[PATCH v2 net-next 10/11] ipv4: tcp: set SOCK_USE_WRITE_QUEUE for ip_send_unicast_reply()

[Eric Dumazet]

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 <eduma...@google.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