netdev - Re: [PATCH-2.4] Fix divide by 0 in vegas_cong

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Date:	Wed, 30 May 2007 09:34:47 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>"@smtp.osdl.org
Cc:	Lior Dotan <liodot@...il.com>, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH-2.4] Fix divide by 0 in vegas_cong_avoid()

On Wed, 30 May 2007 10:49:54 +0300 (EEST)
"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi> wrote:

> On Tue, 29 May 2007, Stephen Hemminger wrote:
> 
> > On Tue, 29 May 2007 20:23:45 +0200
> > "Lior Dotan" <liodot@...il.com> wrote:
> > 
> > > NTP was not running. I'm not sure what do you mean by fixing the -1.
> > > The trace shows that vegas_cong_avoid() is called with -1, and the
> > > only way it can happen is from tcp_clean_rtx_queue() and the patch
> > > should eliminate this. Another way of solving this is by checking
> > > vegas_rtt_calc() and see if it gets -1 and handle it there.
> > > Another thing that I don't understand is that some places like
> > > tcp_ack() declare seq_rtt as signed and some vegas_cong_avoid()
> > > declare it as unsigned. Shouldn't it be declared always as signed?
> > > 
> > > On 5/29/07, Stephen Hemminger <shemminger@...ux-foundation.org> wrote:
> > > > On Tue, 29 May 2007 12:18:19 +0300
> > > > "Lior Dotan" <liodot@...il.com> wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > I had a divide by zero on kernel 2.4.33 running with Vegas enabled.
> > 
> > I don't think anyone has backported the other vegas fixes from 2.6.21
> > to 2.4.
> > 
> > 
> > For 2.6.22, rtt_calc doesn't exist, instead pkts_acked is called.
> 
> I tried to figure this one out yesterday, and it seems to me that divide 
> by zero should not occur with 2.6.22 (nor with 2.6.21 code), no matter
> how I try to approach vegas...
> 
> > The following should be added to avoid getting bogus timestamp values 
> > from retransmits.
> 
> ...but this is unreliable timestamps problem is a real one that originates 
> from API merge commit 164891aadf1721fca4dce473bb0e0998181537c6 of yours 
> (see some though about it below). I suppose there are now similar concerns 
> about timestamp validity in other cc modules than vegas too.
> 
> > --- a/net/ipv4/tcp_vegas.c	2007-05-02 12:26:35.000000000 -0700
> > +++ b/net/ipv4/tcp_vegas.c	2007-05-29 14:06:26.000000000 -0700
> > @@ -117,6 +117,10 @@ void tcp_vegas_pkts_acked(struct sock *s
> >  	struct vegas *vegas = inet_csk_ca(sk);
> >  	u32 vrtt;
> >  
> > +	/* Ignore retransmits */
> > +	if (unlikely(cnt == 0))
> > +		return;
> > +
> >  	/* Never allow zero rtt or baseRTT */
> >  	vrtt = ktime_to_us(net_timedelta(last)) + 1;
> 
> ...I don't think this works, because cnt won't be zero ever because to 
> make the call some skbs were cleaned from the queue (isn't that what 
> pkts_acked stands for?). There is as if (acked&FLAG_ACKED) guard before 
> making the pkts_acked call to cc modules, thus delta in packets_out will 
> always be greater than 0. Hmm, now that I realize this, I would say that
> checks for > 0 in pkts_acked are entirely bogus (in the current code), 
> hmm, that means I have more things to cleanup in tcp-2.6, at least, bic, 
> cubic and westwood do them... :-).
> 
> I think the code did a right thing before your api merge, since it called 
> rtt callback only if FLAG_RETRANS_DATA_ACKED was not set (and pkts_acked 
> always), i.e., when TCP cannot know if it was the rexmission that 
> triggered the cumulative ACK or any original transmission, no new RTT 
> measurement should be made. Consider also the fact that, there might be a 
> non rexmitted skb after rexmitted one, which Lior's patch doesn't do 
> right way at all since the FLAG_DATA_ACKED would again get set (no 
> div-by-zero would have occured then but an unreliable timestamp would
> have been givento vegas in 2.4).
> 
> The number of packets that got cumulatively acked is never a right metric 
> to measure whether the cumulative ACK originated from rexmitted skbs or 
> not. ...Perhaps the FLAG_RETRANS_DATA_ACKED flag needs to be passed 
> somehow to cc modules?
> 
> 

What about the mixed case where some retransmitted data and new
data was covered by one ack.  I would rather keep the merge, but think
about the cases more carefully.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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