Hello Proces have a established tcp socket. It calls sys_revcmsg on that socket. That function goes all the way to tcp_recvmsg function. Function tcp_recvmsg.c (defined in net/ipv4/tcp.c) blocks it self on function sk_wait_data if socket recv queue is empty. I wanted to know how socked is checked in time. Soo i Looked up this sk_wait_data function (http://lxr.free-electrons.com/source/net/core/sock.c#L1933): 1933 int sk_wait_data(struct sock *sk, long *timeo) 1934 { 1935 int rc; 1936 DEFINE_WAIT(wait); 1937 1938 prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE); 1939 set_bit(SOCK_ASYNC_WAITDATA, &sk->sk_socket->flags); 1940 rc = sk_wait_event(sk, timeo, !skb_queue_empty(&sk->sk_receive_queue)); 1941 clear_bit(SOCK_ASYNC_WAITDATA, &sk->sk_socket->flags); 1942 finish_wait(sk_sleep(sk), &wait); 1943 return rc; 1944 } This function blocks it self on the sk_wait_event macro. Here is it definition (http://lxr.free-electrons.com/source/include/net/sock.h#L893): 893 #define sk_wait_event(__sk, __timeo, __condition) \ 894 ({ int __rc; \ 895 release_sock(__sk); \ 896 __rc = __condition; \ 897 if (!__rc) { \ 898 *(__timeo) = schedule_timeout(*(__timeo)); \ 899 } \ 900 lock_sock(__sk); \ 901 __rc = __condition; \ 902 __rc; \ 903 }) This macro is blocked in the schedule_timeout all the time. I know this because I added printk functions with some marks easly to target the line blocking the socket. It hangs on this schedule_timeout and it magicaly released when some data apear in the revc queue. How does the kernel know when to "unblock" the process? Function name: sk_wait_data and macro name: sk_wait_event tells me that there should be some queue checning from time to time... Can someone explain this to me?