netdev - Re: scp stalls mysteriously

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Message-id: <4B17ACEA.9060804@tvk.rwth-aachen.de>
Date:	Thu, 03 Dec 2009 13:19:54 +0100
From:	Damian Lukowski <damian@....rwth-aachen.de>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Frederic Leroy <fredo@...rox.org>, Netdev <netdev@...r.kernel.org>,
	Asdo <asdo@...ftmail.org>, David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Greg KH <gregkh@...e.de>
Subject: Re: scp stalls mysteriously

Damian Lukowski schrieb:
> Ilpo Järvinen schrieb:
>> I've added Greg as CC to make him aware of this issue in early as it now 
>> affects 2.6.32 too (rather important to get dealt quickly in stable once 
>> we have a tested solution since TCP is pretty broken with the silent 
>> deaths this problem seems to cause). ...One possibility would be to just 
>> queue the tested revert to stable and sort this thing out for 2.6.33 in 
>> net-2.6.
>>
>> Opinions, Dave?, Greg?
>>
>> Now back to the issue...
>>
>> You said in the other mail that "All further test are on linus-stable 
>> tree.", which has this contradiction that Linus does not maintain stable 
>> trees. Which exactly was the tree used for the .9. test, Linus' tree or 
>> the 2.6.31 stable tree? I suppose the former since the revert wouldn't 
>> apply to 2.6.31 so I just want to confirm.
>>
>>
>> On Thu, 3 Dec 2009, Frederic Leroy wrote:
>>> On Wed, Dec 02, 2009 at 08:17:44PM +0100, Damian Lukowski wrote:
>>>> could you please printk retrans_stamp just before the return in 
>>>> include/net/tcp.h:retransmits_timed_out()?
>>>> If the value is not monotonically increasing but is reset to 0 at some
>>>> point, this might lead to problems in tcp_write_timeout().
>>>> It's the only idea I have now.
>>> Your idea is good.
>>> Only one out of 4 value is not null.
>>>
>>> Logs corresponding on http://wwW.starox.org/pub/scp_stall is .10
>>>
>>> I make 2 attempts. Printk corresponding to .10 are those after the line 
>>> "wlan1 enter promiscuous mode"
>> Nice thinking indeed Damian, thanks. ...But but, where exactly did you 
>> print? ...There are multiple returns and the return false branch is 
>> expected to have a zero retrans_stamp in a typical case but that is not
>> a problem because we never use the value.
> 
> Yes, it's the retrans_stamp in the subtraction I suspected to be 0.
> I also suspect this to happen only in the ca_state < CA_Loss case,
> so one first solution might be to return true whenever retrans_stamp == 0.

return false, of course.

> Unluckily, I still cannot reproduce the scp stalls here, so it would be nice
> if Frederic printed retrans_stamp together with icsk_ca_state and
> icsk_retransmits, please.
> 
> Damian
> 
>> ...Anyway, if I'm wrong with my suspicion and it still holds that we have 
>> zero retrans_stamp in the substraction too, it could have something to do 
>> with this snippet:
>>
>> static void tcp_try_to_open(struct sock *sk, int flag)
>> {
>>         struct tcp_sock *tp = tcp_sk(sk);
>>
>>         tcp_verify_left_out(tp);
>>
>>         if (!tp->frto_counter && tp->retrans_out == 0)
>>                 tp->retrans_stamp = 0;
>>
>> ...It bit me last time when FRTO was enabled after very small modification 
>> (without running a full verification after the trivial looking 
>> modification). ...So I've worked around this clearing for FRTO as you 
>> can see :-).
>>
>>
>> Also, we have the another mystery to be solved, the fast retransmission is 
>> not triggered for some reason (or alternatively not captured in to a 
>> log), even in the working .9. case. It would be easy to see whether it 
>> works at all from TCP point of view by looking into mibs once you have 
>> have some transfers in a working configuration:
>>
>> grep -A1 TCP /proc/net/netstat
>>
>> ...luckily this fast retransmit issue is less crucial as almost all people 
>> are pretty happy already if their RTO-based recovery works even if the 
>> fast recovery would not. So figuring it out can be postponed (if one has 
>> to prioritize) until the silent death issue is out of the way.
>>
>>
> 
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