netdev - Re: [PATCH 3/4 net-next] tcp: measure RTT from new SACK

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Date:	Sun, 21 Jul 2013 23:32:33 -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 3/4 net-next] tcp: measure RTT from new SACK

On Sun, Jul 21, 2013 at 1:39 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> @@ -1148,6 +1149,7 @@ static u8 tcp_sacktag_one(struct sock *sk,
>                                                            state->reord);
>                                 if (!after(end_seq, tp->high_seq))
>                                         state->flag |= FLAG_ORIG_SACK_ACKED;
> +                               state->rtt = tcp_time_stamp - lsndtime;

To be safer we should probably take the RTT sample for the left-most
SACKed segment, just like tcp_clean_rtx_queue() takes a seq_rtt RTT
sample for the left-most cumulatively ACKed segment. Since
tcp_sacktag_write_queue() already sorts the SACK blocks, I think that
means tcp_sacktag_one() just needs to only set state->rtt if it is
still less than 0 (analagous to the update logic in
tcp_clean_rtx_queue()).

Also, FWIW I think the "lsndtime" variable name here is a little confusing,
since it is being used for the send time of the particular skb, which is
quite different from the similarly-named existing variable, tp->lsndttime.
Maybe "xmit_stamp"?

Otherwise, looks good. This is going to be nice to have.

neal
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