[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>
diff --git a/net/ipv4/tcp_highspeed.c b/net/ipv4/tcp_highspeed.c
index aaa1538..fa3e1aa 100644
--- a/net/ipv4/tcp_highspeed.c
+++ b/net/ipv4/tcp_highspeed.c
@@ -139,14 +139,19 @@
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--;
}