tcp: allow dctcp alpha to drop to zero

agshew · davem330 · commit c80dbe046129 · 2015-10-23T02:46:52.000-07:00
If alpha is strictly reduced by alpha &gt;&gt; dctcp_shift_g and if alpha is less
than 1 &lt;&lt; dctcp_shift_g, then alpha may never reach zero. For example,
given shift_g=4 and alpha=15, alpha &gt;&gt; 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 &gt;= 240.

Signed-off-by: Andrew G. Shewmaker &lt;agshew@gmail.com&gt;
Acked-by: Florian Westphal &lt;fw@strlen.de&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git 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.
