netdev - [PATCH 2/2 net-next] tcp: sk_add_backlog() is too agressive for TCP

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Date:	Mon, 23 Apr 2012 11:38:54 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>, Tom Herbert <therbert@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Rick Jones <rick.jones2@...com>
Subject: [PATCH 2/2 net-next] tcp: sk_add_backlog() is too agressive for TCP

From: Eric Dumazet <edumazet@...gle.com>

While investigating TCP performance problems on 10Gb+ links, we found a
tcp sender was dropping lot of incoming ACKS because of sk_rcvbuf limit
in sk_add_backlog(), especially if receiver doesnt use GRO/LRO and sends
one ACK every two MSS segments.

A sender usually tweaks sk_sndbuf, but sk_rcvbuf stays at its default
value (87380), allowing a too small backlog.

A TCP ACK, even being small, can consume nearly same truesize space than
outgoing packets. Using sk_rcvbuf + sk_sndbuf as a limit makes sense and
is fast to compute.

Performance results on netperf, single flow, receiver with disabled
GRO/LRO : 7500 Mbits instead of 6050 Mbits, no more TCPBacklogDrop
increments at sender.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Tom Herbert <therbert@...gle.com>
Cc: Maciej Żenczykowski <maze@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc: Rick Jones <rick.jones2@...com>
---
 net/ipv4/tcp_ipv4.c |    3 ++-
 net/ipv6/tcp_ipv6.c |    3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 917607e..cf97e98 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1752,7 +1752,8 @@ process:
 			if (!tcp_prequeue(sk, skb))
 				ret = tcp_v4_do_rcv(sk, skb);
 		}
-	} else if (unlikely(sk_add_backlog(sk, skb, sk->sk_rcvbuf))) {
+	} else if (unlikely(sk_add_backlog(sk, skb,
+					   sk->sk_rcvbuf + sk->sk_sndbuf))) {
 		bh_unlock_sock(sk);
 		NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
 		goto discard_and_relse;
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index b04e6d8..5fb19d3 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1654,7 +1654,8 @@ process:
 			if (!tcp_prequeue(sk, skb))
 				ret = tcp_v6_do_rcv(sk, skb);
 		}
-	} else if (unlikely(sk_add_backlog(sk, skb, sk->sk_rcvbuf))) {
+	} else if (unlikely(sk_add_backlog(sk, skb,
+					   sk->sk_rcvbuf + sk->sk_sndbuf))) {
 		bh_unlock_sock(sk);
 		NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
 		goto discard_and_relse;


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