[TCP]: tcp_memory_pressure and tcp_socket are__read_mostly candidates

tcp_memory_pressure and tcp_socket currently share a cache line with tcp_memory_allocated, tcp_sockets_allocated.
(Very hot cache line)
It makes sense to declare these variables as __read_mostly, to avoid false sharing on SMP.

ffffffff8081d9c0 B tcp_orphan_count
ffffffff8081d9c4 B tcp_memory_allocated
ffffffff8081d9c8 B tcp_sockets_allocated
ffffffff8081d9cc B tcp_memory_pressure
ffffffff8081d9d0 b tcp_md5sig_users
ffffffff8081d9d8 b tcp_md5sig_pool
ffffffff8081d9e0 b warntime.31570
ffffffff8081d9e8 b tcp_socket
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -297,7 +297,7 @@ EXPORT_SYMBOL(tcp_sockets_allocated);
  * All the sk_stream_mem_schedule() is of this nature: accounting
  * is strict, actions are advisory and have some latency.
  */
-int tcp_memory_pressure;
+int tcp_memory_pressure __read_mostly;
 
 EXPORT_SYMBOL(tcp_memory_pressure);
 

net/ipv4/tcp_ipv4.c

@@ -88,7 +88,7 @@ int sysctl_tcp_low_latency __read_mostly;
 #define ICMP_MIN_LENGTH 8
 
 /* Socket used for sending RSTs */
-static struct socket *tcp_socket;
+static struct socket *tcp_socket __read_mostly;
 
 void tcp_v4_send_check(struct sock *sk, int len, struct sk_buff *skb);