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[TCP] tcp_highspeed: Fix AI updates.

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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>
This commit is contained in:
Xiaoliang (David) Wei 2006-07-11 13:03:28 -07:00 committed by David S. Miller
parent 781b456a98
commit 6150c22e2a
1 changed files with 9 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
net/ipv4/tcp_highspeed.c
View File

@ -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--;
}