netdev - Re: [PATCH 2/5] tcp: use DSACKs that arrive when packets

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Date:	Thu, 17 Nov 2011 03:34:17 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Neal Cardwell <ncardwell@...gle.com>
cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH 2/5] tcp: use DSACKs that arrive when packets_out is 0

On Wed, 16 Nov 2011, Neal Cardwell wrote:

> The bug: Senders ignored DSACKs after recovery when there were no
> outstanding packets (a common scenario for HTTP servers).
> 
> The change: when there are no outstanding packets (the "no_queue" goto
> label), call tcp_fastretrans_alert() in order to use DSACKs to undo
> congestion window reductions.
> 
> Other patches in this series will provide other changes that are
> necessary to fully fix this problem.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> ---
>  net/ipv4/tcp_input.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index f772aaa..b49e418 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3788,6 +3788,10 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>  	return 1;
>  
>  no_queue:
> +	/* If data was DSACKed, see if we can undo a cwnd reduction. */
> +	if (flag & FLAG_DSACKING_ACK)
> +		tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
> +				      is_dupack, flag);

Acked-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>

...however, there could be other undo related gotchas now added by this 
patch due to complexity of tcp_fastretrans_alert. It would probably be 
safer to just add the dsack undo related code here?

-- 
 i.