Commit 6150c22e authored Jul 11, 2006 by Xiaoliang (David) Wei Committed by David S. Miller Jul 12, 2006

[TCP] tcp_highspeed: Fix AI updates.



I think there is still a problem with the AIMD parameter update in
HighSpeed TCP code.

Line 125~138 of the code (net/ipv4/tcp_highspeed.c):

	/* Update AIMD parameters */
	if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
		while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
		       ca->ai < HSTCP_AIMD_MAX - 1)
			ca->ai++;
	} else if (tp->snd_cwnd < hstcp_aimd_vals[ca->ai].cwnd) {
		while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
		       ca->ai > 0)
			ca->ai--;

In fact, the second part (decreasing ca->ai) never decreases since the
while loop's inequality is in the reverse direction. This leads to
unfairness with multiple flows (once a flow happens to enjoy a higher
ca->ai, it keeps enjoying that even its cwnd decreases)

Here is a tentative fix (I also added a comment, trying to keep the
change clear):

Acked-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 781b456a

net/ipv4/tcp_highspeed.c

+9 −4

Original line number	Diff line number	Diff line
		@@ -139,14 +139,19 @@ static void hstcp_cong_avoid(struct sock *sk, u32 adk, u32 rtt,
		tp->snd_cwnd++;
		}
		} else {
		/* Update AIMD parameters */
		/* Update AIMD parameters.
		*
		* We want to guarantee that:
		* hstcp_aimd_vals[ca->ai-1].cwnd <
		* snd_cwnd <=
		* hstcp_aimd_vals[ca->ai].cwnd
		*/
		if (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd) {
		while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
		ca->ai < HSTCP_AIMD_MAX - 1)
		ca->ai++;
		} else if (tp->snd_cwnd < hstcp_aimd_vals[ca->ai].cwnd) {
		while (tp->snd_cwnd > hstcp_aimd_vals[ca->ai].cwnd &&
		ca->ai > 0)
		} else if (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd) {
		while (ca->ai && tp->snd_cwnd <= hstcp_aimd_vals[ca->ai-1].cwnd)
		ca->ai--;
		}