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>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e886e2f7fa8d03edc8644179a6f1ef7ca6a374f6..e7b54d29690e947f006f99e5d6d7f53b8cc89feb 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -474,8 +474,11 @@ 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;