netdev - Re: TCP keepalive timer problem

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Date:	Thu, 27 Aug 2009 16:49:00 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Li_Xin2@....com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: TCP keepalive timer problem

Andi Kleen a écrit :
> On Thu, Aug 27, 2009 at 04:17:10PM +0200, Eric Dumazet wrote:
>> Andi Kleen a écrit :
>>> Eric Dumazet <eric.dumazet@...il.com> writes:
>>>> Now, 7200 seconds might be inappropriate for special needs, and considering
>>>> there is no way to change tcp_retries2 for a given socket (only choice being the global
>>>> tcp_retries2 setting), I would vote for a change in our stack, to *relax* RFC,
>>>> and get smaller keepalive timers if possible.
>>> I think the better fix would be to just to only do that when
>>> tcp_retries2 > keep alive time. So keep the existing behaviour
>>> with default keep alive, but switch when the user defined
>>> a very short keep alive.
>>>
>> tcp_retries2 is a number of retries, its difficult to derive a time from it.
> 
> That shouldn't be too hard. 
> 
>> Also, it's not clear what behavior you are refering to.
>> Imagine we can be smart and compute tcp_retries2_time (in jiffies) from tcp_retries2
>> If keepalive_timer fires and we have packets in flight, what heuristic do you suggest ?
> 
> I didn't suggest to change something at firing time, just pattern
> the code you removed with if (keepalive_time > retries2 time)
> 
> That's not perfect, but likely good enough.
> 
> 
>> if (tp->packets_out || tcp_send_head(sk))
>> 	if (tcp_retries2_time < keepalive_time_when(tp))
>> 		goto resched;
>> elapsed = tcp_time_stamp - tp->rcv_tstamp;
>> ...
>>
>> What would be the gain ?
>> Arming timer exactly every keepalive_time_when(tp)
>> instead of keepalive_time_when(tp) - (tcp_time_stamp - tp->rcv_tstamp) ?
> 
> The gain would be that you don't send unnecessary packets by default (following the RFC), but 
> still give expected behaviour to users who explicitely set short keepalives.
> 

Yep, so to recap we have two changes :

1) The one I sent (taking into account the time of last ACK we received) to compute the
timer delays.

2) The one you suggest, avoiding to send a probe if we have packets in flight, relying
on normal retransmits timers.

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