netdev - Re: [PATCH 1/4 net-next] tcp: consolidate SYNACK RTT sampling

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Date:	Sun, 21 Jul 2013 22:45:13 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/4 net-next] tcp: consolidate SYNACK RTT sampling

On Sun, Jul 21, 2013 at 1:39 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> This patch series improve RTT sampling in three ways:
> 1. Sample RTT during fast recovery and reordering events.
> 2. Favor ack-based RTT to timestamps because of broken TS ECR fields
> 3. Consolidate the RTT measurement logic.
>
> The first patch consolidates SYNACK and other RTT measurement to use a
> central function tcp_ack_update_rtt(). A (small) bonus is now SYNACK
> RTT measurement happens after PAWS check, potentially reducing the
> impact of RTO seeding on bad TCP timestamps values.
>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>

The idea seems very nice to me, but I think a line or two got dropped
in a rebase. :-)

In the previous version of this patch you moved the TFO-specific call
to tcp_synack_rtt_meas() out to a spot in tcp_rcv_state_process()
where both TFO and non-TFO passive connections would hit it. In this
version of the patch it removes the existing calls to
tcp_synack_rtt_meas() but does not add this new call to replace them.
(And it does not seem to be added back in a later patch in the
series.)

Other than that, looks great.

neal
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