From c80dbe04612986fd6104b4a1be21681b113b5ac9 Mon Sep 17 00:00:00 2001 From: Andrew Shewmaker Date: Sun, 18 Oct 2015 21:59:08 -0700 Subject: [PATCH] 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 Acked-by: Florian Westphal Signed-off-by: David S. Miller --- 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 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. -- 2.30.2