netdev - Re: [PATCH net-next] tcp: better retrans tracking for defer-accept

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Date:	Sat, 03 Nov 2012 14:46:59 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	ja@....bg, subramanian.vijay@...il.com, netdev@...r.kernel.org,
	ncardwell@...gle.com, venkat.x.venkatsubra@...cle.com,
	enh@...gle.com, ycheng@...gle.com
Subject: Re: [PATCH net-next] tcp: better retrans tracking for defer-accept

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 27 Oct 2012 16:18:57 +0200

> From: Eric Dumazet <edumazet@...gle.com>
> 
> For passive TCP connections using TCP_DEFER_ACCEPT facility,
> we incorrectly increment req->retrans each time timeout triggers
> while no SYNACK is sent.
> 
> SYNACK are not sent for TCP_DEFER_ACCEPT that were established (for wich
> we received the ACK from client). Only the last SYNACK is
> sent so that we can receive again an ACK from client, to move the
> req into accept queue. We plan to change this later to avoid
> the useless retransmit (and potential problem as this SYNACK could be
> lost)
> 
> TCP_INFO later gives wrong information to user, claiming imaginary
> retransmits.
> 
> Decouple req->retrans field into two independent fields :
> 
> num_retrans : number of retransmit
> num_timeout : number of timeouts
> 
> num_timeout is the counter that is incremented at each timeout,
> regardless of actual SYNACK being sent or not, and used to
> compute the exponential timeout.
> 
> Introduce inet_rtx_syn_ack() helper to increment num_retrans
> only if ->rtx_syn_ack() succeeded.
> 
> Use inet_rtx_syn_ack() from tcp_check_req() to increment num_retrans
> when we re-send a SYNACK in answer to a (retransmitted) SYN.
> Prior to this patch, we were not counting these retransmits.
> 
> Change tcp_v[46]_rtx_synack() to increment TCP_MIB_RETRANSSEGS
> only if a synack packet was successfully queued.
> 
> Reported-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.
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