text: /*
   * 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 = div_u64(local_clock(), NSEC_PER_USEC);
  
  		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);
  }
  
  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");
  