netdev - Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

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Date:	Tue, 10 Apr 2012 20:45:46 +0200
From:	Eric Dumazet <edumazet@...gle.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

On Tue, Apr 10, 2012 at 7:59 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> 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@...gle.com>
> ---
>  net/ipv4/tcp_input.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index e886e2f..e7b54d2 100644
> --- 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;
> --
> 1.7.7.3
>

Acked-by: Eric Dumazet <edumazet@...gle.com>
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