Merge branch 'tcp_cubic-various-fixes'

Eric Dumazet says:

====================
tcp_cubic: various fixes

This patch series converts tcp_cubic to usec clock resolution
for Hystart logic.

This makes Hystart more relevant for data-center flows.
Prior to this series, Hystart was not kicking, or was
kicking without good reason, since the 1ms clock was too coarse.

Last patch also fixes an issue with Hystart vs TCP pacing.

v2: removed a last-minute debug chunk from last patch
====================

Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/tcp_cubic.c

···
 
 /* 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;
···
 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;
···
 		 " 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 {
···
 	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 */
···
 	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)
···
 	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 = 0;
+	ca->curr_rtt = ~0U;
 	ca->sample_cnt = 0;
 }
 
···
 	 */
 
 	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);
···
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bictcp *ca = inet_csk_ca(sk);
-
-	if (ca->found & hystart_detect)
-		return;
+	u32 threshold;
 
 	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) {
-				ca->found |= HYSTART_ACK_TRAIN;
+
+			threshold = ca->delay_min;
+			/* Hystart ack train triggers if we get ack past
+			 * ca->delay_min/2.
+			 * Pacing might have delayed packets up to RTT/2
+			 * during slow start.
+			 */
+			if (sk->sk_pacing_status == SK_PACING_NONE)
+				threshold >>= 1;
+
+			if ((s32)(now - ca->round_start) > threshold) {
+				ca->found = 1;
 				NET_INC_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTTRAINDETECT);
 				NET_ADD_STATS(sock_net(sk),
···
 	if (hystart_detect & HYSTART_DELAY) {
 		/* obtain the minimum delay of more than sampling packets */
 		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
-			if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
+			if (ca->curr_rtt > delay)
 				ca->curr_rtt = delay;
 
 			ca->sample_cnt++;
 		} else {
 			if (ca->curr_rtt > ca->delay_min +
 			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
-				ca->found |= HYSTART_DELAY;
+				ca->found = 1;
 				NET_INC_STATS(sock_net(sk),
 					      LINUX_MIB_TCPHYSTARTDELAYDETECT);
 				NET_ADD_STATS(sock_net(sk),
···
 	}
 }
 
-/* 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);
···
 	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;
 
 	/* first time call or link delay decreases */
-	if (ca->delay_min == 0 || ca->delay_min > delay)
-		ca->delay_min = delay;
+	if (ca->delay_min == 0 || ca->delay_min > delay) {
+		unsigned long rate = READ_ONCE(sk->sk_pacing_rate);
+
+		/* Account for TSO/GRO delays.
+		 * Otherwise short RTT flows could get too small ssthresh,
+		 * since during slow start we begin with small TSO packets
+		 * and could lower ca->delay_min too much.
+		 * Ideally even with a very small RTT we would like to have
+		 * at least one TSO packet being sent and received by GRO,
+		 * and another one in qdisc layer.
+		 * We apply another 100% factor because @rate is doubled at
+		 * this point.
+		 * We cap the cushion to 1ms.
+		 */
+		if (rate)
+			delay += min_t(u64, USEC_PER_MSEC,
+				       div64_ul((u64)GSO_MAX_SIZE *
+						4 * USEC_PER_SEC, rate));
+		if (ca->delay_min == 0 || ca->delay_min > delay)
+			ca->delay_min = delay;
+	}
 
 	/* hystart triggers when cwnd is larger than some threshold */
-	if (hystart && tcp_in_slow_start(tp) &&
+	if (!ca->found && hystart && tcp_in_slow_start(tp) &&
 	    tp->snd_cwnd >= hystart_low_window)
 		hystart_update(sk, delay);
 }