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 7092a61c4dc8465fcf17ff71b289cf25bbb8b559..7e538f71f5fbae087c3e3e4367d60e08cd609ac5 100644 (file)
--- 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.