From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity Date: Tue, 15 Dec 2015 13:41:44 +0100 Message-ID: <20151215124144.GD6357@twins.programming.kicks-ass.net> References: <1449641971-20827-1-git-send-email-smuckle@linaro.org> <1449641971-20827-10-git-send-email-smuckle@linaro.org> <20151214151729.GQ6357@twins.programming.kicks-ass.net> <20151214165128.GU6357@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from bombadil.infradead.org ([198.137.202.9]:56936 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753685AbbLOMlu (ORCPT ); Tue, 15 Dec 2015 07:41:50 -0500 Content-Disposition: inline In-Reply-To: Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Vincent Guittot Cc: Steve Muckle , Ingo Molnar , linux-kernel , "linux-pm@vger.kernel.org" , Morten Rasmussen , Dietmar Eggemann , Juri Lelli , Patrick Bellasi , Michael Turquette , Luca Abeni On Tue, Dec 15, 2015 at 05:43:44AM +0100, Vincent Guittot wrote: > On 14 December 2015 at 17:51, Peter Zijlstra wrote: > > No, since the WCET can and _will_ happen, its the best you can do with > > cpufreq. If you were to set it lower you could not be able to execute > > correctly in your 'never' tail cases. > > In the context of frequency scaling, This mean that we will never > reach low frequency Only if you've stuffed your machine full of deadline tasks, if you take Luca's example of the I/B frame decoder thingy, then even the WCET for the I frames should not be very much (albeit significantly more than B frames). So while the WCET is pessimistic compared to the avg case, most CPUs can do video decoding without much effort at all, so even the WCET for the I-frames might allow us to drop to the lowest cpufreq. Now, if you were to decode 10 streams at the same time, different story of course ;-)