netdev - Re: [PATCH net-next] tcp: introduce TCPSpuriousRtxHostQueues SNMP counter

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Message-Id: <20130418.145745.1194444222242790469.davem@davemloft.net>
Date:	Thu, 18 Apr 2013 14:57:45 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ycheng@...gle.com, ncardwell@...gle.com,
	therbert@...gle.com, willemb@...gle.com
Subject: Re: [PATCH net-next] tcp: introduce TCPSpuriousRtxHostQueues SNMP
 counter

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 18 Apr 2013 09:52:51 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Host queues (Qdisc + NIC) can hold packets so long that TCP can
> eventually retransmit a packet before the first transmit even left
> the host.
> 
> Its not clear right now if we could avoid this in the first place :
> 
> - We could arm RTO timer not at the time we enqueue packets, but
>   at the time we TX complete them (tcp_wfree())
> 
> - Cancel the sending of the new copy of the packet if prior one
>   is still in queue.
> 
> This patch adds instrumentation so that we can at least see how
> often this problem happens.
> 
> TCPSpuriousRtxHostQueues SNMP counter is incremented every time
> we detect the fast clone is not yet freed in tcp_transmit_skb()
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks.
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