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tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

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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>
This commit is contained in:
Neal Cardwell 2012-04-10 07:59:20 +00:00 committed by David S. Miller
parent 5fb84b1428
commit 18a223e0b9
1 changed files with 5 additions and 2 deletions
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7
net/ipv4/tcp_input.c
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@ -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;