Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754057AbcCAOQH (ORCPT ); Tue, 1 Mar 2016 09:16:07 -0500 Received: from foss.arm.com ([217.140.101.70]:51434 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753513AbcCAOQE (ORCPT ); Tue, 1 Mar 2016 09:16:04 -0500 Date: Tue, 1 Mar 2016 14:17:06 +0000 From: Juri Lelli To: Peter Zijlstra Cc: Vincent Guittot , Steve Muckle , "Rafael J. Wysocki" , "Rafael J. Wysocki" , Linux PM list , Linux Kernel Mailing List , Srinivas Pandruvada , Viresh Kumar , Thomas Gleixner Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks Message-ID: <20160301141706.GJ18792@e106622-lin> References: <5387313.xAhVpzgZCg@vostro.rjw.lan> <56BA8C29.4090905@linaro.org> <20160211115959.GI6357@twins.programming.kicks-ass.net> <20160211122429.GM11415@e106622-lin> <20160211152625.GM6357@twins.programming.kicks-ass.net> <20160212140415.GS6357@twins.programming.kicks-ass.net> <20160301135811.GQ6356@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160301135811.GQ6356@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1507 Lines: 34 On 01/03/16 14:58, Peter Zijlstra wrote: > On Fri, Feb 12, 2016 at 03:48:54PM +0100, Vincent Guittot wrote: > > > Another point to take into account is that the RT tasks will "steal" > > the compute capacity that has been requested by the cfs tasks. > > > > Let takes the example of a CPU with 3 OPP on which run 2 rt tasks A > > and B and 1 cfs task C. > > > Let assume that the real time constraint of RT task A is too agressive > > for the lowest OPP0 and that the change of the frequency of the core > > is too slow compare to this constraint but the real time constraint of > > RT task B can be handle whatever the OPP. System don't have other > > choice than setting the cpufreq min freq to OPP1 to be sure that > > constraint of task A will be covered at anytime. > > > Then, we still have 2 > > possible OPPs. The CFS task asks for compute capacity that fits in > > OPP1 but a part of this capacity will be stolen by RT tasks. If we > > monitor the load of RT tasks and request capacity for these RT tasks > > according to their current utilization, we can decide to switch to > > highest OPP2 to ensure that task C will have enough remaining > > capacity. A lot of embedded platform faces such kind of use cases > > Still doesn't make sense. How would you know the constraint of RT task > A, and that it cannot be satisfied by OPP0 ? The only information you > have in the task model is a static priority. > But, can't we have the problem Vincent describes if we s/RT/DL/ ? Thanks, - Juri