From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: Re: tc linklayer ADSL calc broken after commit 56b765b79 (htb: improved accuracy at high rates) Date: Thu, 30 May 2013 11:15:47 +0200 Message-ID: <20130530111547.07397287@redhat.com> References: <20130529151330.22c5c89e@redhat.com> <1369842724.5109.44.camel@edumazet-glaptop> <20130529155034.334092c5@nehalam.linuxnetplumber.net> <1369869506.5109.87.camel@edumazet-glaptop> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Stephen Hemminger , Jesper Dangaard Brouer , David Miller , j.vimal@gmail.com, Michal Soltys , Mike Frysinger , Jussi Kivilinna , Patrick McHardy , Jiri Pirko , Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNl?= =?UTF-8?B?bg==?= , Dave Taht , netdev@vger.kernel.org, bloat@lists.bufferbloat.net, Dan Siemon , Jim Gettys , Steven Barth , Felix Fietkau , Jiri Benc , "russell-tcatm@stuart.id.au" To: Eric Dumazet Return-path: Received: from mx1.redhat.com ([209.132.183.28]:37580 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030276Ab3E3JQd (ORCPT ); Thu, 30 May 2013 05:16:33 -0400 In-Reply-To: <1369869506.5109.87.camel@edumazet-glaptop> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 29 May 2013 16:18:26 -0700 Eric Dumazet wrote: > On Wed, 2013-05-29 at 15:50 -0700, Stephen Hemminger wrote: > > On Wed, 29 May 2013 08:52:04 -0700 > > Eric Dumazet 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