net/ipv4/tcp_cdg.c - linux - Git at Google

 /*
  * 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 <linux/sched/clock.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 cdg_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 cdg_minmax rtt;
 	struct cdg_minmax rtt_prev;
 	struct cdg_minmax *gradients;
 	struct cdg_minmax gsum;
 	bool gfilled;
 	u8  tail;
 	u8  state;
 	u8  delack;
 	u32 rtt_seq;
 	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) {
 		u32 now_us = tp->tcp_mstamp;

 		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)) {
 				NET_INC_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTTRAINDETECT);
 				NET_ADD_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTTRAINCWND,
 					      tp->snd_cwnd);
 				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) {
 				NET_INC_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTDELAYDETECT);
 				NET_ADD_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTDELAYCWND,
 					      tp->snd_cwnd);
 				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;

 	if (tcp_in_slow_start(tp) && hystart_detect)
 		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, const struct ack_sample *sample)
 {
 	struct cdg *ca = inet_csk_ca(sk);
 	struct tcp_sock *tp = tcp_sk(sk);

 	if (sample->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 (sample->pkts_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, sample->rtt_us);
 			ca->delack--;
 			return;
 		} else if (sample->pkts_acked > 1 && ca->delack < 5) {
 			ca->delack++;
 		}
 	}

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

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

 	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 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 cdg_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);
 }

 static 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_reno_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");
	/*
	* 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 <linux/sched/clock.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 cdg_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 cdg_minmax rtt;
	struct cdg_minmax rtt_prev;
	struct cdg_minmax *gradients;
	struct cdg_minmax gsum;
	bool gfilled;
	u8 tail;
	u8 state;
	u8 delack;
	u32 rtt_seq;
	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) {
	u32 now_us = tp->tcp_mstamp;

	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)) {
	NET_INC_STATS(sock_net(sk),
	LINUX_MIB_TCPHYSTARTTRAINDETECT);
	NET_ADD_STATS(sock_net(sk),
	LINUX_MIB_TCPHYSTARTTRAINCWND,
	tp->snd_cwnd);
	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) {
	NET_INC_STATS(sock_net(sk),
	LINUX_MIB_TCPHYSTARTDELAYDETECT);
	NET_ADD_STATS(sock_net(sk),
	LINUX_MIB_TCPHYSTARTDELAYCWND,
	tp->snd_cwnd);
	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;

	if (tcp_in_slow_start(tp) && hystart_detect)
	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, const struct ack_sample sample)
	{
	struct cdg *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	if (sample->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 (sample->pkts_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, sample->rtt_us);
	ca->delack--;
	return;
	} else if (sample->pkts_acked > 1 && ca->delack < 5) {
	ca->delack++;
	}
	}

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

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

	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 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 cdg_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);
	}

	static 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_reno_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");