tcp: smoother receiver autotuning

Back in linux-3.13 (commit b0983d3c ("tcp: fix dynamic right sizing"))
I addressed the pressing issues we had with receiver autotuning.

But DRS suffers from extra latencies caused by rcv_rtt_est.rtt_us
drifts. One common problem happens during slow start, since the
apparent RTT measured by the receiver can be inflated by ~50%,
at the end of one packet train.

Also, a single drop can delay read() calls by one RTT, meaning
tcp_rcv_space_adjust() can be called one RTT too late.

By replacing the tri-modal heuristic with a continuous function,
we can offset the effects of not growing 'at the optimal time'.

The curve of the function matches prior behavior if the space
increased by 25% and 50% exactly.

Cost of added multiply/divide is small, considering a TCP flow
typically would run this part of the code few times in its life.

I tested this patch with 100 ms RTT / 1% loss link, 100 runs
of (netperf -l 5), and got an average throughput of 4600 Mbit
instead of 1700 Mbit.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -601,26 +601,17 @@ void tcp_rcv_space_adjust(struct sock *sk)
 	if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf &&
 	    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
 		int rcvmem, rcvbuf;
-		u64 rcvwin;
+		u64 rcvwin, grow;
 
 		/* minimal window to cope with packet losses, assuming
 		 * steady state. Add some cushion because of small variations.
 		 */
 		rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
 
-		/* If rate increased by 25%,
-		 *	assume slow start, rcvwin = 3 * copied
-		 * If rate increased by 50%,
-		 *	assume sender can use 2x growth, rcvwin = 4 * copied
-		 */
-		if (copied >=
-		    tp->rcvq_space.space + (tp->rcvq_space.space >> 2)) {
-			if (copied >=
-			    tp->rcvq_space.space + (tp->rcvq_space.space >> 1))
-				rcvwin <<= 1;
-			else
-				rcvwin += (rcvwin >> 1);
-		}
+		/* Accommodate for sender rate increase (eg. slow start) */
+		grow = rcvwin * (copied - tp->rcvq_space.space);
+		do_div(grow, tp->rcvq_space.space);
+		rcvwin += (grow << 1);
 
 		rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
 		while (tcp_win_from_space(sk, rcvmem) < tp->advmss)