netdev - Re: scp stalls mysteriously

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Message-ID: <alpine.DEB.2.00.0912031444150.7024@wel-95.cs.helsinki.fi>
Date:	Thu, 3 Dec 2009 14:49:11 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Damian Lukowski <damian@....rwth-aachen.de>
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

On Thu, 3 Dec 2009, Damian Lukowski wrote:

> > 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.

I suppose adding || !tp->retrans_stamp into the false condition is fine 
as long as we don't then have a connection that can cause a connection 
to hang there forever for some reason (this needs to be understood well 
enough, not just test driven in stables :-)).

> 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.

It wouldn't hurt to know tp->packets_out and tp->retrans_out too, that 
might have some significant w.r.t what happens because of FRTO.


-- 
 i.
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