dctcp: more accurate tracking of packets delivery

After commit e21db6f6 ("tcp: track total bytes delivered with ECN CE marks")
core TCP stack does a very good job tracking ECN signals.

The "sender's best estimate of CE information" Yuchung mentioned in his
patch is indeed the best we can do.

DCTCP can use tp->delivered_ce and tp->delivered to not duplicate the logic,
and use the existing best estimate.

This solves some problems, since current DCTCP logic does not deal with losses
and/or GRO or ack aggregation very well.

This also removes a dubious use of inet_csk(sk)->icsk_ack.rcv_mss
(this should have been tp->mss_cache), and a 64 bit divide.

Finally, we can see that the DCTCP logic, calling dctcp_update_alpha() for
every ACK could be done differently, calling it only once per RTT.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Lawrence Brakmo <brakmo@fb.com>
Cc: Abdul Kabbani <akabbani@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_dctcp.c

@@ -49,9 +49,8 @@
 #define DCTCP_MAX_ALPHA	1024U
 
 struct dctcp {
-	u32 acked_bytes_ecn;
-	u32 acked_bytes_total;
-	u32 prior_snd_una;
+	u32 old_delivered;
+	u32 old_delivered_ce;
 	u32 prior_rcv_nxt;
 	u32 dctcp_alpha;
 	u32 next_seq;
@@ -73,8 +72,8 @@ static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
 {
 	ca->next_seq = tp->snd_nxt;
 
-	ca->acked_bytes_ecn = 0;
-	ca->acked_bytes_total = 0;
+	ca->old_delivered = tp->delivered;
+	ca->old_delivered_ce = tp->delivered_ce;
 }
 
 static void dctcp_init(struct sock *sk)
@@ -86,7 +85,6 @@ static void dctcp_init(struct sock *sk)
 	     sk->sk_state == TCP_CLOSE)) {
 		struct dctcp *ca = inet_csk_ca(sk);
 
-		ca->prior_snd_una = tp->snd_una;
 		ca->prior_rcv_nxt = tp->rcv_nxt;
 
 		ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
@@ -118,37 +116,25 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct dctcp *ca = inet_csk_ca(sk);
-	u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
-
-	/* If ack did not advance snd_una, count dupack as MSS size.
-	 * If ack did update window, do not count it at all.
-	 */
-	if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
-		acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
-	if (acked_bytes) {
-		ca->acked_bytes_total += acked_bytes;
-		ca->prior_snd_una = tp->snd_una;
-
-		if (flags & CA_ACK_ECE)
-			ca->acked_bytes_ecn += acked_bytes;
-	}
 
 	/* Expired RTT */
 	if (!before(tp->snd_una, ca->next_seq)) {
-		u64 bytes_ecn = ca->acked_bytes_ecn;
+		u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce;
 		u32 alpha = ca->dctcp_alpha;
 
 		/* alpha = (1 - g) * alpha + g * F */
 
 		alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
-		if (bytes_ecn) {
+		if (delivered_ce) {
+			u32 delivered = tp->delivered - ca->old_delivered;
+
 			/* If dctcp_shift_g == 1, a 32bit value would overflow
-			 * after 8 Mbytes.
+			 * after 8 M packets.
 			 */
-			bytes_ecn <<= (10 - dctcp_shift_g);
-			do_div(bytes_ecn, max(1U, ca->acked_bytes_total));
+			delivered_ce <<= (10 - dctcp_shift_g);
+			delivered_ce /= max(1U, delivered);
 
-			alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
+			alpha = min(alpha + delivered_ce, DCTCP_MAX_ALPHA);
 		}
 		/* dctcp_alpha can be read from dctcp_get_info() without
 		 * synchro, so we ask compiler to not use dctcp_alpha
@@ -200,6 +186,7 @@ static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 			     union tcp_cc_info *info)
 {
 	const struct dctcp *ca = inet_csk_ca(sk);
+	const struct tcp_sock *tp = tcp_sk(sk);
 
 	/* Fill it also in case of VEGASINFO due to req struct limits.
 	 * We can still correctly retrieve it later.
@@ -211,8 +198,10 @@ static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
 			info->dctcp.dctcp_enabled = 1;
 			info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
 			info->dctcp.dctcp_alpha = ca->dctcp_alpha;
-			info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn;
-			info->dctcp.dctcp_ab_tot = ca->acked_bytes_total;
+			info->dctcp.dctcp_ab_ecn = tp->mss_cache *
+						   (tp->delivered_ce - ca->old_delivered_ce);
+			info->dctcp.dctcp_ab_tot = tp->mss_cache *
+						   (tp->delivered - ca->old_delivered);
 		}
 
 		*attr = INET_DIAG_DCTCPINFO;