netdev - [PATCH net-next] tcp: remove loop to compute wscale

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Message-Id: <20181129155620.106334-1-edumazet@google.com>
Date:   Thu, 29 Nov 2018 07:56:20 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next] tcp: remove loop to compute wscale

We can remove the loop and conditional branches
and compute wscale efficiently thanks to ilog2()

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv4/tcp_output.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 3ae4a277991a3473f61b1f73acd5ee355cf6a5df..d3b691f3a9e896ce340ae081a0d24092024361c8 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -233,16 +233,14 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
 	if (init_rcv_wnd)
 		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
 
-	(*rcv_wscale) = 0;
+	*rcv_wscale = 0;
 	if (wscale_ok) {
 		/* Set window scaling on max possible window */
 		space = max_t(u32, space, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
 		space = max_t(u32, space, sysctl_rmem_max);
 		space = min_t(u32, space, *window_clamp);
-		while (space > U16_MAX && (*rcv_wscale) < TCP_MAX_WSCALE) {
-			space >>= 1;
-			(*rcv_wscale)++;
-		}
+		*rcv_wscale = clamp_t(int, ilog2(space) - 15,
+				      0, TCP_MAX_WSCALE);
 	}
 	/* Set the clamp no higher than max representable value */
 	(*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
-- 
2.20.0.rc0.387.gc7a69e6b6c-goog