tcp_cdg.c 11.1 KB
Newer Older
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147
/*
 * CAIA Delay-Gradient (CDG) congestion control
 *
 * This implementation is based on the paper:
 *   D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
 *   delay gradients." In IFIP Networking, pages 328-341. Springer, 2011.
 *
 * Scavenger traffic (Less-than-Best-Effort) should disable coexistence
 * heuristics using parameters use_shadow=0 and use_ineff=0.
 *
 * Parameters window, backoff_beta, and backoff_factor are crucial for
 * throughput and delay. Future work is needed to determine better defaults,
 * and to provide guidelines for use in different environments/contexts.
 *
 * Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/.
 * Parameter window is only configurable when loading tcp_cdg as a module.
 *
 * Notable differences from paper/FreeBSD:
 *   o Using Hybrid Slow start and Proportional Rate Reduction.
 *   o Add toggle for shadow window mechanism. Suggested by David Hayes.
 *   o Add toggle for non-congestion loss tolerance.
 *   o Scaling parameter G is changed to a backoff factor;
 *     conversion is given by: backoff_factor = 1000/(G * window).
 *   o Limit shadow window to 2 * cwnd, or to cwnd when application limited.
 *   o More accurate e^-x.
 */
#include <linux/kernel.h>
#include <linux/random.h>
#include <linux/module.h>
#include <net/tcp.h>

#define HYSTART_ACK_TRAIN	1
#define HYSTART_DELAY		2

static int window __read_mostly = 8;
static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
static unsigned int backoff_factor __read_mostly = 42;
static unsigned int hystart_detect __read_mostly = 3;
static unsigned int use_ineff __read_mostly = 5;
static bool use_shadow __read_mostly = true;
static bool use_tolerance __read_mostly;

module_param(window, int, 0444);
MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
module_param(backoff_beta, uint, 0644);
MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
module_param(backoff_factor, uint, 0644);
MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
module_param(hystart_detect, uint, 0644);
MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
		 "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
module_param(use_ineff, uint, 0644);
MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
module_param(use_shadow, bool, 0644);
MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
module_param(use_tolerance, bool, 0644);
MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");

struct minmax {
	union {
		struct {
			s32 min;
			s32 max;
		};
		u64 v64;
	};
};

enum cdg_state {
	CDG_UNKNOWN = 0,
	CDG_NONFULL = 1,
	CDG_FULL    = 2,
	CDG_BACKOFF = 3,
};

struct cdg {
	struct minmax rtt;
	struct minmax rtt_prev;
	struct minmax *gradients;
	struct minmax gsum;
	bool gfilled;
	u8  tail;
	u8  state;
	u8  delack;
	u32 rtt_seq;
	u32 undo_cwnd;
	u32 shadow_wnd;
	u16 backoff_cnt;
	u16 sample_cnt;
	s32 delay_min;
	u32 last_ack;
	u32 round_start;
};

/**
 * nexp_u32 - negative base-e exponential
 * @ux: x in units of micro
 *
 * Returns exp(ux * -1e-6) * U32_MAX.
 */
static u32 __pure nexp_u32(u32 ux)
{
	static const u16 v[] = {
		/* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
		65535,
		65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
		61378, 57484, 50423, 38795, 22965, 8047,  987,   14,
	};
	u32 msb = ux >> 8;
	u32 res;
	int i;

	/* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
	if (msb > U16_MAX)
		return 0;

	/* Scale first eight bits linearly: */
	res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);

	/* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
	for (i = 1; msb; i++, msb >>= 1) {
		u32 y = v[i & -(msb & 1)] + U32_C(1);

		res = ((u64)res * y) >> 16;
	}

	return res;
}

/* Based on the HyStart algorithm (by Ha et al.) that is implemented in
 * tcp_cubic. Differences/experimental changes:
 *   o Using Hayes' delayed ACK filter.
 *   o Using a usec clock for the ACK train.
 *   o Reset ACK train when application limited.
 *   o Invoked at any cwnd (i.e. also when cwnd < 16).
 *   o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
 */
static void tcp_cdg_hystart_update(struct sock *sk)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
	if (ca->delay_min == 0)
		return;

	if (hystart_detect & HYSTART_ACK_TRAIN) {
148
		u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);
149 150 151 152 153 154 155 156 157

		if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) {
			ca->last_ack = now_us;
			ca->round_start = now_us;
		} else if (before(now_us, ca->last_ack + 3000)) {
			u32 base_owd = max(ca->delay_min / 2U, 125U);

			ca->last_ack = now_us;
			if (after(now_us, ca->round_start + base_owd)) {
158 159 160 161 162
				__NET_INC_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTTRAINDETECT);
				__NET_ADD_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTTRAINCWND,
						tp->snd_cwnd);
163 164 165 166 167 168 169 170 171 172 173 174 175 176
				tp->snd_ssthresh = tp->snd_cwnd;
				return;
			}
		}
	}

	if (hystart_detect & HYSTART_DELAY) {
		if (ca->sample_cnt < 8) {
			ca->sample_cnt++;
		} else {
			s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
					 125U);

			if (ca->rtt.min > thresh) {
177 178 179 180 181
				__NET_INC_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTDELAYDETECT);
				__NET_ADD_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTDELAYCWND,
						tp->snd_cwnd);
182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266
				tp->snd_ssthresh = tp->snd_cwnd;
			}
		}
	}
}

static s32 tcp_cdg_grad(struct cdg *ca)
{
	s32 gmin = ca->rtt.min - ca->rtt_prev.min;
	s32 gmax = ca->rtt.max - ca->rtt_prev.max;
	s32 grad;

	if (ca->gradients) {
		ca->gsum.min += gmin - ca->gradients[ca->tail].min;
		ca->gsum.max += gmax - ca->gradients[ca->tail].max;
		ca->gradients[ca->tail].min = gmin;
		ca->gradients[ca->tail].max = gmax;
		ca->tail = (ca->tail + 1) & (window - 1);
		gmin = ca->gsum.min;
		gmax = ca->gsum.max;
	}

	/* We keep sums to ignore gradients during cwnd reductions;
	 * the paper's smoothed gradients otherwise simplify to:
	 * (rtt_latest - rtt_oldest) / window.
	 *
	 * We also drop division by window here.
	 */
	grad = gmin > 0 ? gmin : gmax;

	/* Extrapolate missing values in gradient window: */
	if (!ca->gfilled) {
		if (!ca->gradients && window > 1)
			grad *= window; /* Memory allocation failed. */
		else if (ca->tail == 0)
			ca->gfilled = true;
		else
			grad = (grad * window) / (int)ca->tail;
	}

	/* Backoff was effectual: */
	if (gmin <= -32 || gmax <= -32)
		ca->backoff_cnt = 0;

	if (use_tolerance) {
		/* Reduce small variations to zero: */
		gmin = DIV_ROUND_CLOSEST(gmin, 64);
		gmax = DIV_ROUND_CLOSEST(gmax, 64);

		if (gmin > 0 && gmax <= 0)
			ca->state = CDG_FULL;
		else if ((gmin > 0 && gmax > 0) || gmax < 0)
			ca->state = CDG_NONFULL;
	}
	return grad;
}

static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	if (prandom_u32() <= nexp_u32(grad * backoff_factor))
		return false;

	if (use_ineff) {
		ca->backoff_cnt++;
		if (ca->backoff_cnt > use_ineff)
			return false;
	}

	ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
	ca->state = CDG_BACKOFF;
	tcp_enter_cwr(sk);
	return true;
}

/* Not called in CWR or Recovery state. */
static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	u32 prior_snd_cwnd;
	u32 incr;

267
	if (tcp_in_slow_start(tp) && hystart_detect)
268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433
		tcp_cdg_hystart_update(sk);

	if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
		s32 grad = 0;

		if (ca->rtt_prev.v64)
			grad = tcp_cdg_grad(ca);
		ca->rtt_seq = tp->snd_nxt;
		ca->rtt_prev = ca->rtt;
		ca->rtt.v64 = 0;
		ca->last_ack = 0;
		ca->sample_cnt = 0;

		if (grad > 0 && tcp_cdg_backoff(sk, grad))
			return;
	}

	if (!tcp_is_cwnd_limited(sk)) {
		ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
		return;
	}

	prior_snd_cwnd = tp->snd_cwnd;
	tcp_reno_cong_avoid(sk, ack, acked);

	incr = tp->snd_cwnd - prior_snd_cwnd;
	ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
}

static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	if (rtt_us <= 0)
		return;

	/* A heuristic for filtering delayed ACKs, adapted from:
	 * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
	 * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
	 */
	if (tp->sacked_out == 0) {
		if (num_acked == 1 && ca->delack) {
			/* A delayed ACK is only used for the minimum if it is
			 * provenly lower than an existing non-zero minimum.
			 */
			ca->rtt.min = min(ca->rtt.min, rtt_us);
			ca->delack--;
			return;
		} else if (num_acked > 1 && ca->delack < 5) {
			ca->delack++;
		}
	}

	ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
	ca->rtt.max = max(ca->rtt.max, rtt_us);
}

static u32 tcp_cdg_ssthresh(struct sock *sk)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	ca->undo_cwnd = tp->snd_cwnd;

	if (ca->state == CDG_BACKOFF)
		return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);

	if (ca->state == CDG_NONFULL && use_tolerance)
		return tp->snd_cwnd;

	ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
	if (use_shadow)
		return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
	return max(2U, tp->snd_cwnd >> 1);
}

static u32 tcp_cdg_undo_cwnd(struct sock *sk)
{
	struct cdg *ca = inet_csk_ca(sk);

	return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd);
}

static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct minmax *gradients;

	switch (ev) {
	case CA_EVENT_CWND_RESTART:
		gradients = ca->gradients;
		if (gradients)
			memset(gradients, 0, window * sizeof(gradients[0]));
		memset(ca, 0, sizeof(*ca));

		ca->gradients = gradients;
		ca->rtt_seq = tp->snd_nxt;
		ca->shadow_wnd = tp->snd_cwnd;
		break;
	case CA_EVENT_COMPLETE_CWR:
		ca->state = CDG_UNKNOWN;
		ca->rtt_seq = tp->snd_nxt;
		ca->rtt_prev = ca->rtt;
		ca->rtt.v64 = 0;
		break;
	default:
		break;
	}
}

static void tcp_cdg_init(struct sock *sk)
{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	/* We silently fall back to window = 1 if allocation fails. */
	if (window > 1)
		ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
					GFP_NOWAIT | __GFP_NOWARN);
	ca->rtt_seq = tp->snd_nxt;
	ca->shadow_wnd = tp->snd_cwnd;
}

static void tcp_cdg_release(struct sock *sk)
{
	struct cdg *ca = inet_csk_ca(sk);

	kfree(ca->gradients);
}

struct tcp_congestion_ops tcp_cdg __read_mostly = {
	.cong_avoid = tcp_cdg_cong_avoid,
	.cwnd_event = tcp_cdg_cwnd_event,
	.pkts_acked = tcp_cdg_acked,
	.undo_cwnd = tcp_cdg_undo_cwnd,
	.ssthresh = tcp_cdg_ssthresh,
	.release = tcp_cdg_release,
	.init = tcp_cdg_init,
	.owner = THIS_MODULE,
	.name = "cdg",
};

static int __init tcp_cdg_register(void)
{
	if (backoff_beta > 1024 || window < 1 || window > 256)
		return -ERANGE;
	if (!is_power_of_2(window))
		return -EINVAL;

	BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
	tcp_register_congestion_control(&tcp_cdg);
	return 0;
}

static void __exit tcp_cdg_unregister(void)
{
	tcp_unregister_congestion_control(&tcp_cdg);
}

module_init(tcp_cdg_register);
module_exit(tcp_cdg_unregister);
MODULE_AUTHOR("Kenneth Klette Jonassen");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP CDG");