// SPDX-License-Identifier: GPL-2.0-only
/*
 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
 * Home page:
 *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
 * This is from the implementation of CUBIC TCP in
 * Sangtae Ha, Injong Rhee and Lisong Xu,
 *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
 *  in ACM SIGOPS Operating System Review, July 2008.
 * Available from:
 *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
 *
 * CUBIC integrates a new slow start algorithm, called HyStart.
 * The details of HyStart are presented in
 *  Sangtae Ha and Injong Rhee,
 *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
 * Available from:
 *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
 *
 * All testing results are available from:
 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
 *
 * Unless CUBIC is enabled and congestion window is large
 * this behaves the same as the original Reno.
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/math64.h>
#include <net/tcp.h>

#define BICTCP_BETA_SCALE    1024       /* Scale factor beta calculation
                                         * max_cwnd = snd_cwnd * beta
                                         */
#define BICTCP_HZ               10      /* BIC HZ 2^10 = 1024 */

/* Two methods of hybrid slow start */
#define HYSTART_ACK_TRAIN       0x1
#define HYSTART_DELAY           0x2

/* Number of delay samples for detecting the increase of delay */
#define HYSTART_MIN_SAMPLES     8
#define HYSTART_DELAY_MIN       (4000U) /* 4 ms */
#define HYSTART_DELAY_MAX       (16000U)        /* 16 ms */
#define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)

static int fast_convergence __read_mostly = 1;
static int beta __read_mostly = 717;    /* = 717/1024 (BICTCP_BETA_SCALE) */
static int initial_ssthresh __read_mostly;
static int bic_scale __read_mostly = 41;
static int tcp_friendliness __read_mostly = 1;

static int hystart __read_mostly = 1;
static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
static int hystart_low_window __read_mostly = 16;
static int hystart_ack_delta_us __read_mostly = 2000;

static u32 cube_rtt_scale __read_mostly;
static u32 beta_scale __read_mostly;
static u64 cube_factor __read_mostly;

/* Note parameters that are used for precomputing scale factors are read-only */
module_param(fast_convergence, int, 0644);
MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
module_param(beta, int, 0644);
MODULE_PARM_DESC(beta, "beta for multiplicative increase");
module_param(initial_ssthresh, int, 0644);
MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
module_param(bic_scale, int, 0444);
MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
module_param(tcp_friendliness, int, 0644);
MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
module_param(hystart, int, 0644);
MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
module_param(hystart_detect, int, 0644);
MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms"
                 " 1: packet-train 2: delay 3: both packet-train and delay");
module_param(hystart_low_window, int, 0644);
MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
module_param(hystart_ack_delta_us, int, 0644);
MODULE_PARM_DESC(hystart_ack_delta_us, "spacing between ack's indicating train (usecs)");

/* BIC TCP Parameters */
struct bictcp {
        u32     cnt;            /* increase cwnd by 1 after ACKs */
        u32     last_max_cwnd;  /* last maximum snd_cwnd */
        u32     last_cwnd;      /* the last snd_cwnd */
        u32     last_time;      /* time when updated last_cwnd */
        u32     bic_origin_point;/* origin point of bic function */
        u32     bic_K;          /* time to origin point
                                   from the beginning of the current epoch */
        u32     delay_min;      /* min delay (usec) */
        u32     epoch_start;    /* beginning of an epoch */
        u32     ack_cnt;        /* number of acks */
        u32     tcp_cwnd;       /* estimated tcp cwnd */
        u16     unused;
        u8      sample_cnt;     /* number of samples to decide curr_rtt */
        u8      found;          /* the exit point is found? */
        u32     round_start;    /* beginning of each round */
        u32     end_seq;        /* end_seq of the round */
        u32     last_ack;       /* last time when the ACK spacing is close */
        u32     curr_rtt;       /* the minimum rtt of current round */
};

static inline void bictcp_reset(struct bictcp *ca)
{
        memset(ca, 0, offsetof(struct bictcp, unused));
        ca->found = 0;
}

static inline u32 bictcp_clock_us(const struct sock *sk)
{
        return tcp_sk(sk)->tcp_mstamp;
}

static inline void bictcp_hystart_reset(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct bictcp *ca = inet_csk_ca(sk);

        ca->round_start = ca->last_ack = bictcp_clock_us(sk);
        ca->end_seq = tp->snd_nxt;
        ca->curr_rtt = ~0U;
        ca->sample_cnt = 0;
}

static void cubictcp_init(struct sock *sk)
{
        struct bictcp *ca = inet_csk_ca(sk);

        bictcp_reset(ca);

        if (hystart)
                bictcp_hystart_reset(sk);

        if (!hystart && initial_ssthresh)
                tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
}

static void cubictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
{
        if (event == CA_EVENT_TX_START) {
                struct bictcp *ca = inet_csk_ca(sk);
                u32 now = tcp_jiffies32;
                s32 delta;

                delta = now - tcp_sk(sk)->lsndtime;

                /* We were application limited (idle) for a while.
                 * Shift epoch_start to keep cwnd growth to cubic curve.
                 */
                if (ca->epoch_start && delta > 0) {
                        ca->epoch_start += delta;
                        if (after(ca->epoch_start, now))
                                ca->epoch_start = now;
                }
                return;
        }
}

/* calculate the cubic root of x using a table lookup followed by one
 * Newton-Raphson iteration.
 * Avg err ~= 0.195%
 */
static u32 cubic_root(u64 a)
{
        u32 x, b, shift;
        /*
         * cbrt(x) MSB values for x MSB values in [0..63].
         * Precomputed then refined by hand - Willy Tarreau
         *
         * For x in [0..63],
         *   v = cbrt(x << 18) - 1
         *   cbrt(x) = (v[x] + 10) >> 6
         */
        static const u8 v[] = {
                /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
                /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
                /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
                /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
                /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
                /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
                /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
                /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
        };

        b = fls64(a);
        if (b < 7) {
                /* a in [0..63] */
                return ((u32)v[(u32)a] + 35) >> 6;
        }

        b = ((b * 84) >> 8) - 1;
        shift = (a >> (b * 3));

        x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;

        /*
         * Newton-Raphson iteration
         *                         2
         * x    = ( 2 * x  +  a / x  ) / 3
         *  k+1          k         k
         */
        x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
        x = ((x * 341) >> 10);
        return x;
}

/*
 * Compute congestion window to use.
 */
static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
{
        u32 delta, bic_target, max_cnt;
        u64 offs, t;

        ca->ack_cnt += acked;   /* count the number of ACKed packets */

        if (ca->last_cwnd == cwnd &&
            (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
                return;

        /* The CUBIC function can update ca->cnt at most once per jiffy.
         * On all cwnd reduction events, ca->epoch_start is set to 0,
         * which will force a recalculation of ca->cnt.
         */
        if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
                goto tcp_friendliness;

        ca->last_cwnd = cwnd;
        ca->last_time = tcp_jiffies32;

        if (ca->epoch_start == 0) {
                ca->epoch_start = tcp_jiffies32;        /* record beginning */
                ca->ack_cnt = acked;                    /* start counting */
                ca->tcp_cwnd = cwnd;                    /* syn with cubic */

                if (ca->last_max_cwnd <= cwnd) {
                        ca->bic_K = 0;
                        ca->bic_origin_point = cwnd;
                } else {
                        /* Compute new K based on
                         * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
                         */
                        ca->bic_K = cubic_root(cube_factor
                                               * (ca->last_max_cwnd - cwnd));
                        ca->bic_origin_point = ca->last_max_cwnd;
                }
        }

        /* cubic function - calc*/
        /* calculate c * time^3 / rtt,
         *  while considering overflow in calculation of time^3
         * (so time^3 is done by using 64 bit)
         * and without the support of division of 64bit numbers
         * (so all divisions are done by using 32 bit)
         *  also NOTE the unit of those veriables
         *        time  = (t - K) / 2^bictcp_HZ
         *        c = bic_scale >> 10
         * rtt  = (srtt >> 3) / HZ
         * !!! The following code does not have overflow problems,
         * if the cwnd < 1 million packets !!!
         */

        t = (s32)(tcp_jiffies32 - ca->epoch_start);
        t += usecs_to_jiffies(ca->delay_min);
        /* change the unit from HZ to bictcp_HZ */
        t <<= BICTCP_HZ;
        do_div(t, HZ);

        if (t < ca->bic_K)              /* t - K */
                offs = ca->bic_K - t;
        else
                offs = t - ca->bic_K;

        /* c/rtt * (t-K)^3 */
        delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
        if (t < ca->bic_K)                            /* below origin*/
                bic_target = ca->bic_origin_point - delta;
        else                                          /* above origin*/
                bic_target = ca->bic_origin_point + delta;

        /* cubic function - calc bictcp_cnt*/
        if (bic_target > cwnd) {
                ca->cnt = cwnd / (bic_target - cwnd);
        } else {
                ca->cnt = 100 * cwnd;              /* very small increment*/
        }

        /*
         * The initial growth of cubic function may be too conservative
         * when the available bandwidth is still unknown.
         */
        if (ca->last_max_cwnd == 0 && ca->cnt > 20)
                ca->cnt = 20;   /* increase cwnd 5% per RTT */

tcp_friendliness:
        /* TCP Friendly */
        if (tcp_friendliness) {
                u32 scale = beta_scale;

                delta = (cwnd * scale) >> 3;
                while (ca->ack_cnt > delta) {           /* update tcp cwnd */
                        ca->ack_cnt -= delta;
                        ca->tcp_cwnd++;
                }

                if (ca->tcp_cwnd > cwnd) {      /* if bic is slower than tcp */
                        delta = ca->tcp_cwnd - cwnd;
                        max_cnt = cwnd / delta;
                        if (ca->cnt > max_cnt)
                                ca->cnt = max_cnt;
                }
        }

        /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
         * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
         */
        ca->cnt = max(ca->cnt, 2U);
}

static void cubictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct bictcp *ca = inet_csk_ca(sk);

        if (!tcp_is_cwnd_limited(sk))
                return;

        if (tcp_in_slow_start(tp)) {
                if (hystart && after(ack, ca->end_seq))
                        bictcp_hystart_reset(sk);
                acked = tcp_slow_start(tp, acked);
                if (!acked)
                        return;
        }
        bictcp_update(ca, tp->snd_cwnd, acked);
        tcp_cong_avoid_ai(tp, ca->cnt, acked);
}

static u32 cubictcp_recalc_ssthresh(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct bictcp *ca = inet_csk_ca(sk);

        ca->epoch_start = 0;    /* end of epoch */

        /* Wmax and fast convergence */
        if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
                ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
                        / (2 * BICTCP_BETA_SCALE);
        else
                ca->last_max_cwnd = tp->snd_cwnd;

        return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
}

static void cubictcp_state(struct sock *sk, u8 new_state)
{
        if (new_state == TCP_CA_Loss) {
                bictcp_reset(inet_csk_ca(sk));
                bictcp_hystart_reset(sk);
        }
}

/* Account for TSO/GRO delays.
 * Otherwise short RTT flows could get too small ssthresh, since during
 * slow start we begin with small TSO packets and ca->delay_min would
 * not account for long aggregation delay when TSO packets get bigger.
 * Ideally even with a very small RTT we would like to have at least one
 * TSO packet being sent and received by GRO, and another one in qdisc layer.
 * We apply another 100% factor because @rate is doubled at this point.
 * We cap the cushion to 1ms.
 */
static u32 hystart_ack_delay(struct sock *sk)
{
        unsigned long rate;

        rate = READ_ONCE(sk->sk_pacing_rate);
        if (!rate)
                return 0;
        return min_t(u64, USEC_PER_MSEC,
                     div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
}

static void hystart_update(struct sock *sk, u32 delay)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct bictcp *ca = inet_csk_ca(sk);
        u32 threshold;

        if (hystart_detect & HYSTART_ACK_TRAIN) {
                u32 now = bictcp_clock_us(sk);

                /* first detection parameter - ack-train detection */
                if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
                        ca->last_ack = now;

                        threshold = ca->delay_min + hystart_ack_delay(sk);

                        /* Hystart ack train triggers if we get ack past
                         * ca->delay_min/2.
                         * Pacing might have delayed packets up to RTT/2
                         * during slow start.
                         */
                        if (sk->sk_pacing_status == SK_PACING_NONE)
                                threshold >>= 1;

                        if ((s32)(now - ca->round_start) > threshold) {
                                ca->found = 1;
                                pr_debug("hystart_ack_train (%u > %u) delay_min %u (+ ack_delay %u) cwnd %u\n",
                                         now - ca->round_start, threshold,
                                         ca->delay_min, hystart_ack_delay(sk), tp->snd_cwnd);
                                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;
                        }
                }
        }

        if (hystart_detect & HYSTART_DELAY) {
                /* obtain the minimum delay of more than sampling packets */
                if (ca->curr_rtt > delay)
                        ca->curr_rtt = delay;
                if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
                        ca->sample_cnt++;
                } else {
                        if (ca->curr_rtt > ca->delay_min +
                            HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
                                ca->found = 1;
                                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 void cubictcp_acked(struct sock *sk, const struct ack_sample *sample)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct bictcp *ca = inet_csk_ca(sk);
        u32 delay;

        /* Some calls are for duplicates without timetamps */
        if (sample->rtt_us < 0)
                return;

        /* Discard delay samples right after fast recovery */
        if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
                return;

        delay = sample->rtt_us;
        if (delay == 0)
                delay = 1;

        /* first time call or link delay decreases */
        if (ca->delay_min == 0 || ca->delay_min > delay)
                ca->delay_min = delay;

        /* hystart triggers when cwnd is larger than some threshold */
        if (!ca->found && tcp_in_slow_start(tp) && hystart &&
            tp->snd_cwnd >= hystart_low_window)
                hystart_update(sk, delay);
}

static struct tcp_congestion_ops cubictcp __read_mostly = {
        .init           = cubictcp_init,
        .ssthresh       = cubictcp_recalc_ssthresh,
        .cong_avoid     = cubictcp_cong_avoid,
        .set_state      = cubictcp_state,
        .undo_cwnd      = tcp_reno_undo_cwnd,
        .cwnd_event     = cubictcp_cwnd_event,
        .pkts_acked     = cubictcp_acked,
        .owner          = THIS_MODULE,
        .name           = "cubic",
};

static int __init cubictcp_register(void)
{
        BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);

        /* Precompute a bunch of the scaling factors that are used per-packet
         * based on SRTT of 100ms
         */

        beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
                / (BICTCP_BETA_SCALE - beta);

        cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */

        /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
         *  so K = cubic_root( (wmax-cwnd)*rtt/c )
         * the unit of K is bictcp_HZ=2^10, not HZ
         *
         *  c = bic_scale >> 10
         *  rtt = 100ms
         *
         * the following code has been designed and tested for
         * cwnd < 1 million packets
         * RTT < 100 seconds
         * HZ < 1,000,00  (corresponding to 10 nano-second)
         */

        /* 1/c * 2^2*bictcp_HZ * srtt */
        cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */

        /* divide by bic_scale and by constant Srtt (100ms) */
        do_div(cube_factor, bic_scale * 10);

        return tcp_register_congestion_control(&cubictcp);
}

static void __exit cubictcp_unregister(void)
{
        tcp_unregister_congestion_control(&cubictcp);
}

module_init(cubictcp_register);
module_exit(cubictcp_unregister);

MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("CUBIC TCP");
MODULE_VERSION("2.3");