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
bictcp_reset(struct bictcp * ca)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
div64_u64(__u64 dividend,__u64 divisor)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)
fls64(__u64 x)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
bictcp_clock_us(const struct sock * sk)156 static __always_inline __u32 bictcp_clock_us(const struct sock *sk)
157 {
158 return tcp_sk(sk)->tcp_mstamp;
159 }
160
bictcp_hystart_reset(struct sock * sk)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 a requirement */
173 SEC("struct_ops/bpf_cubic_init")
BPF_PROG(bpf_cubic_init,struct sock * sk)174 void 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 */
188 SEC("struct_ops/bpf_cubic_cwnd_event")
BPF_PROG(bpf_cubic_cwnd_event,struct sock * sk,enum tcp_ca_event event)189 void 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 */
218 static 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 */
cubic_root(__u64 a)233 static __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 */
bictcp_update(struct bictcp * ca,__u32 cwnd,__u32 acked)266 static __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
352 tcp_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. */
BPF_STRUCT_OPS(bpf_cubic_cong_avoid,struct sock * sk,__u32 ack,__u32 acked)381 void 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
BPF_STRUCT_OPS(bpf_cubic_recalc_ssthresh,struct sock * sk)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
BPF_STRUCT_OPS(bpf_cubic_state,struct sock * sk,__u8 new_state)417 void 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 */
hystart_ack_delay(struct sock * sk)436 static __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
hystart_update(struct sock * sk,__u32 delay)447 static __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
493 int bpf_cubic_acked_called = 0;
494
BPF_STRUCT_OPS(bpf_cubic_acked,struct sock * sk,const struct ack_sample * sample)495 void 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
525 extern __u32 tcp_reno_undo_cwnd(struct sock *sk) __ksym;
526
BPF_STRUCT_OPS(bpf_cubic_undo_cwnd,struct sock * sk)527 __u32 BPF_STRUCT_OPS(bpf_cubic_undo_cwnd, struct sock *sk)
528 {
529 return tcp_reno_undo_cwnd(sk);
530 }
531
532 SEC(".struct_ops")
533 struct 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