LCOV - code coverage report
Current view: top level - net/ipv4 - tcp_cubic.c (source / functions) Hit Total Coverage
Test: landlock.info Lines: 55 176 31.2 %
Date: 2021-04-22 12:43:58 Functions: 6 11 54.5 %

          Line data    Source code
       1             : // SPDX-License-Identifier: GPL-2.0-only
       2             : /*
       3             :  * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
       4             :  * Home page:
       5             :  *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
       6             :  * This is from the implementation of CUBIC TCP in
       7             :  * Sangtae Ha, Injong Rhee and Lisong Xu,
       8             :  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
       9             :  *  in ACM SIGOPS Operating System Review, July 2008.
      10             :  * Available from:
      11             :  *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
      12             :  *
      13             :  * CUBIC integrates a new slow start algorithm, called HyStart.
      14             :  * The details of HyStart are presented in
      15             :  *  Sangtae Ha and Injong Rhee,
      16             :  *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
      17             :  * Available from:
      18             :  *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
      19             :  *
      20             :  * All testing results are available from:
      21             :  * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
      22             :  *
      23             :  * Unless CUBIC is enabled and congestion window is large
      24             :  * this behaves the same as the original Reno.
      25             :  */
      26             : 
      27             : #include <linux/mm.h>
      28             : #include <linux/module.h>
      29             : #include <linux/math64.h>
      30             : #include <net/tcp.h>
      31             : 
      32             : #define BICTCP_BETA_SCALE    1024       /* Scale factor beta calculation
      33             :                                          * max_cwnd = snd_cwnd * beta
      34             :                                          */
      35             : #define BICTCP_HZ               10      /* BIC HZ 2^10 = 1024 */
      36             : 
      37             : /* Two methods of hybrid slow start */
      38             : #define HYSTART_ACK_TRAIN       0x1
      39             : #define HYSTART_DELAY           0x2
      40             : 
      41             : /* Number of delay samples for detecting the increase of delay */
      42             : #define HYSTART_MIN_SAMPLES     8
      43             : #define HYSTART_DELAY_MIN       (4000U) /* 4 ms */
      44             : #define HYSTART_DELAY_MAX       (16000U)        /* 16 ms */
      45             : #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
      46             : 
      47             : static int fast_convergence __read_mostly = 1;
      48             : static int beta __read_mostly = 717;    /* = 717/1024 (BICTCP_BETA_SCALE) */
      49             : static int initial_ssthresh __read_mostly;
      50             : static int bic_scale __read_mostly = 41;
      51             : static int tcp_friendliness __read_mostly = 1;
      52             : 
      53             : static int hystart __read_mostly = 1;
      54             : static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
      55             : static int hystart_low_window __read_mostly = 16;
      56             : static int hystart_ack_delta_us __read_mostly = 2000;
      57             : 
      58             : static u32 cube_rtt_scale __read_mostly;
      59             : static u32 beta_scale __read_mostly;
      60             : static u64 cube_factor __read_mostly;
      61             : 
      62             : /* Note parameters that are used for precomputing scale factors are read-only */
      63             : module_param(fast_convergence, int, 0644);
      64             : MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
      65             : module_param(beta, int, 0644);
      66             : MODULE_PARM_DESC(beta, "beta for multiplicative increase");
      67             : module_param(initial_ssthresh, int, 0644);
      68             : MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
      69             : module_param(bic_scale, int, 0444);
      70             : MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
      71             : module_param(tcp_friendliness, int, 0644);
      72             : MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
      73             : module_param(hystart, int, 0644);
      74             : MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
      75             : module_param(hystart_detect, int, 0644);
      76             : MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms"
      77             :                  " 1: packet-train 2: delay 3: both packet-train and delay");
      78             : module_param(hystart_low_window, int, 0644);
      79             : MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
      80             : module_param(hystart_ack_delta_us, int, 0644);
      81             : MODULE_PARM_DESC(hystart_ack_delta_us, "spacing between ack's indicating train (usecs)");
      82             : 
      83             : /* BIC TCP Parameters */
      84             : struct bictcp {
      85             :         u32     cnt;            /* increase cwnd by 1 after ACKs */
      86             :         u32     last_max_cwnd;  /* last maximum snd_cwnd */
      87             :         u32     last_cwnd;      /* the last snd_cwnd */
      88             :         u32     last_time;      /* time when updated last_cwnd */
      89             :         u32     bic_origin_point;/* origin point of bic function */
      90             :         u32     bic_K;          /* time to origin point
      91             :                                    from the beginning of the current epoch */
      92             :         u32     delay_min;      /* min delay (usec) */
      93             :         u32     epoch_start;    /* beginning of an epoch */
      94             :         u32     ack_cnt;        /* number of acks */
      95             :         u32     tcp_cwnd;       /* estimated tcp cwnd */
      96             :         u16     unused;
      97             :         u8      sample_cnt;     /* number of samples to decide curr_rtt */
      98             :         u8      found;          /* the exit point is found? */
      99             :         u32     round_start;    /* beginning of each round */
     100             :         u32     end_seq;        /* end_seq of the round */
     101             :         u32     last_ack;       /* last time when the ACK spacing is close */
     102             :         u32     curr_rtt;       /* the minimum rtt of current round */
     103             : };
     104             : 
     105           4 : static inline void bictcp_reset(struct bictcp *ca)
     106             : {
     107           4 :         memset(ca, 0, offsetof(struct bictcp, unused));
     108           4 :         ca->found = 0;
     109             : }
     110             : 
     111           4 : static inline u32 bictcp_clock_us(const struct sock *sk)
     112             : {
     113           4 :         return tcp_sk(sk)->tcp_mstamp;
     114             : }
     115             : 
     116           4 : static inline void bictcp_hystart_reset(struct sock *sk)
     117             : {
     118           4 :         struct tcp_sock *tp = tcp_sk(sk);
     119           4 :         struct bictcp *ca = inet_csk_ca(sk);
     120             : 
     121           4 :         ca->round_start = ca->last_ack = bictcp_clock_us(sk);
     122           4 :         ca->end_seq = tp->snd_nxt;
     123           4 :         ca->curr_rtt = ~0U;
     124           4 :         ca->sample_cnt = 0;
     125           4 : }
     126             : 
     127           4 : static void bictcp_init(struct sock *sk)
     128             : {
     129           4 :         struct bictcp *ca = inet_csk_ca(sk);
     130             : 
     131           4 :         bictcp_reset(ca);
     132             : 
     133           4 :         if (hystart)
     134           4 :                 bictcp_hystart_reset(sk);
     135             : 
     136           4 :         if (!hystart && initial_ssthresh)
     137           0 :                 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
     138           4 : }
     139             : 
     140         235 : static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
     141             : {
     142         235 :         if (event == CA_EVENT_TX_START) {
     143         231 :                 struct bictcp *ca = inet_csk_ca(sk);
     144         231 :                 u32 now = tcp_jiffies32;
     145         231 :                 s32 delta;
     146             : 
     147         231 :                 delta = now - tcp_sk(sk)->lsndtime;
     148             : 
     149             :                 /* We were application limited (idle) for a while.
     150             :                  * Shift epoch_start to keep cwnd growth to cubic curve.
     151             :                  */
     152         231 :                 if (ca->epoch_start && delta > 0) {
     153           0 :                         ca->epoch_start += delta;
     154           0 :                         if (after(ca->epoch_start, now))
     155           0 :                                 ca->epoch_start = now;
     156             :                 }
     157         231 :                 return;
     158             :         }
     159             : }
     160             : 
     161             : /* calculate the cubic root of x using a table lookup followed by one
     162             :  * Newton-Raphson iteration.
     163             :  * Avg err ~= 0.195%
     164             :  */
     165           0 : static u32 cubic_root(u64 a)
     166             : {
     167           0 :         u32 x, b, shift;
     168             :         /*
     169             :          * cbrt(x) MSB values for x MSB values in [0..63].
     170             :          * Precomputed then refined by hand - Willy Tarreau
     171             :          *
     172             :          * For x in [0..63],
     173             :          *   v = cbrt(x << 18) - 1
     174             :          *   cbrt(x) = (v[x] + 10) >> 6
     175             :          */
     176           0 :         static const u8 v[] = {
     177             :                 /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
     178             :                 /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
     179             :                 /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
     180             :                 /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
     181             :                 /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
     182             :                 /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
     183             :                 /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
     184             :                 /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
     185             :         };
     186             : 
     187           0 :         b = fls64(a);
     188           0 :         if (b < 7) {
     189             :                 /* a in [0..63] */
     190           0 :                 return ((u32)v[(u32)a] + 35) >> 6;
     191             :         }
     192             : 
     193           0 :         b = ((b * 84) >> 8) - 1;
     194           0 :         shift = (a >> (b * 3));
     195             : 
     196           0 :         x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
     197             : 
     198             :         /*
     199             :          * Newton-Raphson iteration
     200             :          *                         2
     201             :          * x    = ( 2 * x  +  a / x  ) / 3
     202             :          *  k+1          k         k
     203             :          */
     204           0 :         x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
     205           0 :         x = ((x * 341) >> 10);
     206           0 :         return x;
     207             : }
     208             : 
     209             : /*
     210             :  * Compute congestion window to use.
     211             :  */
     212           0 : static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
     213             : {
     214           0 :         u32 delta, bic_target, max_cnt;
     215           0 :         u64 offs, t;
     216             : 
     217           0 :         ca->ack_cnt += acked;        /* count the number of ACKed packets */
     218             : 
     219           0 :         if (ca->last_cwnd == cwnd &&
     220           0 :             (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
     221             :                 return;
     222             : 
     223             :         /* The CUBIC function can update ca->cnt at most once per jiffy.
     224             :          * On all cwnd reduction events, ca->epoch_start is set to 0,
     225             :          * which will force a recalculation of ca->cnt.
     226             :          */
     227           0 :         if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
     228           0 :                 goto tcp_friendliness;
     229             : 
     230           0 :         ca->last_cwnd = cwnd;
     231           0 :         ca->last_time = tcp_jiffies32;
     232             : 
     233           0 :         if (ca->epoch_start == 0) {
     234           0 :                 ca->epoch_start = tcp_jiffies32;     /* record beginning */
     235           0 :                 ca->ack_cnt = acked;                 /* start counting */
     236           0 :                 ca->tcp_cwnd = cwnd;                 /* syn with cubic */
     237             : 
     238           0 :                 if (ca->last_max_cwnd <= cwnd) {
     239           0 :                         ca->bic_K = 0;
     240           0 :                         ca->bic_origin_point = cwnd;
     241             :                 } else {
     242             :                         /* Compute new K based on
     243             :                          * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
     244             :                          */
     245           0 :                         ca->bic_K = cubic_root(cube_factor
     246           0 :                                                * (ca->last_max_cwnd - cwnd));
     247           0 :                         ca->bic_origin_point = ca->last_max_cwnd;
     248             :                 }
     249             :         }
     250             : 
     251             :         /* cubic function - calc*/
     252             :         /* calculate c * time^3 / rtt,
     253             :          *  while considering overflow in calculation of time^3
     254             :          * (so time^3 is done by using 64 bit)
     255             :          * and without the support of division of 64bit numbers
     256             :          * (so all divisions are done by using 32 bit)
     257             :          *  also NOTE the unit of those veriables
     258             :          *        time  = (t - K) / 2^bictcp_HZ
     259             :          *        c = bic_scale >> 10
     260             :          * rtt  = (srtt >> 3) / HZ
     261             :          * !!! The following code does not have overflow problems,
     262             :          * if the cwnd < 1 million packets !!!
     263             :          */
     264             : 
     265           0 :         t = (s32)(tcp_jiffies32 - ca->epoch_start);
     266           0 :         t += usecs_to_jiffies(ca->delay_min);
     267             :         /* change the unit from HZ to bictcp_HZ */
     268           0 :         t <<= BICTCP_HZ;
     269           0 :         do_div(t, HZ);
     270             : 
     271           0 :         if (t < ca->bic_K)                /* t - K */
     272           0 :                 offs = ca->bic_K - t;
     273             :         else
     274           0 :                 offs = t - ca->bic_K;
     275             : 
     276             :         /* c/rtt * (t-K)^3 */
     277           0 :         delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
     278           0 :         if (t < ca->bic_K)                            /* below origin*/
     279           0 :                 bic_target = ca->bic_origin_point - delta;
     280             :         else                                          /* above origin*/
     281           0 :                 bic_target = ca->bic_origin_point + delta;
     282             : 
     283             :         /* cubic function - calc bictcp_cnt*/
     284           0 :         if (bic_target > cwnd) {
     285           0 :                 ca->cnt = cwnd / (bic_target - cwnd);
     286             :         } else {
     287           0 :                 ca->cnt = 100 * cwnd;              /* very small increment*/
     288             :         }
     289             : 
     290             :         /*
     291             :          * The initial growth of cubic function may be too conservative
     292             :          * when the available bandwidth is still unknown.
     293             :          */
     294           0 :         if (ca->last_max_cwnd == 0 && ca->cnt > 20)
     295           0 :                 ca->cnt = 20;        /* increase cwnd 5% per RTT */
     296             : 
     297           0 : tcp_friendliness:
     298             :         /* TCP Friendly */
     299           0 :         if (tcp_friendliness) {
     300           0 :                 u32 scale = beta_scale;
     301             : 
     302           0 :                 delta = (cwnd * scale) >> 3;
     303           0 :                 while (ca->ack_cnt > delta) {             /* update tcp cwnd */
     304           0 :                         ca->ack_cnt -= delta;
     305           0 :                         ca->tcp_cwnd++;
     306             :                 }
     307             : 
     308           0 :                 if (ca->tcp_cwnd > cwnd) {        /* if bic is slower than tcp */
     309           0 :                         delta = ca->tcp_cwnd - cwnd;
     310           0 :                         max_cnt = cwnd / delta;
     311           0 :                         if (ca->cnt > max_cnt)
     312           0 :                                 ca->cnt = max_cnt;
     313             :                 }
     314             :         }
     315             : 
     316             :         /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
     317             :          * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
     318             :          */
     319           0 :         ca->cnt = max(ca->cnt, 2U);
     320             : }
     321             : 
     322         351 : static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
     323             : {
     324         351 :         struct tcp_sock *tp = tcp_sk(sk);
     325         351 :         struct bictcp *ca = inet_csk_ca(sk);
     326             : 
     327         702 :         if (!tcp_is_cwnd_limited(sk))
     328             :                 return;
     329             : 
     330           0 :         if (tcp_in_slow_start(tp)) {
     331           0 :                 if (hystart && after(ack, ca->end_seq))
     332           0 :                         bictcp_hystart_reset(sk);
     333           0 :                 acked = tcp_slow_start(tp, acked);
     334           0 :                 if (!acked)
     335             :                         return;
     336             :         }
     337           0 :         bictcp_update(ca, tp->snd_cwnd, acked);
     338           0 :         tcp_cong_avoid_ai(tp, ca->cnt, acked);
     339             : }
     340             : 
     341           0 : static u32 bictcp_recalc_ssthresh(struct sock *sk)
     342             : {
     343           0 :         const struct tcp_sock *tp = tcp_sk(sk);
     344           0 :         struct bictcp *ca = inet_csk_ca(sk);
     345             : 
     346           0 :         ca->epoch_start = 0; /* end of epoch */
     347             : 
     348             :         /* Wmax and fast convergence */
     349           0 :         if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
     350           0 :                 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
     351           0 :                         / (2 * BICTCP_BETA_SCALE);
     352             :         else
     353           0 :                 ca->last_max_cwnd = tp->snd_cwnd;
     354             : 
     355           0 :         return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
     356             : }
     357             : 
     358           4 : static void bictcp_state(struct sock *sk, u8 new_state)
     359             : {
     360           4 :         if (new_state == TCP_CA_Loss) {
     361           0 :                 bictcp_reset(inet_csk_ca(sk));
     362           0 :                 bictcp_hystart_reset(sk);
     363             :         }
     364           4 : }
     365             : 
     366             : /* Account for TSO/GRO delays.
     367             :  * Otherwise short RTT flows could get too small ssthresh, since during
     368             :  * slow start we begin with small TSO packets and ca->delay_min would
     369             :  * not account for long aggregation delay when TSO packets get bigger.
     370             :  * Ideally even with a very small RTT we would like to have at least one
     371             :  * TSO packet being sent and received by GRO, and another one in qdisc layer.
     372             :  * We apply another 100% factor because @rate is doubled at this point.
     373             :  * We cap the cushion to 1ms.
     374             :  */
     375           0 : static u32 hystart_ack_delay(struct sock *sk)
     376             : {
     377           0 :         unsigned long rate;
     378             : 
     379           0 :         rate = READ_ONCE(sk->sk_pacing_rate);
     380           0 :         if (!rate)
     381             :                 return 0;
     382           0 :         return min_t(u64, USEC_PER_MSEC,
     383             :                      div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
     384             : }
     385             : 
     386           0 : static void hystart_update(struct sock *sk, u32 delay)
     387             : {
     388           0 :         struct tcp_sock *tp = tcp_sk(sk);
     389           0 :         struct bictcp *ca = inet_csk_ca(sk);
     390           0 :         u32 threshold;
     391             : 
     392           0 :         if (hystart_detect & HYSTART_ACK_TRAIN) {
     393           0 :                 u32 now = bictcp_clock_us(sk);
     394             : 
     395             :                 /* first detection parameter - ack-train detection */
     396           0 :                 if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
     397           0 :                         ca->last_ack = now;
     398             : 
     399           0 :                         threshold = ca->delay_min + hystart_ack_delay(sk);
     400             : 
     401             :                         /* Hystart ack train triggers if we get ack past
     402             :                          * ca->delay_min/2.
     403             :                          * Pacing might have delayed packets up to RTT/2
     404             :                          * during slow start.
     405             :                          */
     406           0 :                         if (sk->sk_pacing_status == SK_PACING_NONE)
     407           0 :                                 threshold >>= 1;
     408             : 
     409           0 :                         if ((s32)(now - ca->round_start) > threshold) {
     410           0 :                                 ca->found = 1;
     411           0 :                                 pr_debug("hystart_ack_train (%u > %u) delay_min %u (+ ack_delay %u) cwnd %u\n",
     412             :                                          now - ca->round_start, threshold,
     413             :                                          ca->delay_min, hystart_ack_delay(sk), tp->snd_cwnd);
     414           0 :                                 NET_INC_STATS(sock_net(sk),
     415             :                                               LINUX_MIB_TCPHYSTARTTRAINDETECT);
     416           0 :                                 NET_ADD_STATS(sock_net(sk),
     417             :                                               LINUX_MIB_TCPHYSTARTTRAINCWND,
     418             :                                               tp->snd_cwnd);
     419           0 :                                 tp->snd_ssthresh = tp->snd_cwnd;
     420             :                         }
     421             :                 }
     422             :         }
     423             : 
     424           0 :         if (hystart_detect & HYSTART_DELAY) {
     425             :                 /* obtain the minimum delay of more than sampling packets */
     426           0 :                 if (ca->curr_rtt > delay)
     427           0 :                         ca->curr_rtt = delay;
     428           0 :                 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
     429           0 :                         ca->sample_cnt++;
     430             :                 } else {
     431           0 :                         if (ca->curr_rtt > ca->delay_min +
     432           0 :                             HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
     433           0 :                                 ca->found = 1;
     434           0 :                                 NET_INC_STATS(sock_net(sk),
     435             :                                               LINUX_MIB_TCPHYSTARTDELAYDETECT);
     436           0 :                                 NET_ADD_STATS(sock_net(sk),
     437             :                                               LINUX_MIB_TCPHYSTARTDELAYCWND,
     438             :                                               tp->snd_cwnd);
     439           0 :                                 tp->snd_ssthresh = tp->snd_cwnd;
     440             :                         }
     441             :                 }
     442             :         }
     443           0 : }
     444             : 
     445         351 : static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
     446             : {
     447         351 :         const struct tcp_sock *tp = tcp_sk(sk);
     448         351 :         struct bictcp *ca = inet_csk_ca(sk);
     449         351 :         u32 delay;
     450             : 
     451             :         /* Some calls are for duplicates without timetamps */
     452         351 :         if (sample->rtt_us < 0)
     453             :                 return;
     454             : 
     455             :         /* Discard delay samples right after fast recovery */
     456         351 :         if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
     457             :                 return;
     458             : 
     459         351 :         delay = sample->rtt_us;
     460         351 :         if (delay == 0)
     461             :                 delay = 1;
     462             : 
     463             :         /* first time call or link delay decreases */
     464         351 :         if (ca->delay_min == 0 || ca->delay_min > delay)
     465          16 :                 ca->delay_min = delay;
     466             : 
     467             :         /* hystart triggers when cwnd is larger than some threshold */
     468         351 :         if (!ca->found && tcp_in_slow_start(tp) && hystart &&
     469         351 :             tp->snd_cwnd >= hystart_low_window)
     470           0 :                 hystart_update(sk, delay);
     471             : }
     472             : 
     473             : static struct tcp_congestion_ops cubictcp __read_mostly = {
     474             :         .init           = bictcp_init,
     475             :         .ssthresh       = bictcp_recalc_ssthresh,
     476             :         .cong_avoid     = bictcp_cong_avoid,
     477             :         .set_state      = bictcp_state,
     478             :         .undo_cwnd      = tcp_reno_undo_cwnd,
     479             :         .cwnd_event     = bictcp_cwnd_event,
     480             :         .pkts_acked     = bictcp_acked,
     481             :         .owner          = THIS_MODULE,
     482             :         .name           = "cubic",
     483             : };
     484             : 
     485           1 : static int __init cubictcp_register(void)
     486             : {
     487           1 :         BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
     488             : 
     489             :         /* Precompute a bunch of the scaling factors that are used per-packet
     490             :          * based on SRTT of 100ms
     491             :          */
     492             : 
     493           1 :         beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
     494           1 :                 / (BICTCP_BETA_SCALE - beta);
     495             : 
     496           1 :         cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
     497             : 
     498             :         /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
     499             :          *  so K = cubic_root( (wmax-cwnd)*rtt/c )
     500             :          * the unit of K is bictcp_HZ=2^10, not HZ
     501             :          *
     502             :          *  c = bic_scale >> 10
     503             :          *  rtt = 100ms
     504             :          *
     505             :          * the following code has been designed and tested for
     506             :          * cwnd < 1 million packets
     507             :          * RTT < 100 seconds
     508             :          * HZ < 1,000,00  (corresponding to 10 nano-second)
     509             :          */
     510             : 
     511             :         /* 1/c * 2^2*bictcp_HZ * srtt */
     512           1 :         cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
     513             : 
     514             :         /* divide by bic_scale and by constant Srtt (100ms) */
     515           1 :         do_div(cube_factor, bic_scale * 10);
     516             : 
     517           1 :         return tcp_register_congestion_control(&cubictcp);
     518             : }
     519             : 
     520           0 : static void __exit cubictcp_unregister(void)
     521             : {
     522           0 :         tcp_unregister_congestion_control(&cubictcp);
     523           0 : }
     524             : 
     525             : module_init(cubictcp_register);
     526             : module_exit(cubictcp_unregister);
     527             : 
     528             : MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
     529             : MODULE_LICENSE("GPL");
     530             : MODULE_DESCRIPTION("CUBIC TCP");
     531             : MODULE_VERSION("2.3");

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