netdev - Re: scp stalls mysteriously

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Message-ID: <20091203131127.131e9122@houba>
Date:	Thu, 3 Dec 2009 13:11:27 +0100
From:	Frederic Leroy <fredo@...rox.org>
To:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
Cc:	Damian Lukowski <damian@....rwth-aachen.de>,
	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

Le Thu, 3 Dec 2009 12:29:39 +0200 (EET),
"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> a écrit :

> 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

Sorry I'm confused and so confuse you.
For .9 .10 and now I'm only using : 
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
 
> 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.

I didn't keep the source of the old 2.6.31 kernel I have. 
So it's either 
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
or
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-2.6-stable.git

> 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.

Here is the code :
http://www.starox.org/pub/scp_stall/printk_retrans_stamp.patch

> ...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

I will try this evening. I can do test only outside office hours.

-- 
Frédéric Leroy
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