From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Re: [PATCH 0/6] Add qdisc->ops->peek() support. Date: Fri, 17 Oct 2008 16:12:03 +0200 Message-ID: <48F89D33.9090809@trash.net> References: <20081016220905.GA2747@ami.dom.local> <48F88613.1060404@trash.net> <20081017130333.GA8297@ff.dom.local> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: David Miller , netdev@vger.kernel.org, Herbert Xu To: Jarek Poplawski Return-path: Received: from stinky.trash.net ([213.144.137.162]:33329 "EHLO stinky.trash.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754326AbYJQOMK (ORCPT ); Fri, 17 Oct 2008 10:12:10 -0400 In-Reply-To: <20081017130333.GA8297@ff.dom.local> Sender: netdev-owner@vger.kernel.org List-ID: Jarek Poplawski wrote: > On Fri, Oct 17, 2008 at 02:33:23PM +0200, Patrick McHardy wrote: >>> @@ -233,7 +233,9 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch) >>> */ >>> cb->time_to_send = psched_get_time(); >>> q->counter = 0; >>> - ret = q->qdisc->ops->requeue(skb, q->qdisc); >>> + q->qdisc->flags |= TCQ_F_REQUEUE; >>> + ret = qdisc_equeue(skb, q->qdisc); >>> + q->qdisc->flags &= ~TCQ_F_REQUEUE; >> Well, the inner qdisc would still need to logic to order packets >> apprioriately. > > I'm not sure I was understood: the idea is to do something like > in this example in tfifo_enqueue() in all leaf qdiscs like fifo > etc. too, so to redirect their ->enqueue() to their ->requeue() > which usually is qdisc_requeue() (or to it directly if needed). Yes, I misunderstood this, I though the intention was to get rid of requeue entirely. >> Its probably not that hard, but as I said, I don't >> think its necessary at all. It only makes a difference with a >> non-work-conserving inner qdisc, but a lot of the functionality of >> netem requires the inner tfifo anyways and rate-limiting is usually >> done on top of netem. So I would suggest so either hard-wire the >> tfifo qdisc or at least make the assumption that inner qdiscs are >> work-conserving. > > Of course, I can do it like this, but wouldn't it break backward > compatibility for some users? Some general thoughts ... We've never had any systematic checks for useful and non-useful combination of qdiscs, which is causing a lot of these complications. Think of all the multiq work that was required to make it work properly with non-work-conserving qdiscs - while at the same time, using a non-work-conserving qdisc (which require a global view) defeats basically all of the benefits. So it would be really useful to come up with a systematic definition of valid combinations instead of trying handling lots of purely theoretical case that don't make sense. One more example - all the qdiscs implement ->drop(), yet its only needed by CBQ and it doesn't make any sense at all to use lets say HFSC as child of CBQ. About this specific case - yes, it would break compatibility for users using f.i. TBF as child of netem. But if you look at the netem_enqueue() function, it in fact assumes that the inner qdisc is a tfifo, so we'd be breaking an already broken case. We can of course be nice and warn about it for a few releases, but I believe there is some real potential for simplification that makes it worth it.