From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755414Ab2DWOnl (ORCPT ); Mon, 23 Apr 2012 10:43:41 -0400 Received: from mail-bk0-f46.google.com ([209.85.214.46]:48139 "EHLO mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753899Ab2DWOnj (ORCPT ); Mon, 23 Apr 2012 10:43:39 -0400 Message-ID: <4F956A95.5030304@gmail.com> Date: Mon, 23 Apr 2012 16:43:33 +0200 From: Juri Lelli User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1 MIME-Version: 1.0 To: Peter Zijlstra CC: tglx@linutronix.de, mingo@redhat.com, rostedt@goodmis.org, cfriesen@nortel.com, oleg@redhat.com, fweisbec@gmail.com, darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch, linux-kernel@vger.kernel.org, claudio@evidence.eu.com, michael@amarulasolutions.com, fchecconi@gmail.com, tommaso.cucinotta@sssup.it, nicola.manica@disi.unitn.it, luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com, paulmck@linux.vnet.ibm.com, raistlin@linux.it, insop.song@ericsson.com, liming.wang@windriver.com Subject: Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation. References: <1333696481-3433-1-git-send-email-juri.lelli@gmail.com> <1333696481-3433-6-git-send-email-juri.lelli@gmail.com> <1335182113.28150.132.camel@twins> In-Reply-To: <1335182113.28150.132.camel@twins> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/23/2012 01:55 PM, Peter Zijlstra wrote: > On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote: >> +/* >> + * Here we check if --at time t-- an entity (which is probably being >> + * [re]activated or, in general, enqueued) can use its remaining runtime >> + * and its current deadline _without_ exceeding the bandwidth it is >> + * assigned (function returns true if it can). >> + * >> + * For this to hold, we must check if: >> + * runtime / (deadline - t)< dl_runtime / dl_deadline . > > It might be good to put a few words in as to why that is.. I know I > always forget (but know where to find it by now), also might be good to > refer those papers Tommaso listed when Steven asked this a while back. > Ok, I'll fix the comment, extend it and add T.'s references in the Documentation. >> + */ >> +static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t) >> +{ >> + u64 left, right; >> + >> + /* >> + * left and right are the two sides of the equation above, >> + * after a bit of shuffling to use multiplications instead >> + * of divisions. >> + * >> + * Note that none of the time values involved in the two >> + * multiplications are absolute: dl_deadline and dl_runtime >> + * are the relative deadline and the maximum runtime of each >> + * instance, runtime is the runtime left for the last instance >> + * and (deadline - t), since t is rq->clock, is the time left >> + * to the (absolute) deadline. Therefore, overflowing the u64 >> + * type is very unlikely to occur in both cases. >> + */ >> + left = dl_se->dl_deadline * dl_se->runtime; >> + right = (dl_se->deadline - t) * dl_se->dl_runtime; > > > From what I can see there are no constraints on the values in > __setparam_dl() so the above left term can be constructed to be an > overflow. > Yes, could happen :-\. > Ideally we'd use u128 here, but I don't think people will let us :/ > Do we need to do something about that? If we cannot go for bigger space probably limit dl_deadline (or warn the user).. >> + return dl_time_before(right, left); >> +}