From 6150c22e2ac3f7dbe73e7ae7817785070d0cff1f Mon Sep 17 00:00:00 2001 From: "Xiaoliang (David) Wei" Date: Tue, 11 Jul 2006 13:03:28 -0700 Subject: [PATCH] [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 Signed-off-by: David S. Miller --- net/ipv4/tcp_highspeed.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/net/ipv4/tcp_highspeed.c b/net/ipv4/tcp_highspeed.c index aaa1538c0692..fa3e1aad660c 100644 --- a/net/ipv4/tcp_highspeed.c +++ b/net/ipv4/tcp_highspeed.c @@ -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--; }