434 строки
11 KiB
C
434 строки
11 KiB
C
/*
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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 <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 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 minmax rtt;
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struct minmax rtt_prev;
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struct minmax *gradients;
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struct 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 undo_cwnd;
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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 = div_u64(local_clock(), NSEC_PER_USEC);
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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_BH(sock_net(sk),
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LINUX_MIB_TCPHYSTARTTRAINDETECT);
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NET_ADD_STATS_BH(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_BH(sock_net(sk),
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LINUX_MIB_TCPHYSTARTDELAYDETECT);
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NET_ADD_STATS_BH(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 (tp->snd_cwnd < tp->snd_ssthresh && 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, u32 num_acked, s32 rtt_us)
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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 (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 (num_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, rtt_us);
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ca->delack--;
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return;
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} else if (num_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, rtt_us);
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ca->rtt.max = max(ca->rtt.max, 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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ca->undo_cwnd = tp->snd_cwnd;
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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 u32 tcp_cdg_undo_cwnd(struct sock *sk)
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{
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struct cdg *ca = inet_csk_ca(sk);
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return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd);
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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 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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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_cdg_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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