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426 lines
11 KiB
426 lines
11 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* |
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* CAIA Delay-Gradient (CDG) congestion control |
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* |
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* This implementation is based on the paper: |
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* D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using |
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* delay gradients." In IFIP Networking, pages 328-341. Springer, 2011. |
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* |
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* Scavenger traffic (Less-than-Best-Effort) should disable coexistence |
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* heuristics using parameters use_shadow=0 and use_ineff=0. |
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* |
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* Parameters window, backoff_beta, and backoff_factor are crucial for |
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* throughput and delay. Future work is needed to determine better defaults, |
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* and to provide guidelines for use in different environments/contexts. |
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* |
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* Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/. |
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* Parameter window is only configurable when loading tcp_cdg as a module. |
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* |
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* Notable differences from paper/FreeBSD: |
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* o Using Hybrid Slow start and Proportional Rate Reduction. |
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* o Add toggle for shadow window mechanism. Suggested by David Hayes. |
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* o Add toggle for non-congestion loss tolerance. |
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* o Scaling parameter G is changed to a backoff factor; |
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* conversion is given by: backoff_factor = 1000/(G * window). |
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* o Limit shadow window to 2 * cwnd, or to cwnd when application limited. |
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* o More accurate e^-x. |
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*/ |
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#include <linux/kernel.h> |
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#include <linux/random.h> |
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#include <linux/module.h> |
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#include <linux/sched/clock.h> |
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#include <net/tcp.h> |
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#define HYSTART_ACK_TRAIN 1 |
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#define HYSTART_DELAY 2 |
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static int window __read_mostly = 8; |
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static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */ |
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static unsigned int backoff_factor __read_mostly = 42; |
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static unsigned int hystart_detect __read_mostly = 3; |
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static unsigned int use_ineff __read_mostly = 5; |
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static bool use_shadow __read_mostly = true; |
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static bool use_tolerance __read_mostly; |
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module_param(window, int, 0444); |
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MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)"); |
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module_param(backoff_beta, uint, 0644); |
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MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)"); |
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module_param(backoff_factor, uint, 0644); |
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MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor"); |
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module_param(hystart_detect, uint, 0644); |
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MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start " |
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"(0: disabled, 1: ACK train, 2: delay threshold, 3: both)"); |
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module_param(use_ineff, uint, 0644); |
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MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)"); |
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module_param(use_shadow, bool, 0644); |
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MODULE_PARM_DESC(use_shadow, "use shadow window heuristic"); |
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module_param(use_tolerance, bool, 0644); |
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MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic"); |
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struct cdg_minmax { |
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union { |
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struct { |
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s32 min; |
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s32 max; |
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}; |
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u64 v64; |
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}; |
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}; |
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enum cdg_state { |
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CDG_UNKNOWN = 0, |
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CDG_NONFULL = 1, |
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CDG_FULL = 2, |
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CDG_BACKOFF = 3, |
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}; |
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struct cdg { |
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struct cdg_minmax rtt; |
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struct cdg_minmax rtt_prev; |
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struct cdg_minmax *gradients; |
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struct cdg_minmax gsum; |
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bool gfilled; |
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u8 tail; |
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u8 state; |
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u8 delack; |
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u32 rtt_seq; |
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u32 shadow_wnd; |
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u16 backoff_cnt; |
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u16 sample_cnt; |
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s32 delay_min; |
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u32 last_ack; |
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u32 round_start; |
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}; |
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/** |
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* nexp_u32 - negative base-e exponential |
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* @ux: x in units of micro |
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* |
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* Returns exp(ux * -1e-6) * U32_MAX. |
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*/ |
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static u32 __pure nexp_u32(u32 ux) |
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{ |
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static const u16 v[] = { |
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/* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */ |
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65535, |
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65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422, |
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61378, 57484, 50423, 38795, 22965, 8047, 987, 14, |
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}; |
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u32 msb = ux >> 8; |
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u32 res; |
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int i; |
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/* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */ |
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if (msb > U16_MAX) |
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return 0; |
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/* Scale first eight bits linearly: */ |
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res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000); |
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/* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */ |
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for (i = 1; msb; i++, msb >>= 1) { |
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u32 y = v[i & -(msb & 1)] + U32_C(1); |
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res = ((u64)res * y) >> 16; |
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} |
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return res; |
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} |
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/* Based on the HyStart algorithm (by Ha et al.) that is implemented in |
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* tcp_cubic. Differences/experimental changes: |
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* o Using Hayes' delayed ACK filter. |
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* o Using a usec clock for the ACK train. |
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* o Reset ACK train when application limited. |
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* o Invoked at any cwnd (i.e. also when cwnd < 16). |
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* o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh). |
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*/ |
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static void tcp_cdg_hystart_update(struct sock *sk) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min); |
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if (ca->delay_min == 0) |
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return; |
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if (hystart_detect & HYSTART_ACK_TRAIN) { |
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u32 now_us = tp->tcp_mstamp; |
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if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) { |
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ca->last_ack = now_us; |
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ca->round_start = now_us; |
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} else if (before(now_us, ca->last_ack + 3000)) { |
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u32 base_owd = max(ca->delay_min / 2U, 125U); |
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ca->last_ack = now_us; |
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if (after(now_us, ca->round_start + base_owd)) { |
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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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return; |
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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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if (ca->sample_cnt < 8) { |
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ca->sample_cnt++; |
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} else { |
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s32 thresh = max(ca->delay_min + ca->delay_min / 8U, |
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125U); |
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if (ca->rtt.min > thresh) { |
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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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static s32 tcp_cdg_grad(struct cdg *ca) |
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{ |
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s32 gmin = ca->rtt.min - ca->rtt_prev.min; |
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s32 gmax = ca->rtt.max - ca->rtt_prev.max; |
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s32 grad; |
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if (ca->gradients) { |
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ca->gsum.min += gmin - ca->gradients[ca->tail].min; |
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ca->gsum.max += gmax - ca->gradients[ca->tail].max; |
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ca->gradients[ca->tail].min = gmin; |
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ca->gradients[ca->tail].max = gmax; |
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ca->tail = (ca->tail + 1) & (window - 1); |
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gmin = ca->gsum.min; |
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gmax = ca->gsum.max; |
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} |
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/* We keep sums to ignore gradients during cwnd reductions; |
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* the paper's smoothed gradients otherwise simplify to: |
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* (rtt_latest - rtt_oldest) / window. |
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* |
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* We also drop division by window here. |
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*/ |
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grad = gmin > 0 ? gmin : gmax; |
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/* Extrapolate missing values in gradient window: */ |
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if (!ca->gfilled) { |
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if (!ca->gradients && window > 1) |
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grad *= window; /* Memory allocation failed. */ |
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else if (ca->tail == 0) |
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ca->gfilled = true; |
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else |
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grad = (grad * window) / (int)ca->tail; |
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} |
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/* Backoff was effectual: */ |
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if (gmin <= -32 || gmax <= -32) |
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ca->backoff_cnt = 0; |
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if (use_tolerance) { |
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/* Reduce small variations to zero: */ |
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gmin = DIV_ROUND_CLOSEST(gmin, 64); |
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gmax = DIV_ROUND_CLOSEST(gmax, 64); |
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if (gmin > 0 && gmax <= 0) |
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ca->state = CDG_FULL; |
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else if ((gmin > 0 && gmax > 0) || gmax < 0) |
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ca->state = CDG_NONFULL; |
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} |
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return grad; |
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} |
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static bool tcp_cdg_backoff(struct sock *sk, u32 grad) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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if (prandom_u32() <= nexp_u32(grad * backoff_factor)) |
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return false; |
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if (use_ineff) { |
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ca->backoff_cnt++; |
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if (ca->backoff_cnt > use_ineff) |
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return false; |
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} |
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ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd); |
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ca->state = CDG_BACKOFF; |
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tcp_enter_cwr(sk); |
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return true; |
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} |
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/* Not called in CWR or Recovery state. */ |
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static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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u32 prior_snd_cwnd; |
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u32 incr; |
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if (tcp_in_slow_start(tp) && hystart_detect) |
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tcp_cdg_hystart_update(sk); |
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if (after(ack, ca->rtt_seq) && ca->rtt.v64) { |
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s32 grad = 0; |
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if (ca->rtt_prev.v64) |
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grad = tcp_cdg_grad(ca); |
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ca->rtt_seq = tp->snd_nxt; |
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ca->rtt_prev = ca->rtt; |
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ca->rtt.v64 = 0; |
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ca->last_ack = 0; |
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ca->sample_cnt = 0; |
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if (grad > 0 && tcp_cdg_backoff(sk, grad)) |
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return; |
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} |
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if (!tcp_is_cwnd_limited(sk)) { |
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ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd); |
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return; |
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} |
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prior_snd_cwnd = tp->snd_cwnd; |
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tcp_reno_cong_avoid(sk, ack, acked); |
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incr = tp->snd_cwnd - prior_snd_cwnd; |
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ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr); |
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} |
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static void tcp_cdg_acked(struct sock *sk, const struct ack_sample *sample) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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if (sample->rtt_us <= 0) |
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return; |
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/* A heuristic for filtering delayed ACKs, adapted from: |
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* D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support |
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* delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010. |
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*/ |
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if (tp->sacked_out == 0) { |
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if (sample->pkts_acked == 1 && ca->delack) { |
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/* A delayed ACK is only used for the minimum if it is |
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* provenly lower than an existing non-zero minimum. |
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*/ |
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ca->rtt.min = min(ca->rtt.min, sample->rtt_us); |
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ca->delack--; |
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return; |
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} else if (sample->pkts_acked > 1 && ca->delack < 5) { |
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ca->delack++; |
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} |
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} |
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ca->rtt.min = min_not_zero(ca->rtt.min, sample->rtt_us); |
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ca->rtt.max = max(ca->rtt.max, sample->rtt_us); |
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} |
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static u32 tcp_cdg_ssthresh(struct sock *sk) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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if (ca->state == CDG_BACKOFF) |
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return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10); |
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if (ca->state == CDG_NONFULL && use_tolerance) |
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return tp->snd_cwnd; |
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ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd); |
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if (use_shadow) |
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return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1); |
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return max(2U, tp->snd_cwnd >> 1); |
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} |
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static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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struct cdg_minmax *gradients; |
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switch (ev) { |
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case CA_EVENT_CWND_RESTART: |
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gradients = ca->gradients; |
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if (gradients) |
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memset(gradients, 0, window * sizeof(gradients[0])); |
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memset(ca, 0, sizeof(*ca)); |
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ca->gradients = gradients; |
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ca->rtt_seq = tp->snd_nxt; |
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ca->shadow_wnd = tp->snd_cwnd; |
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break; |
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case CA_EVENT_COMPLETE_CWR: |
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ca->state = CDG_UNKNOWN; |
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ca->rtt_seq = tp->snd_nxt; |
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ca->rtt_prev = ca->rtt; |
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ca->rtt.v64 = 0; |
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break; |
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default: |
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break; |
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} |
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} |
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static void tcp_cdg_init(struct sock *sk) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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struct tcp_sock *tp = tcp_sk(sk); |
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/* We silently fall back to window = 1 if allocation fails. */ |
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if (window > 1) |
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ca->gradients = kcalloc(window, sizeof(ca->gradients[0]), |
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GFP_NOWAIT | __GFP_NOWARN); |
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ca->rtt_seq = tp->snd_nxt; |
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ca->shadow_wnd = tp->snd_cwnd; |
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} |
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static void tcp_cdg_release(struct sock *sk) |
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{ |
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struct cdg *ca = inet_csk_ca(sk); |
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kfree(ca->gradients); |
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} |
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static struct tcp_congestion_ops tcp_cdg __read_mostly = { |
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.cong_avoid = tcp_cdg_cong_avoid, |
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.cwnd_event = tcp_cdg_cwnd_event, |
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.pkts_acked = tcp_cdg_acked, |
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.undo_cwnd = tcp_reno_undo_cwnd, |
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.ssthresh = tcp_cdg_ssthresh, |
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.release = tcp_cdg_release, |
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.init = tcp_cdg_init, |
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.owner = THIS_MODULE, |
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.name = "cdg", |
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}; |
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static int __init tcp_cdg_register(void) |
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{ |
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if (backoff_beta > 1024 || window < 1 || window > 256) |
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return -ERANGE; |
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if (!is_power_of_2(window)) |
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return -EINVAL; |
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BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE); |
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tcp_register_congestion_control(&tcp_cdg); |
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return 0; |
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} |
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static void __exit tcp_cdg_unregister(void) |
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{ |
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tcp_unregister_congestion_control(&tcp_cdg); |
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} |
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module_init(tcp_cdg_register); |
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module_exit(tcp_cdg_unregister); |
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MODULE_AUTHOR("Kenneth Klette Jonassen"); |
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MODULE_LICENSE("GPL"); |
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MODULE_DESCRIPTION("TCP CDG");
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