// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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");