From cff04e2da308c522f654237b45dd64248fe8d1fa Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 23 Dec 2019 12:27:52 -0800 Subject: tcp_cubic: switch bictcp_clock() to usec resolution Current 1ms clock feeds ca->round_start, ca->delay_min, ca->last_ack. This is quite problematic for data-center flows, where delay_min is way below 1 ms. This means Hystart Train detection triggers every time jiffies value is updated, since "((s32)(now - ca->round_start) > ca->delay_min >> 4)" expression becomes true. This kind of random behavior can be solved by reusing the existing usec timestamp that TCP keeps in tp->tcp_mstamp Note that a followup patch will tweak things a bit, because during slow start, GRO aggregation on receivers naturally increases the RTT as TSO packets gradually come to ~64KB size. To recap, right after this patch CUBIC Hystart train detection is more aggressive, since short RTT flows might exit slow start at cwnd = 20, instead of being possibly unbounded. Following patch will address this problem. Signed-off-by: Eric Dumazet Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_cubic.c | 35 ++++++++++++++--------------------- 1 file changed, 14 insertions(+), 21 deletions(-) diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 5eff762acd7f..068775b91fb5 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -40,8 +40,8 @@ /* Number of delay samples for detecting the increase of delay */ #define HYSTART_MIN_SAMPLES 8 -#define HYSTART_DELAY_MIN (4U<<3) -#define HYSTART_DELAY_MAX (16U<<3) +#define HYSTART_DELAY_MIN (4000U) /* 4 ms */ +#define HYSTART_DELAY_MAX (16000U) /* 16 ms */ #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX) static int fast_convergence __read_mostly = 1; @@ -53,7 +53,7 @@ static int tcp_friendliness __read_mostly = 1; static int hystart __read_mostly = 1; static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY; static int hystart_low_window __read_mostly = 16; -static int hystart_ack_delta __read_mostly = 2; +static int hystart_ack_delta_us __read_mostly = 2000; static u32 cube_rtt_scale __read_mostly; static u32 beta_scale __read_mostly; @@ -77,8 +77,8 @@ MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms" " 1: packet-train 2: delay 3: both packet-train and delay"); module_param(hystart_low_window, int, 0644); MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start"); -module_param(hystart_ack_delta, int, 0644); -MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)"); +module_param(hystart_ack_delta_us, int, 0644); +MODULE_PARM_DESC(hystart_ack_delta_us, "spacing between ack's indicating train (usecs)"); /* BIC TCP Parameters */ struct bictcp { @@ -89,7 +89,7 @@ struct bictcp { u32 bic_origin_point;/* origin point of bic function */ u32 bic_K; /* time to origin point from the beginning of the current epoch */ - u32 delay_min; /* min delay (msec << 3) */ + u32 delay_min; /* min delay (usec) */ u32 epoch_start; /* beginning of an epoch */ u32 ack_cnt; /* number of acks */ u32 tcp_cwnd; /* estimated tcp cwnd */ @@ -117,13 +117,9 @@ static inline void bictcp_reset(struct bictcp *ca) ca->found = 0; } -static inline u32 bictcp_clock(void) +static inline u32 bictcp_clock_us(const struct sock *sk) { -#if HZ < 1000 - return ktime_to_ms(ktime_get_real()); -#else - return jiffies_to_msecs(jiffies); -#endif + return tcp_sk(sk)->tcp_mstamp; } static inline void bictcp_hystart_reset(struct sock *sk) @@ -131,7 +127,7 @@ static inline void bictcp_hystart_reset(struct sock *sk) struct tcp_sock *tp = tcp_sk(sk); struct bictcp *ca = inet_csk_ca(sk); - ca->round_start = ca->last_ack = bictcp_clock(); + ca->round_start = ca->last_ack = bictcp_clock_us(sk); ca->end_seq = tp->snd_nxt; ca->curr_rtt = ~0U; ca->sample_cnt = 0; @@ -276,7 +272,7 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked) */ t = (s32)(tcp_jiffies32 - ca->epoch_start); - t += msecs_to_jiffies(ca->delay_min >> 3); + t += usecs_to_jiffies(ca->delay_min); /* change the unit from HZ to bictcp_HZ */ t <<= BICTCP_HZ; do_div(t, HZ); @@ -382,12 +378,12 @@ static void hystart_update(struct sock *sk, u32 delay) struct bictcp *ca = inet_csk_ca(sk); if (hystart_detect & HYSTART_ACK_TRAIN) { - u32 now = bictcp_clock(); + u32 now = bictcp_clock_us(sk); /* first detection parameter - ack-train detection */ - if ((s32)(now - ca->last_ack) <= hystart_ack_delta) { + if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) { ca->last_ack = now; - if ((s32)(now - ca->round_start) > ca->delay_min >> 4) { + if ((s32)(now - ca->round_start) > ca->delay_min >> 1) { ca->found = 1; NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHYSTARTTRAINDETECT); @@ -421,9 +417,6 @@ static void hystart_update(struct sock *sk, u32 delay) } } -/* Track delayed acknowledgment ratio using sliding window - * ratio = (15*ratio + sample) / 16 - */ static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) { const struct tcp_sock *tp = tcp_sk(sk); @@ -438,7 +431,7 @@ static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ) return; - delay = (sample->rtt_us << 3) / USEC_PER_MSEC; + delay = sample->rtt_us; if (delay == 0) delay = 1; -- cgit v1.2.3-59-g8ed1b