netdev - [PATCH] allow dctcp alpha to drop to zero

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Message-ID: <20151019045908.GB11368@mininet-vm>
Date:	Sun, 18 Oct 2015 21:59:08 -0700
From:	Andrew Shewmaker <agshew@...il.com>
To:	netdev@...r.kernel.org
Cc:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Florian Westphal <fw@...len.de>,
	Glenn Judd <glenn.judd@...ganstanley.com>
Subject: [PATCH] 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@...il.com>
---
 net/ipv4/tcp_dctcp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_dctcp.c b/net/ipv4/tcp_dctcp.c
index 7092a61..7e538f7 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.
-- 
1.9.1

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