netdev - [PATCH v2 net-next 10/11] ipv4: tcp: set SOCK_USE_WRITE_QUEUE for ip_send_unicast

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Date:	Fri,  1 Apr 2016 08:52:21 -0700
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>,
	Tom Herbert <tom@...bertland.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: [PATCH v2 net-next 10/11] ipv4: tcp: set SOCK_USE_WRITE_QUEUE for ip_send_unicast_reply()

TCP uses per cpu 'sockets' to send some packets :
- RST packets ( tcp_v4_send_reset()) )
- ACK packets for SYN_RECV and TIMEWAIT sockets

By setting SOCK_USE_WRITE_QUEUE flag, we tell sock_wfree()
to not call sk_write_space() since these internal sockets
do not care.

This gives a small performance improvement, merely by allowing
cpu to properly predict the sock_wfree() conditional branch,
and avoiding one atomic operation.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_ipv4.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index f3ce0afe70aa..456ff3d6a132 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2384,6 +2384,7 @@ static int __net_init tcp_sk_init(struct net *net)
 					   IPPROTO_TCP, net);
 		if (res)
 			goto fail;
+		sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);
 		*per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk;
 	}
 
-- 
2.8.0.rc3.226.g39d4020