netdev - Re: [PATCH] tcp: bug fix in proportional rate reduction.

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Date:	Mon, 20 May 2013 21:46:51 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Nandita Dukkipati <nanditad@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: bug fix in proportional rate reduction.

On Mon, May 20, 2013 at 8:22 PM, Nandita Dukkipati <nanditad@...gle.com> wrote:
> This patch is a fix for a bug triggering newly_acked_sacked < 0
> in tcp_ack(.).
>
> The bug is triggered by sacked_out decreasing relative to prior_sacked,
> but packets_out remaining the same as pior_packets. This is because the
> snapshot of prior_packets is taken after tcp_sacktag_write_queue() while
> prior_sacked is captured before tcp_sacktag_write_queue(). The problem
> is: tcp_sacktag_write_queue (tcp_match_skb_to_sack() -> tcp_fragment)
> adjusts the pcount for packets_out and sacked_out (MSS change or other
> reason). As a result, this delta in pcount is reflected in
> (prior_sacked - sacked_out) but not in (prior_packets - packets_out).
>
> This patch does the following:
> 1) initializes prior_packets at the start of tcp_ack() so as to
> capture the delta in packets_out created by tcp_fragment.
> 2) introduces a new "previous_packets_out" variable that snapshots
> packets_out right before tcp_clean_rtx_queue, so pkts_acked can be
> correctly computed as before.
>
> Signed-off-by: Nandita Dukkipati <nanditad@...gle.com>
> ---
>  net/ipv4/tcp_input.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)

Acked-by: Neal Cardwell <ncardwell@...gle.com>

neal
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