From: Andrew Shewmaker <agshew@gmail.com>
Date: Mon, 19 Oct 2015 04:59:08 +0000 (-0700)
Subject: tcp: allow dctcp alpha to drop to zero
X-Git-Tag: firefly_0821_release~176^2~865^2~23
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=c80dbe04612986fd6104b4a1be21681b113b5ac9;p=firefly-linux-kernel-4.4.55.git

tcp: allow dctcp alpha to drop to zero

If alpha is strictly reduced by alpha >> dctcp_shift_g and if alpha is less
than 1 << dctcp_shift_g, then alpha may never reach zero. For example,
given shift_g=4 and alpha=15, alpha >> dctcp_shift_g yields 0 and alpha
remains 15. The effect isn't noticeable in this case below cwnd=137, but
could gradually drive uncongested flows with leftover alpha down to
cwnd=137. A larger dctcp_shift_g would have a greater effect.

This change causes alpha=15 to drop to 0 instead of being decrementing by 1
as it would when alpha=16. However, it requires one less conditional to
implement since it doesn't have to guard against subtracting 1 from 0U. A
decay of 15 is not unreasonable since an equal or greater amount occurs at
alpha >= 240.

Signed-off-by: Andrew G. Shewmaker <agshew@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
---

diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 7092a61c4dc8..7e538f71f5fb 100644
--- a/net/ipv4/tcp_dctcp.c
+++ b/net/ipv4/tcp_dctcp.c
@@ -209,7 +209,7 @@ static void dctcp_update_alpha(struct sock *sk, u32 flags)
 
 		/* alpha = (1 - g) * alpha + g * F */
 
-		alpha -= alpha >> dctcp_shift_g;
+		alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
 		if (bytes_ecn) {
 			/* If dctcp_shift_g == 1, a 32bit value would overflow
 			 * after 8 Mbytes.