tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample
authorNeal Cardwell <ncardwell@google.com>
Tue, 10 Apr 2012 07:59:20 +0000 (07:59 +0000)
committerVarun Wadekar <vwadekar@nvidia.com>
Wed, 18 Apr 2012 14:07:04 +0000 (19:07 +0530)
commitca58601fa43bf67e64f64ecd6184c473fe00ec70
tree6687227cde97b583178a2a8b1a5e9f8547f4f388
parent1dc7f9d13948e9cb083519b8d18e67b4d064370a
tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

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>
net/ipv4/tcp_input.c