netdev - Re: tc linklayer ADSL calc broken after commit 56b765b79 (htb: improved accuracy at high rates)

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Date:	Thu, 30 May 2013 11:15:47 +0200
From:	Jesper Dangaard Brouer <jbrouer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	David Miller <davem@...hat.com>, j.vimal@...il.com,
	Michal Soltys <soltys@....info>,
	Mike Frysinger <vapier@...too.org>,
	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Patrick McHardy <kaber@...sh.net>,
	Jiri Pirko <jpirko@...hat.com>,
	Toke Høiland-Jørgensen 
	<toke@...e.dk>, Dave Taht <dave.taht@...il.com>,
	netdev@...r.kernel.org, bloat@...ts.bufferbloat.net,
	Dan Siemon <dan@...erfire.com>,
	Jim Gettys <jg@...edesktop.org>,
	Steven Barth <cyrus@...nwrt.org>, Felix Fietkau <nbd@....name>,
	Jiri Benc <jbenc@...hat.com>,
	"russell-tcatm@...art.id.au" <russell-tcatm@...art.id.au>
Subject: Re: tc linklayer ADSL calc broken after commit 56b765b79 (htb:
 improved accuracy at high rates)


On Wed, 29 May 2013 16:18:26 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-05-29 at 15:50 -0700, Stephen Hemminger wrote:
> > On Wed, 29 May 2013 08:52:04 -0700
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > On Wed, 2013-05-29 at 15:13 +0200, Jesper Dangaard Brouer wrote:
> > > > I recently discovered that the (traffic control) tc linklayer
> > > > calculations for ATM/ADSL have been broken by:
> > > >  commit 56b765b79 (htb: improved accuracy at high rates).
> > > > 
> > > > Thus, people shaping on ADSL links, using e.g.:
> > > >  tc class add ... htb rate X ceil Y linklayer atm overhead 10
> > > > 
> > > > Will no-longer get ATM cell tax/overhead adjusted.
> > > > 
> 
> Adding the logic on the kernel is doable, by adding some clean
> attributes so that tc can setup the feature, and report the attributes
> back.

Yes, doing the logic in the kernel might be a better solution.
But the question is how do we keep iproute2 backward compatible with
older kernels?



> cpus are fast today and can perform the atm cell/overhead faster than
> a table lookup.

Do remember that the target CPU is small embedded router boxes.
BUT I do agree that, the following code required, is probably faster
than a table lookup:

 int pkt_len = skb->len + (encap_overhead * gso_segments);
 int wire_sz = DIV_ROUND_UP(pkt_len,48)*53;

(I suspect, that the compiler might even optimize and remove any
real divisions, I bet Eric can tell us.)

Looking at how simple the above code is, I'm a little appalled by all
the table lookup infrastructure and hacks we added, to
support/implement this.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer
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