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531 lines
15 KiB
531 lines
15 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* |
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* TCP CUBIC: Binary Increase Congestion control for TCP v2.3 |
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* Home page: |
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* http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC |
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* This is from the implementation of CUBIC TCP in |
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* Sangtae Ha, Injong Rhee and Lisong Xu, |
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* "CUBIC: A New TCP-Friendly High-Speed TCP Variant" |
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* in ACM SIGOPS Operating System Review, July 2008. |
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* Available from: |
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* http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf |
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* |
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* CUBIC integrates a new slow start algorithm, called HyStart. |
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* The details of HyStart are presented in |
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* Sangtae Ha and Injong Rhee, |
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* "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008. |
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* Available from: |
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* http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf |
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* |
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* All testing results are available from: |
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* http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing |
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* |
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* Unless CUBIC is enabled and congestion window is large |
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* this behaves the same as the original Reno. |
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*/ |
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|
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#include <linux/mm.h> |
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#include <linux/module.h> |
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#include <linux/math64.h> |
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#include <net/tcp.h> |
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|
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#define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation |
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* max_cwnd = snd_cwnd * beta |
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*/ |
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#define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */ |
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|
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/* Two methods of hybrid slow start */ |
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#define HYSTART_ACK_TRAIN 0x1 |
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#define HYSTART_DELAY 0x2 |
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|
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/* Number of delay samples for detecting the increase of delay */ |
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#define HYSTART_MIN_SAMPLES 8 |
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#define HYSTART_DELAY_MIN (4000U) /* 4 ms */ |
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#define HYSTART_DELAY_MAX (16000U) /* 16 ms */ |
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#define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX) |
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|
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static int fast_convergence __read_mostly = 1; |
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static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */ |
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static int initial_ssthresh __read_mostly; |
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static int bic_scale __read_mostly = 41; |
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static int tcp_friendliness __read_mostly = 1; |
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|
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static int hystart __read_mostly = 1; |
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static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY; |
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static int hystart_low_window __read_mostly = 16; |
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static int hystart_ack_delta_us __read_mostly = 2000; |
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|
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static u32 cube_rtt_scale __read_mostly; |
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static u32 beta_scale __read_mostly; |
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static u64 cube_factor __read_mostly; |
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|
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/* Note parameters that are used for precomputing scale factors are read-only */ |
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module_param(fast_convergence, int, 0644); |
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MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); |
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module_param(beta, int, 0644); |
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MODULE_PARM_DESC(beta, "beta for multiplicative increase"); |
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module_param(initial_ssthresh, int, 0644); |
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MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); |
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module_param(bic_scale, int, 0444); |
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MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); |
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module_param(tcp_friendliness, int, 0644); |
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MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); |
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module_param(hystart, int, 0644); |
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MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm"); |
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module_param(hystart_detect, int, 0644); |
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MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms" |
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" 1: packet-train 2: delay 3: both packet-train and delay"); |
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module_param(hystart_low_window, int, 0644); |
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MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start"); |
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module_param(hystart_ack_delta_us, int, 0644); |
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MODULE_PARM_DESC(hystart_ack_delta_us, "spacing between ack's indicating train (usecs)"); |
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|
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/* BIC TCP Parameters */ |
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struct bictcp { |
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u32 cnt; /* increase cwnd by 1 after ACKs */ |
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u32 last_max_cwnd; /* last maximum snd_cwnd */ |
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u32 last_cwnd; /* the last snd_cwnd */ |
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u32 last_time; /* time when updated last_cwnd */ |
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u32 bic_origin_point;/* origin point of bic function */ |
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u32 bic_K; /* time to origin point |
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from the beginning of the current epoch */ |
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u32 delay_min; /* min delay (usec) */ |
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u32 epoch_start; /* beginning of an epoch */ |
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u32 ack_cnt; /* number of acks */ |
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u32 tcp_cwnd; /* estimated tcp cwnd */ |
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u16 unused; |
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u8 sample_cnt; /* number of samples to decide curr_rtt */ |
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u8 found; /* the exit point is found? */ |
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u32 round_start; /* beginning of each round */ |
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u32 end_seq; /* end_seq of the round */ |
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u32 last_ack; /* last time when the ACK spacing is close */ |
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u32 curr_rtt; /* the minimum rtt of current round */ |
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}; |
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|
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static inline void bictcp_reset(struct bictcp *ca) |
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{ |
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memset(ca, 0, offsetof(struct bictcp, unused)); |
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ca->found = 0; |
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} |
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|
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static inline u32 bictcp_clock_us(const struct sock *sk) |
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{ |
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return tcp_sk(sk)->tcp_mstamp; |
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} |
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|
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static inline void bictcp_hystart_reset(struct sock *sk) |
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{ |
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struct tcp_sock *tp = tcp_sk(sk); |
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struct bictcp *ca = inet_csk_ca(sk); |
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|
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ca->round_start = ca->last_ack = bictcp_clock_us(sk); |
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ca->end_seq = tp->snd_nxt; |
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ca->curr_rtt = ~0U; |
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ca->sample_cnt = 0; |
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} |
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|
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static void cubictcp_init(struct sock *sk) |
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{ |
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struct bictcp *ca = inet_csk_ca(sk); |
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|
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bictcp_reset(ca); |
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|
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if (hystart) |
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bictcp_hystart_reset(sk); |
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|
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if (!hystart && initial_ssthresh) |
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tcp_sk(sk)->snd_ssthresh = initial_ssthresh; |
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} |
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|
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static void cubictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event) |
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{ |
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if (event == CA_EVENT_TX_START) { |
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struct bictcp *ca = inet_csk_ca(sk); |
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u32 now = tcp_jiffies32; |
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s32 delta; |
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|
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delta = now - tcp_sk(sk)->lsndtime; |
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|
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/* We were application limited (idle) for a while. |
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* Shift epoch_start to keep cwnd growth to cubic curve. |
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*/ |
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if (ca->epoch_start && delta > 0) { |
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ca->epoch_start += delta; |
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if (after(ca->epoch_start, now)) |
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ca->epoch_start = now; |
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} |
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return; |
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} |
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} |
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|
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/* calculate the cubic root of x using a table lookup followed by one |
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* Newton-Raphson iteration. |
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* Avg err ~= 0.195% |
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*/ |
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static u32 cubic_root(u64 a) |
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{ |
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u32 x, b, shift; |
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/* |
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* cbrt(x) MSB values for x MSB values in [0..63]. |
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* Precomputed then refined by hand - Willy Tarreau |
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* |
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* For x in [0..63], |
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* v = cbrt(x << 18) - 1 |
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* cbrt(x) = (v[x] + 10) >> 6 |
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*/ |
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static const u8 v[] = { |
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/* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, |
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/* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, |
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/* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, |
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/* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, |
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/* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, |
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/* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, |
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/* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, |
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/* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, |
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}; |
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b = fls64(a); |
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if (b < 7) { |
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/* a in [0..63] */ |
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return ((u32)v[(u32)a] + 35) >> 6; |
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} |
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b = ((b * 84) >> 8) - 1; |
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shift = (a >> (b * 3)); |
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x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; |
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|
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/* |
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* Newton-Raphson iteration |
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* 2 |
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* x = ( 2 * x + a / x ) / 3 |
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* k+1 k k |
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*/ |
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x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1))); |
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x = ((x * 341) >> 10); |
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return x; |
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} |
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|
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/* |
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* Compute congestion window to use. |
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*/ |
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static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked) |
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{ |
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u32 delta, bic_target, max_cnt; |
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u64 offs, t; |
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ca->ack_cnt += acked; /* count the number of ACKed packets */ |
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if (ca->last_cwnd == cwnd && |
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(s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32) |
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return; |
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|
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/* The CUBIC function can update ca->cnt at most once per jiffy. |
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* On all cwnd reduction events, ca->epoch_start is set to 0, |
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* which will force a recalculation of ca->cnt. |
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*/ |
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if (ca->epoch_start && tcp_jiffies32 == ca->last_time) |
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goto tcp_friendliness; |
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ca->last_cwnd = cwnd; |
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ca->last_time = tcp_jiffies32; |
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if (ca->epoch_start == 0) { |
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ca->epoch_start = tcp_jiffies32; /* record beginning */ |
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ca->ack_cnt = acked; /* start counting */ |
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ca->tcp_cwnd = cwnd; /* syn with cubic */ |
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if (ca->last_max_cwnd <= cwnd) { |
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ca->bic_K = 0; |
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ca->bic_origin_point = cwnd; |
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} else { |
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/* Compute new K based on |
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* (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) |
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*/ |
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ca->bic_K = cubic_root(cube_factor |
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* (ca->last_max_cwnd - cwnd)); |
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ca->bic_origin_point = ca->last_max_cwnd; |
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} |
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} |
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/* cubic function - calc*/ |
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/* calculate c * time^3 / rtt, |
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* while considering overflow in calculation of time^3 |
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* (so time^3 is done by using 64 bit) |
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* and without the support of division of 64bit numbers |
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* (so all divisions are done by using 32 bit) |
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* also NOTE the unit of those veriables |
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* time = (t - K) / 2^bictcp_HZ |
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* c = bic_scale >> 10 |
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* rtt = (srtt >> 3) / HZ |
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* !!! The following code does not have overflow problems, |
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* if the cwnd < 1 million packets !!! |
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*/ |
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t = (s32)(tcp_jiffies32 - ca->epoch_start); |
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t += usecs_to_jiffies(ca->delay_min); |
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/* change the unit from HZ to bictcp_HZ */ |
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t <<= BICTCP_HZ; |
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do_div(t, HZ); |
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if (t < ca->bic_K) /* t - K */ |
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offs = ca->bic_K - t; |
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else |
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offs = t - ca->bic_K; |
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|
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/* c/rtt * (t-K)^3 */ |
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delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); |
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if (t < ca->bic_K) /* below origin*/ |
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bic_target = ca->bic_origin_point - delta; |
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else /* above origin*/ |
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bic_target = ca->bic_origin_point + delta; |
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|
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/* cubic function - calc bictcp_cnt*/ |
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if (bic_target > cwnd) { |
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ca->cnt = cwnd / (bic_target - cwnd); |
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} else { |
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ca->cnt = 100 * cwnd; /* very small increment*/ |
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} |
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/* |
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* The initial growth of cubic function may be too conservative |
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* when the available bandwidth is still unknown. |
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*/ |
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if (ca->last_max_cwnd == 0 && ca->cnt > 20) |
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ca->cnt = 20; /* increase cwnd 5% per RTT */ |
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tcp_friendliness: |
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/* TCP Friendly */ |
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if (tcp_friendliness) { |
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u32 scale = beta_scale; |
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|
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delta = (cwnd * scale) >> 3; |
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while (ca->ack_cnt > delta) { /* update tcp cwnd */ |
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ca->ack_cnt -= delta; |
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ca->tcp_cwnd++; |
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} |
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if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */ |
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delta = ca->tcp_cwnd - cwnd; |
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max_cnt = cwnd / delta; |
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if (ca->cnt > max_cnt) |
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ca->cnt = max_cnt; |
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} |
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} |
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|
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/* The maximum rate of cwnd increase CUBIC allows is 1 packet per |
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* 2 packets ACKed, meaning cwnd grows at 1.5x per RTT. |
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*/ |
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ca->cnt = max(ca->cnt, 2U); |
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} |
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static void cubictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked) |
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{ |
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struct tcp_sock *tp = tcp_sk(sk); |
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struct bictcp *ca = inet_csk_ca(sk); |
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|
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if (!tcp_is_cwnd_limited(sk)) |
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return; |
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if (tcp_in_slow_start(tp)) { |
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if (hystart && after(ack, ca->end_seq)) |
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bictcp_hystart_reset(sk); |
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acked = tcp_slow_start(tp, acked); |
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if (!acked) |
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return; |
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} |
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bictcp_update(ca, tp->snd_cwnd, acked); |
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tcp_cong_avoid_ai(tp, ca->cnt, acked); |
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} |
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|
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static u32 cubictcp_recalc_ssthresh(struct sock *sk) |
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{ |
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const struct tcp_sock *tp = tcp_sk(sk); |
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struct bictcp *ca = inet_csk_ca(sk); |
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|
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ca->epoch_start = 0; /* end of epoch */ |
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|
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/* Wmax and fast convergence */ |
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if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) |
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ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) |
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/ (2 * BICTCP_BETA_SCALE); |
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else |
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ca->last_max_cwnd = tp->snd_cwnd; |
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|
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return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); |
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} |
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|
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static void cubictcp_state(struct sock *sk, u8 new_state) |
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{ |
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if (new_state == TCP_CA_Loss) { |
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bictcp_reset(inet_csk_ca(sk)); |
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bictcp_hystart_reset(sk); |
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} |
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} |
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|
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/* Account for TSO/GRO delays. |
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* Otherwise short RTT flows could get too small ssthresh, since during |
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* slow start we begin with small TSO packets and ca->delay_min would |
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* not account for long aggregation delay when TSO packets get bigger. |
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* Ideally even with a very small RTT we would like to have at least one |
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* TSO packet being sent and received by GRO, and another one in qdisc layer. |
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* We apply another 100% factor because @rate is doubled at this point. |
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* We cap the cushion to 1ms. |
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*/ |
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static u32 hystart_ack_delay(struct sock *sk) |
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{ |
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unsigned long rate; |
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|
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rate = READ_ONCE(sk->sk_pacing_rate); |
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if (!rate) |
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return 0; |
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return min_t(u64, USEC_PER_MSEC, |
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div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate)); |
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} |
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|
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static void hystart_update(struct sock *sk, u32 delay) |
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{ |
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struct tcp_sock *tp = tcp_sk(sk); |
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struct bictcp *ca = inet_csk_ca(sk); |
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u32 threshold; |
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|
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if (hystart_detect & HYSTART_ACK_TRAIN) { |
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u32 now = bictcp_clock_us(sk); |
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|
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/* first detection parameter - ack-train detection */ |
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if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) { |
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ca->last_ack = now; |
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|
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threshold = ca->delay_min + hystart_ack_delay(sk); |
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|
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/* Hystart ack train triggers if we get ack past |
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* ca->delay_min/2. |
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* Pacing might have delayed packets up to RTT/2 |
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* during slow start. |
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*/ |
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if (sk->sk_pacing_status == SK_PACING_NONE) |
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threshold >>= 1; |
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|
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if ((s32)(now - ca->round_start) > threshold) { |
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ca->found = 1; |
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pr_debug("hystart_ack_train (%u > %u) delay_min %u (+ ack_delay %u) cwnd %u\n", |
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now - ca->round_start, threshold, |
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ca->delay_min, hystart_ack_delay(sk), tp->snd_cwnd); |
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NET_INC_STATS(sock_net(sk), |
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LINUX_MIB_TCPHYSTARTTRAINDETECT); |
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NET_ADD_STATS(sock_net(sk), |
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LINUX_MIB_TCPHYSTARTTRAINCWND, |
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tp->snd_cwnd); |
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tp->snd_ssthresh = tp->snd_cwnd; |
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} |
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} |
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} |
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|
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if (hystart_detect & HYSTART_DELAY) { |
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/* obtain the minimum delay of more than sampling packets */ |
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if (ca->curr_rtt > delay) |
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ca->curr_rtt = delay; |
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if (ca->sample_cnt < HYSTART_MIN_SAMPLES) { |
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ca->sample_cnt++; |
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} else { |
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if (ca->curr_rtt > ca->delay_min + |
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HYSTART_DELAY_THRESH(ca->delay_min >> 3)) { |
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ca->found = 1; |
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NET_INC_STATS(sock_net(sk), |
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LINUX_MIB_TCPHYSTARTDELAYDETECT); |
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NET_ADD_STATS(sock_net(sk), |
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LINUX_MIB_TCPHYSTARTDELAYCWND, |
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tp->snd_cwnd); |
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tp->snd_ssthresh = tp->snd_cwnd; |
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} |
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} |
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} |
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} |
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|
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static void cubictcp_acked(struct sock *sk, const struct ack_sample *sample) |
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{ |
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const struct tcp_sock *tp = tcp_sk(sk); |
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struct bictcp *ca = inet_csk_ca(sk); |
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u32 delay; |
|
|
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/* Some calls are for duplicates without timetamps */ |
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if (sample->rtt_us < 0) |
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return; |
|
|
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/* Discard delay samples right after fast recovery */ |
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if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ) |
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return; |
|
|
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delay = sample->rtt_us; |
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if (delay == 0) |
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delay = 1; |
|
|
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/* first time call or link delay decreases */ |
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if (ca->delay_min == 0 || ca->delay_min > delay) |
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ca->delay_min = delay; |
|
|
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/* hystart triggers when cwnd is larger than some threshold */ |
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if (!ca->found && tcp_in_slow_start(tp) && hystart && |
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tp->snd_cwnd >= hystart_low_window) |
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hystart_update(sk, delay); |
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} |
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|
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static struct tcp_congestion_ops cubictcp __read_mostly = { |
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.init = cubictcp_init, |
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.ssthresh = cubictcp_recalc_ssthresh, |
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.cong_avoid = cubictcp_cong_avoid, |
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.set_state = cubictcp_state, |
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.undo_cwnd = tcp_reno_undo_cwnd, |
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.cwnd_event = cubictcp_cwnd_event, |
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.pkts_acked = cubictcp_acked, |
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.owner = THIS_MODULE, |
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.name = "cubic", |
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}; |
|
|
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static int __init cubictcp_register(void) |
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{ |
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BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); |
|
|
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/* Precompute a bunch of the scaling factors that are used per-packet |
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* based on SRTT of 100ms |
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*/ |
|
|
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beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3 |
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/ (BICTCP_BETA_SCALE - beta); |
|
|
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cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ |
|
|
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/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 |
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* so K = cubic_root( (wmax-cwnd)*rtt/c ) |
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* the unit of K is bictcp_HZ=2^10, not HZ |
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* |
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* c = bic_scale >> 10 |
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* rtt = 100ms |
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* |
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* the following code has been designed and tested for |
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* cwnd < 1 million packets |
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* RTT < 100 seconds |
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* HZ < 1,000,00 (corresponding to 10 nano-second) |
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*/ |
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/* 1/c * 2^2*bictcp_HZ * srtt */ |
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cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ |
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/* divide by bic_scale and by constant Srtt (100ms) */ |
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do_div(cube_factor, bic_scale * 10); |
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return tcp_register_congestion_control(&cubictcp); |
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} |
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static void __exit cubictcp_unregister(void) |
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{ |
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tcp_unregister_congestion_control(&cubictcp); |
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} |
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module_init(cubictcp_register); |
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module_exit(cubictcp_unregister); |
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MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); |
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MODULE_LICENSE("GPL"); |
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MODULE_DESCRIPTION("CUBIC TCP"); |
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MODULE_VERSION("2.3");
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