netdev - [RFC PATCH 25/28] tcp: try to avoid safer when ACKs are thinned

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Date:   Wed, 18 Mar 2020 11:38:06 +0200
From:   Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
To:     netdev@...r.kernel.org
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Olivier Tilmans <olivier.tilmans@...ia-bell-labs.com>
Subject: [RFC PATCH 25/28] tcp: try to avoid safer when ACKs are thinned

From: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>

Add newly acked pkts EWMA. When ACK thinning occurs, select
between safer and unsafe cep delta based on it.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>
---
 include/linux/tcp.h  |  1 +
 net/ipv4/tcp.c       |  1 +
 net/ipv4/tcp_input.c | 20 +++++++++++++++++++-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 64db51e5d45e..22be7cf2e084 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -332,6 +332,7 @@ struct tcp_sock {
 		prev_ecnfield:2,/* ECN bits from the previous segment */
 		accecn_opt_demand:2,/* Demand AccECN option for n next ACKs */
 		estimate_ecnfield:2;/* ECN field for AccECN delivered estimates */
+	u16	pkts_acked_ewma;/* EWMA of packets acked for AccECN cep heuristic */
 	u64	accecn_opt_tstamp;	/* Last AccECN option sent timestamp */
 	u32	lost;		/* Total data packets lost incl. rexmits */
 	u32	app_limited;	/* limited until "delivered" reaches this val */
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 09f73f81e6fa..4a22c19fa6d5 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2629,6 +2629,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tcp_accecn_init_counters(tp);
 	tp->prev_ecnfield = 0;
 	tp->accecn_opt_tstamp = 0;
+	tp->pkts_acked_ewma = 0;
 	tcp_set_ca_state(sk, TCP_CA_Open);
 	tp->is_sack_reneg = 0;
 	tcp_clear_retrans(tp);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 6bc9995202c8..f5476e6b1479 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -514,6 +514,10 @@ static bool tcp_accecn_process_option(struct tcp_sock *tp,
 	return res;
 }
 
+#define PKTS_ACKED_WEIGHT	6
+#define PKTS_ACKED_PREC		6
+#define ACK_COMP_THRESH		4
+
 /* Returns the ECN CE delta */
 static u32 tcp_accecn_process(struct tcp_sock *tp, const struct sk_buff *skb,
 			      u32 delivered_pkts, u32 delivered_bytes, int flag)
@@ -529,6 +533,19 @@ static u32 tcp_accecn_process(struct tcp_sock *tp, const struct sk_buff *skb,
 
 	opt_deltas_valid = tcp_accecn_process_option(tp, skb, delivered_bytes);
 
+	if (delivered_pkts) {
+		if (!tp->pkts_acked_ewma) {
+			tp->pkts_acked_ewma = delivered_pkts << PKTS_ACKED_PREC;
+		} else {
+			u32 ewma = tp->pkts_acked_ewma;
+
+			ewma = (((ewma << PKTS_ACKED_WEIGHT) - ewma) +
+				(delivered_pkts << PKTS_ACKED_PREC)) >>
+			       PKTS_ACKED_WEIGHT;
+			tp->pkts_acked_ewma = min_t(u32, ewma, 0xFFFFU);
+		}
+	}
+
 	if (!(flag & FLAG_SLOWPATH)) {
 		/* AccECN counter might overflow on large ACKs */
 		if (delivered_pkts <= TCP_ACCECN_CEP_ACE_MASK)
@@ -555,7 +572,8 @@ static u32 tcp_accecn_process(struct tcp_sock *tp, const struct sk_buff *skb,
 			return safe_delta;
 		if (d_ceb < safe_delta * tp->mss_cache >> TCP_ACCECN_SAFETY_SHIFT)
 			return delta;
-	}
+	} else if (tp->pkts_acked_ewma > (ACK_COMP_THRESH << PKTS_ACKED_PREC))
+		return delta;
 
 	return safe_delta;
 }
-- 
2.20.1