tcp: increase the default TCP scaling ratio

After commit dfa2f048 ("tcp: get rid of sysctl_tcp_adv_win_scale"),
we noticed an application-level timeout due to reduced throughput.

Before the commit, for a client that sets SO_RCVBUF to 65k, it takes
around 22 seconds to transfer 10M data. After the commit, it takes 40
seconds. Because our application has a 30-second timeout, this
regression broke the application.

The reason that it takes longer to transfer data is that
tp->scaling_ratio is initialized to a value that results in ~0.25 of
rcvbuf. In our case, SO_RCVBUF is set to 65536 by the application, which
translates to 2 * 65536 = 131,072 bytes in rcvbuf and hence a ~28k
initial receive window.

Later, even though the scaling_ratio is updated to a more accurate
skb->len/skb->truesize, which is ~0.66 in our environment, the window
stays at ~0.25 * rcvbuf. This is because tp->window_clamp does not
change together with the tp->scaling_ratio update when autotuning is
disabled due to SO_RCVBUF. As a result, the window size is capped at the
initial window_clamp, which is also ~0.25 * rcvbuf, and never grows
bigger.

Most modern applications let the kernel do autotuning, and benefit from
the increased scaling_ratio. But there are applications such as kafka
that has a default setting of SO_RCVBUF=64k.

This patch increases the initial scaling_ratio from ~25% to 50% in order
to make it backward compatible with the original default
sysctl_tcp_adv_win_scale for applications setting SO_RCVBUF.

Fixes: dfa2f048 ("tcp: get rid of sysctl_tcp_adv_win_scale")
Signed-off-by: Hechao Li <hli@netflix.com>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/netdev/20240402215405.432863-1-hli@netflix.com/Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1539,11 +1539,10 @@ static inline int tcp_space_from_win(const struct sock *sk, int win)
 	return __tcp_space_from_win(tcp_sk(sk)->scaling_ratio, win);
 }
 
-/* Assume a conservative default of 1200 bytes of payload per 4K page.
+/* Assume a 50% default for skb->len/skb->truesize ratio.
  * This may be adjusted later in tcp_measure_rcv_mss().
  */
-#define TCP_DEFAULT_SCALING_RATIO ((1200 << TCP_RMEM_TO_WIN_SCALE) / \
-				   SKB_TRUESIZE(4096))
+#define TCP_DEFAULT_SCALING_RATIO (1 << (TCP_RMEM_TO_WIN_SCALE - 1))
 
 static inline void tcp_scaling_ratio_init(struct sock *sk)
 {