netdev - Re: [PATCH 1/1] net: tcp: RTO restart

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Date:	Fri, 21 Feb 2014 05:14:26 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Per Hurtig <per.hurtig@....se>
Cc:	Andreas Petlund <apetlund@...ula.no>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Anna Brunström <anna.brunstrom@....se>,
	Michael Welzl <michawe@....uio.no>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: [PATCH 1/1] net: tcp: RTO restart

On Fri, 2014-02-21 at 11:49 +0100, Per Hurtig wrote:
> On 19 Feb 2014, at 19:01, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > On Wed, 2014-02-19 at 18:17 +0100, Andreas Petlund wrote:
> >> I’ve applied and tested the patch on VMs with both 32 bit and 64 bit kernel.
> >> 
> >> Acked-by: Andreas Petlund <apetlund@...ula.no>
> >> Tested-by: Andreas Petlund <apetlund@...ula.no>
> > 
> > Sure the patch applies, but what experimental results did you get ?
> > 
> > My concern is that TCP_SKB_CB(skb)->when doesn't account for sojourn
> > time in host queues (qdisc and device). 
> > 
> 
> Correct me if I’m wrong, but I was under the impression that the sojourn was
> accounted for in the RTT calculations, and therefore rather inappropriate to
> be accounted for again? If this assumption is wrong, how would you propose that
> we account for the sojourn time?

This is a generic problem. 

Thats why I added LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES snmp counter,
so that we can experiment with the idea of not firing RTO while original
packet is still in the host qdisc.

       struct sk_buff *skb = tcp_write_queue_head(sk);
       const u32 rto_time_stamp = TCP_SKB_CB(skb)->when;
       s32 delta = (s32)(tcp_time_stamp - rto_time_stamp);

       if (delta > 0)
            rto -= delta;

Can trivially gives rto < 0, resulting in a timer of 120 seconds
(TCP_RTO_MAX)




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