	delay_min;	/* min delay (usec) */
90	__u32	epoch_start;	/* beginning of an epoch */
91	__u32	ack_cnt;	/* number of acks */
92	__u32	tcp_cwnd;	/* estimated tcp cwnd */
93	__u16	unused;
94	__u8	sample_cnt;	/* number of samples to decide curr_rtt */
95	__u8	found;		/* the exit point is found? */
96	__u32	round_start;	/* beginning of each round */
97	__u32	end_seq;	/* end_seq of the round */
98	__u32	last_ack;	/* last time when the ACK spacing is close */
99	__u32	curr_rtt;	/* the minimum rtt of current round */
100};
101
102static void bictcp_reset(struct bpf_bictcp *ca)
103{
104	ca->cnt = 0;
105	ca->last_max_cwnd = 0;
106	ca->last_cwnd = 0;
107	ca->last_time = 0;
108	ca->bic_origin_point = 0;
109	ca->bic_K = 0;
110	ca->delay_min = 0;
111	ca->epoch_start = 0;
112	ca->ack_cnt = 0;
113	ca->tcp_cwnd = 0;
114	ca->found = 0;
115}
116
117extern unsigned long CONFIG_HZ __kconfig;
118#define HZ CONFIG_HZ
119#define USEC_PER_MSEC	1000UL
120#define USEC_PER_SEC	1000000UL
121#define USEC_PER_JIFFY	(USEC_PER_SEC / HZ)
122
123static __u64 div64_u64(__u64 dividend, __u64 divisor)
124{
125	return dividend / divisor;
126}
127
128#define div64_ul div64_u64
129
130#define BITS_PER_U64 (sizeof(__u64) * 8)
131static int fls64(__u64 x)
132{
133	int num = BITS_PER_U64 - 1;
134
135	if (x == 0)
136		return 0;
137
138	if (!(x & (~0ull << (BITS_PER_U64-32)))) {
139		num -= 32;
140		x <<= 32;
141	}
142	if (!(x & (~0ull << (BITS_PER_U64-16)))) {
143		num -= 16;
144		x <<= 16;
145	}
146	if (!(x & (~0ull << (BITS_PER_U64-8)))) {
147		num -= 8;
148		x <<= 8;
149	}
150	if (!(x & (~0ull << (BITS_PER_U64-4)))) {
151		num -= 4;
152		x <<= 4;
153	}
154	if (!(x & (~0ull << (BITS_PER_U64-2)))) {
155		num -= 2;
156		x <<= 2;
157	}
158	if (!(x & (~0ull << (BITS_PER_U64-1))))
159		num -= 1;
160
161	return num + 1;
162}
163
164static __u32 bictcp_clock_us(const struct sock *sk)
165{
166	return tcp_sk(sk)->tcp_mstamp;
167}
168
169static void bictcp_hystart_reset(struct sock *sk)
170{
171	struct tcp_sock *tp = tcp_sk(sk);
172	struct bpf_bictcp *ca = inet_csk_ca(sk);
173
174	ca->round_start = ca->last_ack = bictcp_clock_us(sk);
175	ca->end_seq = tp->snd_nxt;
176	ca->curr_rtt = ~0U;
177	ca->sample_cnt = 0;
178}
179
180SEC("struct_ops")
181void BPF_PROG(bpf_cubic_init, struct sock *sk)
182{
183	struct bpf_bictcp *ca = inet_csk_ca(sk);
184
185	bictcp_reset(ca);
186
187	if (hystart)
188		bictcp_hystart_reset(sk);
189
190	if (!hystart && initial_ssthresh)
191		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
192}
193
194SEC("struct_ops")
195void BPF_PROG(bpf_cubic_cwnd_event, struct sock *sk, enum tcp_ca_event event)
196{
197	if (event == CA_EVENT_TX_START) {
198		struct bpf_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 */
224static 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 */
239static __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 */
272static void bictcp_update(struct bpf_bictcp *ca, __u32 cwnd, __u32 acked)
273{
274	__u32 delta, bic_target, max_cnt;
275	__u64 offs, t;
276
277	ca->ack_cnt += acked;	/* count the number of ACKed packets */
278
279	if (ca->last_cwnd == cwnd &&
280	    (__s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
281		return;
282
283	/* The CUBIC function can update ca->cnt at most once per jiffy.
284	 * On all cwnd reduction events, ca->epoch_start is set to 0,
285	 * which will force a recalculation of ca->cnt.
286	 */
287	if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
288		goto tcp_friendliness;
289
290	ca->last_cwnd = cwnd;
291	ca->last_time = tcp_jiffies32;
292
293	if (ca->epoch_start == 0) {
294		ca->epoch_start = tcp_jiffies32;	/* record beginning */
295		ca->ack_cnt = acked;			/* start counting */
296		ca->tcp_cwnd = cwnd;			/* syn with cubic */
297
298		if (ca->last_max_cwnd <= cwnd) {
299			ca->bic_K = 0;
300			ca->bic_origin_point = cwnd;
301		} else {
302			/* Compute new K based on
303			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
304			 */
305			ca->bic_K = cubic_root(cube_factor
306					       * (ca->last_max_cwnd - cwnd));
307			ca->bic_origin_point = ca->last_max_cwnd;
308		}
309	}
310
311	/* cubic function - calc*/
312	/* calculate c * time^3 / rtt,
313	 *  while considering overflow in calculation of time^3
314	 * (so time^3 is done by using 64 bit)
315	 * and without the support of division of 64bit numbers
316	 * (so all divisions are done by using 32 bit)
317	 *  also NOTE the unit of those veriables
318	 *	  time  = (t - K) / 2^bictcp_HZ
319	 *	  c = bic_scale >> 10
320	 * rtt  = (srtt >> 3) / HZ
321	 * !!! The following code does not have overflow problems,
322	 * if the cwnd < 1 million packets !!!
323	 */
324
325	t = (__s32)(tcp_jiffies32 - ca->epoch_start) * USEC_PER_JIFFY;
326	t += ca->delay_min;
327	/* change the unit from usec to bictcp_HZ */
328	t <<= BICTCP_HZ;
329	t /= USEC_PER_SEC;
330
331	if (t < ca->bic_K)		/* t - K */
332		offs = ca->bic_K - t;
333	else
334		offs = t - ca->bic_K;
335
336	/* c/rtt * (t-K)^3 */
337	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
338	if (t < ca->bic_K)                            /* below origin*/
339		bic_target = ca->bic_origin_point - delta;
340	else                                          /* above origin*/
341		bic_target = ca->bic_origin_point + delta;
342
343	/* cubic function - calc bictcp_cnt*/
344	if (bic_target > cwnd) {
345		ca->cnt = cwnd / (bic_target - cwnd);
346	} else {
347		ca->cnt = 100 * cwnd;              /* very small increment*/
348	}
349
350	/*
351	 * The initial growth of cubic function may be too conservative
352	 * when the available bandwidth is still unknown.
353	 */
354	if (ca->last_max_cwnd == 0 && ca->cnt > 20)
355		ca->cnt = 20;	/* increase cwnd 5% per RTT */
356
357tcp_friendliness:
358	/* TCP Friendly */
359	if (tcp_friendliness) {
360		__u32 scale = beta_scale;
361		__u32 n;
362
363		/* update tcp cwnd */
364		delta = (cwnd * scale) >> 3;
365		if (ca->ack_cnt > delta && delta) {
366			n = ca->ack_cnt / delta;
367			ca->ack_cnt -= n * delta;
368			ca->tcp_cwnd += n;
369		}
370
371		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
372			delta = ca->tcp_cwnd - cwnd;
373			max_cnt = cwnd / delta;
374			if (ca->cnt > max_cnt)
375				ca->cnt = max_cnt;
376		}
377	}
378
379	/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
380	 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
381	 */
382	ca->cnt = max(ca->cnt, 2U);
383}
384
385SEC("struct_ops")
386void BPF_PROG(bpf_cubic_cong_avoid, struct sock *sk, __u32 ack, __u32 acked)
387{
388	struct tcp_sock *tp = tcp_sk(sk);
389	struct bpf_bictcp *ca = inet_csk_ca(sk);
390
391	if (!tcp_is_cwnd_limited(sk))
392		return;
393
394	if (tcp_in_slow_start(tp)) {
395		if (hystart && after(ack, ca->end_seq))
396			bictcp_hystart_reset(sk);
397		acked = tcp_slow_start(tp, acked);
398		if (!acked)
399			return;
400	}
401	bictcp_update(ca, tp->snd_cwnd, acked);
402	tcp_cong_avoid_ai(tp, ca->cnt, acked);
403}
404
405SEC("struct_ops")
406__u32 BPF_PROG(bpf_cubic_recalc_ssthresh, struct sock *sk)
407{
408	const struct tcp_sock *tp = tcp_sk(sk);
409	struct bpf_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
423SEC("struct_ops")
424void BPF_PROG(bpf_cubic_state, struct sock *sk, __u8 new_state)
425{
426	if (new_state == TCP_CA_Loss) {
427		bictcp_reset(inet_csk_ca(sk));
428		bictcp_hystart_reset(sk);
429	}
430}
431
432#define GSO_MAX_SIZE		65536
433
434/* Account for TSO/GRO delays.
435 * Otherwise short RTT flows could get too small ssthresh, since during
436 * slow start we begin with small TSO packets and ca->delay_min would
437 * not account for long aggregation delay when TSO packets get bigger.
438 * Ideally even with a very small RTT we would like to have at least one
439 * TSO packet being sent and received by GRO, and another one in qdisc layer.
440 * We apply another 100% factor because @rate is doubled at this point.
441 * We cap the cushion to 1ms.
442 */
443static __u32 hystart_ack_delay(struct sock *sk)
444{
445	unsigned long rate;
446
447	rate = sk->sk_pacing_rate;
448	if (!rate)
449		return 0;
450	return min((__u64)USEC_PER_MSEC,
451		   div64_ul((__u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
452}
453
454static void hystart_update(struct sock *sk, __u32 delay)
455{
456	struct tcp_sock *tp = tcp_sk(sk);
457	struct bpf_bictcp *ca = inet_csk_ca(sk);
458	__u32 threshold;
459
460	if (hystart_detect & HYSTART_ACK_TRAIN) {
461		__u32 now = bictcp_clock_us(sk);
462
463		/* first detection parameter - ack-train detection */
464		if ((__s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
465			ca->last_ack = now;
466
467			threshold = ca->delay_min + hystart_ack_delay(sk);
468
469			/* Hystart ack train triggers if we get ack past
470			 * ca->delay_min/2.
471			 * Pacing might have delayed packets up to RTT/2
472			 * during slow start.
473			 */
474			if (sk->sk_pacing_status == SK_PACING_NONE)
475				threshold >>= 1;
476
477			if ((__s32)(now - ca->round_start) > threshold) {
478				ca->found = 1;
479				tp->snd_ssthresh = tp->snd_cwnd;
480			}
481		}
482	}
483
484	if (hystart_detect & HYSTART_DELAY) {
485		/* obtain the minimum delay of more than sampling packets */
486		if (ca->curr_rtt > delay)
487			ca->curr_rtt = delay;
488		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
489			ca->sample_cnt++;
490		} else {
491			if (ca->curr_rtt > ca->delay_min +
492			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
493				ca->found = 1;
494				tp->snd_ssthresh = tp->snd_cwnd;
495			}
496		}
497	}
498}
499
500int bpf_cubic_acked_called = 0;
501
502SEC("struct_ops")
503void BPF_PROG(bpf_cubic_acked, struct sock *sk, const struct ack_sample *sample)
504{
505	const struct tcp_sock *tp = tcp_sk(sk);
506	struct bpf_bictcp *ca = inet_csk_ca(sk);
507	__u32 delay;
508
509	bpf_cubic_acked_called = 1;
510	/* Some calls are for duplicates without timetamps */
511	if (sample->rtt_us < 0)
512		return;
513
514	/* Discard delay samples right after fast recovery */
515	if (ca->epoch_start && (__s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
516		return;
517
518	delay = sample->rtt_us;
519	if (delay == 0)
520		delay = 1;
521
522	/* first time call or link delay decreases */
523	if (ca->delay_min == 0 || ca->delay_min > delay)
524		ca->delay_min = delay;
525
526	/* hystart triggers when cwnd is larger than some threshold */
527	if (!ca->found && tcp_in_slow_start(tp) && hystart &&
528	    tp->snd_cwnd >= hystart_low_window)
529		hystart_update(sk, delay);
530}
531
532extern __u32 tcp_reno_undo_cwnd(struct sock *sk) __ksym;
533
534SEC("struct_ops")
535__u32 BPF_PROG(bpf_cubic_undo_cwnd, struct sock *sk)
536{
537	return tcp_reno_undo_cwnd(sk);
538}
539
540SEC(".struct_ops")
541struct tcp_congestion_ops cubic = {
542	.init		= (void *)bpf_cubic_init,
543	.ssthresh	= (void *)bpf_cubic_recalc_ssthresh,
544	.cong_avoid	= (void *)bpf_cubic_cong_avoid,
545	.set_state	= (void *)bpf_cubic_state,
546	.undo_cwnd	= (void *)bpf_cubic_undo_cwnd,
547	.cwnd_event	= (void *)bpf_cubic_cwnd_event,
548	.pkts_acked     = (void *)bpf_cubic_acked,
549	.name		= "bpf_cubic",
550};
551