linux-stable/net/ipv4/tcp_cdg.c
Yuchung Cheng 76174004a0 tcp: do not slow start when cwnd equals ssthresh
In the original design slow start is only used to raise cwnd
when cwnd is stricly below ssthresh. It makes little sense
to slow start when cwnd == ssthresh: especially
when hystart has set ssthresh in the initial ramp, or after
recovery when cwnd resets to ssthresh. Not doing so will
also help reduce the buffer bloat slightly.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Nandita Dukkipati <nanditad@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2015-07-09 14:22:52 -07:00

433 lines
11 KiB
C

/*
* CAIA Delay-Gradient (CDG) congestion control
*
* This implementation is based on the paper:
* D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
* delay gradients." In IFIP Networking, pages 328-341. Springer, 2011.
*
* Scavenger traffic (Less-than-Best-Effort) should disable coexistence
* heuristics using parameters use_shadow=0 and use_ineff=0.
*
* Parameters window, backoff_beta, and backoff_factor are crucial for
* throughput and delay. Future work is needed to determine better defaults,
* and to provide guidelines for use in different environments/contexts.
*
* Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/.
* Parameter window is only configurable when loading tcp_cdg as a module.
*
* Notable differences from paper/FreeBSD:
* o Using Hybrid Slow start and Proportional Rate Reduction.
* o Add toggle for shadow window mechanism. Suggested by David Hayes.
* o Add toggle for non-congestion loss tolerance.
* o Scaling parameter G is changed to a backoff factor;
* conversion is given by: backoff_factor = 1000/(G * window).
* o Limit shadow window to 2 * cwnd, or to cwnd when application limited.
* o More accurate e^-x.
*/
#include <linux/kernel.h>
#include <linux/random.h>
#include <linux/module.h>
#include <net/tcp.h>
#define HYSTART_ACK_TRAIN 1
#define HYSTART_DELAY 2
static int window __read_mostly = 8;
static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
static unsigned int backoff_factor __read_mostly = 42;
static unsigned int hystart_detect __read_mostly = 3;
static unsigned int use_ineff __read_mostly = 5;
static bool use_shadow __read_mostly = true;
static bool use_tolerance __read_mostly;
module_param(window, int, 0444);
MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
module_param(backoff_beta, uint, 0644);
MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
module_param(backoff_factor, uint, 0644);
MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
module_param(hystart_detect, uint, 0644);
MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
"(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
module_param(use_ineff, uint, 0644);
MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
module_param(use_shadow, bool, 0644);
MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
module_param(use_tolerance, bool, 0644);
MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
struct minmax {
union {
struct {
s32 min;
s32 max;
};
u64 v64;
};
};
enum cdg_state {
CDG_UNKNOWN = 0,
CDG_NONFULL = 1,
CDG_FULL = 2,
CDG_BACKOFF = 3,
};
struct cdg {
struct minmax rtt;
struct minmax rtt_prev;
struct minmax *gradients;
struct minmax gsum;
bool gfilled;
u8 tail;
u8 state;
u8 delack;
u32 rtt_seq;
u32 undo_cwnd;
u32 shadow_wnd;
u16 backoff_cnt;
u16 sample_cnt;
s32 delay_min;
u32 last_ack;
u32 round_start;
};
/**
* nexp_u32 - negative base-e exponential
* @ux: x in units of micro
*
* Returns exp(ux * -1e-6) * U32_MAX.
*/
static u32 __pure nexp_u32(u32 ux)
{
static const u16 v[] = {
/* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
65535,
65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
61378, 57484, 50423, 38795, 22965, 8047, 987, 14,
};
u32 msb = ux >> 8;
u32 res;
int i;
/* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
if (msb > U16_MAX)
return 0;
/* Scale first eight bits linearly: */
res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);
/* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
for (i = 1; msb; i++, msb >>= 1) {
u32 y = v[i & -(msb & 1)] + U32_C(1);
res = ((u64)res * y) >> 16;
}
return res;
}
/* Based on the HyStart algorithm (by Ha et al.) that is implemented in
* tcp_cubic. Differences/experimental changes:
* o Using Hayes' delayed ACK filter.
* o Using a usec clock for the ACK train.
* o Reset ACK train when application limited.
* o Invoked at any cwnd (i.e. also when cwnd < 16).
* o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
*/
static void tcp_cdg_hystart_update(struct sock *sk)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
if (ca->delay_min == 0)
return;
if (hystart_detect & HYSTART_ACK_TRAIN) {
u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);
if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) {
ca->last_ack = now_us;
ca->round_start = now_us;
} else if (before(now_us, ca->last_ack + 3000)) {
u32 base_owd = max(ca->delay_min / 2U, 125U);
ca->last_ack = now_us;
if (after(now_us, ca->round_start + base_owd)) {
NET_INC_STATS_BH(sock_net(sk),
LINUX_MIB_TCPHYSTARTTRAINDETECT);
NET_ADD_STATS_BH(sock_net(sk),
LINUX_MIB_TCPHYSTARTTRAINCWND,
tp->snd_cwnd);
tp->snd_ssthresh = tp->snd_cwnd;
return;
}
}
}
if (hystart_detect & HYSTART_DELAY) {
if (ca->sample_cnt < 8) {
ca->sample_cnt++;
} else {
s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
125U);
if (ca->rtt.min > thresh) {
NET_INC_STATS_BH(sock_net(sk),
LINUX_MIB_TCPHYSTARTDELAYDETECT);
NET_ADD_STATS_BH(sock_net(sk),
LINUX_MIB_TCPHYSTARTDELAYCWND,
tp->snd_cwnd);
tp->snd_ssthresh = tp->snd_cwnd;
}
}
}
}
static s32 tcp_cdg_grad(struct cdg *ca)
{
s32 gmin = ca->rtt.min - ca->rtt_prev.min;
s32 gmax = ca->rtt.max - ca->rtt_prev.max;
s32 grad;
if (ca->gradients) {
ca->gsum.min += gmin - ca->gradients[ca->tail].min;
ca->gsum.max += gmax - ca->gradients[ca->tail].max;
ca->gradients[ca->tail].min = gmin;
ca->gradients[ca->tail].max = gmax;
ca->tail = (ca->tail + 1) & (window - 1);
gmin = ca->gsum.min;
gmax = ca->gsum.max;
}
/* We keep sums to ignore gradients during cwnd reductions;
* the paper's smoothed gradients otherwise simplify to:
* (rtt_latest - rtt_oldest) / window.
*
* We also drop division by window here.
*/
grad = gmin > 0 ? gmin : gmax;
/* Extrapolate missing values in gradient window: */
if (!ca->gfilled) {
if (!ca->gradients && window > 1)
grad *= window; /* Memory allocation failed. */
else if (ca->tail == 0)
ca->gfilled = true;
else
grad = (grad * window) / (int)ca->tail;
}
/* Backoff was effectual: */
if (gmin <= -32 || gmax <= -32)
ca->backoff_cnt = 0;
if (use_tolerance) {
/* Reduce small variations to zero: */
gmin = DIV_ROUND_CLOSEST(gmin, 64);
gmax = DIV_ROUND_CLOSEST(gmax, 64);
if (gmin > 0 && gmax <= 0)
ca->state = CDG_FULL;
else if ((gmin > 0 && gmax > 0) || gmax < 0)
ca->state = CDG_NONFULL;
}
return grad;
}
static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
if (prandom_u32() <= nexp_u32(grad * backoff_factor))
return false;
if (use_ineff) {
ca->backoff_cnt++;
if (ca->backoff_cnt > use_ineff)
return false;
}
ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
ca->state = CDG_BACKOFF;
tcp_enter_cwr(sk);
return true;
}
/* Not called in CWR or Recovery state. */
static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
u32 prior_snd_cwnd;
u32 incr;
if (tcp_in_slow_start(tp) && hystart_detect)
tcp_cdg_hystart_update(sk);
if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
s32 grad = 0;
if (ca->rtt_prev.v64)
grad = tcp_cdg_grad(ca);
ca->rtt_seq = tp->snd_nxt;
ca->rtt_prev = ca->rtt;
ca->rtt.v64 = 0;
ca->last_ack = 0;
ca->sample_cnt = 0;
if (grad > 0 && tcp_cdg_backoff(sk, grad))
return;
}
if (!tcp_is_cwnd_limited(sk)) {
ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
return;
}
prior_snd_cwnd = tp->snd_cwnd;
tcp_reno_cong_avoid(sk, ack, acked);
incr = tp->snd_cwnd - prior_snd_cwnd;
ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
}
static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
if (rtt_us <= 0)
return;
/* A heuristic for filtering delayed ACKs, adapted from:
* D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
* delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
*/
if (tp->sacked_out == 0) {
if (num_acked == 1 && ca->delack) {
/* A delayed ACK is only used for the minimum if it is
* provenly lower than an existing non-zero minimum.
*/
ca->rtt.min = min(ca->rtt.min, rtt_us);
ca->delack--;
return;
} else if (num_acked > 1 && ca->delack < 5) {
ca->delack++;
}
}
ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
ca->rtt.max = max(ca->rtt.max, rtt_us);
}
static u32 tcp_cdg_ssthresh(struct sock *sk)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
ca->undo_cwnd = tp->snd_cwnd;
if (ca->state == CDG_BACKOFF)
return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
if (ca->state == CDG_NONFULL && use_tolerance)
return tp->snd_cwnd;
ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
if (use_shadow)
return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
return max(2U, tp->snd_cwnd >> 1);
}
static u32 tcp_cdg_undo_cwnd(struct sock *sk)
{
struct cdg *ca = inet_csk_ca(sk);
return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd);
}
static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct minmax *gradients;
switch (ev) {
case CA_EVENT_CWND_RESTART:
gradients = ca->gradients;
if (gradients)
memset(gradients, 0, window * sizeof(gradients[0]));
memset(ca, 0, sizeof(*ca));
ca->gradients = gradients;
ca->rtt_seq = tp->snd_nxt;
ca->shadow_wnd = tp->snd_cwnd;
break;
case CA_EVENT_COMPLETE_CWR:
ca->state = CDG_UNKNOWN;
ca->rtt_seq = tp->snd_nxt;
ca->rtt_prev = ca->rtt;
ca->rtt.v64 = 0;
break;
default:
break;
}
}
static void tcp_cdg_init(struct sock *sk)
{
struct cdg *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
/* We silently fall back to window = 1 if allocation fails. */
if (window > 1)
ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
GFP_NOWAIT | __GFP_NOWARN);
ca->rtt_seq = tp->snd_nxt;
ca->shadow_wnd = tp->snd_cwnd;
}
static void tcp_cdg_release(struct sock *sk)
{
struct cdg *ca = inet_csk_ca(sk);
kfree(ca->gradients);
}
struct tcp_congestion_ops tcp_cdg __read_mostly = {
.cong_avoid = tcp_cdg_cong_avoid,
.cwnd_event = tcp_cdg_cwnd_event,
.pkts_acked = tcp_cdg_acked,
.undo_cwnd = tcp_cdg_undo_cwnd,
.ssthresh = tcp_cdg_ssthresh,
.release = tcp_cdg_release,
.init = tcp_cdg_init,
.owner = THIS_MODULE,
.name = "cdg",
};
static int __init tcp_cdg_register(void)
{
if (backoff_beta > 1024 || window < 1 || window > 256)
return -ERANGE;
if (!is_power_of_2(window))
return -EINVAL;
BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
tcp_register_congestion_control(&tcp_cdg);
return 0;
}
static void __exit tcp_cdg_unregister(void)
{
tcp_unregister_congestion_control(&tcp_cdg);
}
module_init(tcp_cdg_register);
module_exit(tcp_cdg_unregister);
MODULE_AUTHOR("Kenneth Klette Jonassen");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP CDG");