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Date: Mon, 23 Dec 2019 12:27:52 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>
Cc: netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Eric Dumazet <eric.dumazet@...il.com>,
Soheil Hassas Yeganeh <soheil@...gle.com>,
Neal Cardwell <ncardwell@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>,
Martin KaFai Lau <kafai@...com>
Subject: [PATCH net-next v2 3/5] 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 <edumazet@...gle.com>
---
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 5eff762acd7fe1510eb39fc789b488de8ca648de..068775b91fb5790e6e60a6490b49e7a266e4ed51 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;
--
2.24.1.735.g03f4e72817-goog
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