[ 072/167] [PATCH 12/26] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

Ben Hutchings
2012-05-09T06:20:32Z

From: Neal Cardwell <ncardwell@google.com>

[ Upstream commit 18a223e0b9ec8979320ba364b47c9772391d6d05 ]

Fix a code path in tcp_rcv_rtt_update() that was comparing scaled and
unscaled RTT samples.

The intent in the code was to only use the 'm' measurement if it was a
new minimum. However, since 'm' had not yet been shifted left 3 bits
but 'new_sample' had, this comparison would nearly always succeed,
leading us to erroneously set our receive-side RTT estimate to the 'm'
sample when that sample could be nearly 8x too high to use.

The overall effect is to often cause the receive-side RTT estimate to
be significantly too large (up to 40% too large for brief periods in
my tests).

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/ipv4/tcp_input.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e4d1e4a..4c46fa7 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -473,8 +473,11 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
if (!win_dep) {
m -= (new_sample >> 3);
new_sample += m;
- } else if (m < new_sample)
- new_sample = m << 3;
+ } else {
+ m <<= 3;
+ if (m < new_sample)
+ new_sample = m;
+ }
} else {
/* No previous measure. */
new_sample = m << 3;
--
1.7.10