static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
{
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned long rtt, delay;

	    /* More than one full frame received... */
	if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
	     /* ... and right edge of window advances far enough.
	      * (tcp_recvmsg() will send ACK otherwise).
	      * If application uses SO_RCVLOWAT, we want send ack now if
	      * we have not received enough bytes to satisfy the condition.
	      */
	    (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
	     __tcp_select_window(sk) >= tp->rcv_wnd)) ||
	    /* We ACK each frame or... */
	    tcp_in_quickack_mode(sk) ||
	    /* Protocol state mandates a one-time immediate ACK */
	    inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOW) {
send_now:
		tcp_send_ack(sk);
		return;
	}

	if (!ofo_possible || RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
		tcp_send_delayed_ack(sk);
		return;
	}

	if (!tcp_is_sack(tp) ||
	    tp->compressed_ack >= sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr)
		goto send_now;

	if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
		tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
		if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
			NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
				      tp->compressed_ack - TCP_FASTRETRANS_THRESH);
		tp->compressed_ack = 0;
	}

	if (++tp->compressed_ack <= TCP_FASTRETRANS_THRESH)
		goto send_now;

	if (hrtimer_is_queued(&tp->compressed_ack_timer))
		return;

	/* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */

	rtt = tp->rcv_rtt_est.rtt_us;
	if (tp->srtt_us && tp->srtt_us < rtt)
		rtt = tp->srtt_us;

	delay = min_t(unsigned long, sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns,
		      rtt * (NSEC_PER_USEC >> 3)/20);
	sock_hold(sk);
	hrtimer_start(&tp->compressed_ack_timer, ns_to_ktime(delay),
		      HRTIMER_MODE_REL_PINNED_SOFT);
}
static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
{
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned long rtt, delay;

	     
	if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
	      
	    (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
	     __tcp_select_window(sk) >= tp->rcv_wnd)) ||
	     
	    tcp_in_quickack_mode(sk) ||
	     
	    inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOW) {
send_now:
		tcp_send_ack(sk);
		return;
	}

	if (!ofo_possible || RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
		tcp_send_delayed_ack(sk);
		return;
	}

	if (!tcp_is_sack(tp) ||
	    tp->compressed_ack >= sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr)
		goto send_now;

	if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {
		tp->compressed_ack_rcv_nxt = tp->rcv_nxt;
		if (tp->compressed_ack > TCP_FASTRETRANS_THRESH)
			NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
				      tp->compressed_ack - TCP_FASTRETRANS_THRESH);
		tp->compressed_ack = 0;
	}

	if (++tp->compressed_ack <= TCP_FASTRETRANS_THRESH)
		goto send_now;

	if (hrtimer_is_queued(&tp->compressed_ack_timer))
		return;

	 

	rtt = tp->rcv_rtt_est.rtt_us;
	if (tp->srtt_us && tp->srtt_us < rtt)
		rtt = tp->srtt_us;

	delay = min_t(unsigned long, sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns,
		      rtt * (NSEC_PER_USEC >> 3)/20);
	sock_hold(sk);
	hrtimer_start(&tp->compressed_ack_timer, ns_to_ktime(delay),
		      HRTIMER_MODE_REL_PINNED_SOFT);
}