From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D9EEFC10DCE for ; Wed, 18 Mar 2020 14:38:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B807E20773 for ; Wed, 18 Mar 2020 14:38:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726984AbgCROi3 (ORCPT ); Wed, 18 Mar 2020 10:38:29 -0400 Received: from foss.arm.com ([217.140.110.172]:50832 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726631AbgCROi3 (ORCPT ); Wed, 18 Mar 2020 10:38:29 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2FF8331B; Wed, 18 Mar 2020 07:38:28 -0700 (PDT) Received: from e123083-lin (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 45E2D3F52E; Wed, 18 Mar 2020 07:38:26 -0700 (PDT) Date: Wed, 18 Mar 2020 15:38:23 +0100 From: Morten Rasmussen To: Daniel Lezcano Cc: Vincent Guittot , Peter Zijlstra , Ingo Molnar , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , linux-kernel , Qais Yousef , Valentin Schneider Subject: Re: [PATCH V2] sched: fair: Use the earliest break even Message-ID: <20200318143823.GC6103@e123083-lin> References: <20200311202625.13629-1-daniel.lezcano@linaro.org> <20200317075607.GE10914@e105550-lin.cambridge.arm.com> <3520b762-08f5-0db8-30cb-372709188bb9@linaro.org> <20200317143053.GF10914@e105550-lin.cambridge.arm.com> <7cd04d35-3522-30fb-82e9-82fdf53d0957@linaro.org> <20200318082452.GA6103@e123083-lin> <798e9bde-a207-3a0e-0f13-0e27d60fd286@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <798e9bde-a207-3a0e-0f13-0e27d60fd286@linaro.org> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 18, 2020 at 11:17:49AM +0100, Daniel Lezcano wrote: > On 18/03/2020 09:24, Morten Rasmussen wrote: > > On Tue, Mar 17, 2020 at 06:07:43PM +0100, Daniel Lezcano wrote: > >> On 17/03/2020 15:30, Morten Rasmussen wrote: > >>> On Tue, Mar 17, 2020 at 02:48:51PM +0100, Daniel Lezcano wrote: > >>>> On 17/03/2020 08:56, Morten Rasmussen wrote: > >>>>> On Thu, Mar 12, 2020 at 11:04:19AM +0100, Daniel Lezcano wrote: > >>>>>>>> In order to be more energy efficient but without impacting the > >>>>>>>> performances, let's use another criteria: the break even deadline. > >>>>>>>> > >>>>>>>> At idle time, when we store the idle state the CPU is entering in, we > >>>>>>>> compute the next deadline where the CPU could be woken up without > >>>>>>>> spending more energy to sleep. > >>>>> > >>>>> I don't follow the argument that sleeping longer should improve energy > >>>>> consumption. > >>>> > >>>> May be it is not explained correctly. > >>>> > >>>> The patch is about selecting a CPU with the smallest break even deadline > >>>> value. In a group of idle CPUs in the same idle state, we will pick the > >>>> one with the smallest break even dead line which is the one with the > >>>> highest probability it already reached its target residency. > >>>> > >>>> It is best effort. > >>> > >>> Indeed. I get what the patch does, I just don't see how the patch > >>> improves energy efficiency. > >> > >> If the CPU is woken up before it reached the break even, the idle state > >> cost in energy is greater than the energy it saved. > >> > >> Am I misunderstanding your point? > > > > Considering just the waking then yes, it reaches energy break-even. > > However, considering all the CPUs in the system, it just moves the idle > > entry/exit energy cost to a different CPU, it doesn't go away. > > > > Whether you have: > > > > |-BE-| > > ____ ____ > > CPU0: ___/ \__/ \___ > > > > CPU1: ____________________ > > > > Or: > > |-BE-| > > ____ > > CPU0: ___/ \___________ > > ____ > > CPU1: ___________/ \___ > > > > _ > > = CPU busy = P_{busy} > > _ = CPU idle = P_{idle} > > / = CPU idle exit = P_{exit} > > \ = CPU idle entry = P_{entry} > > > > The sum of areas under the curves is the same, i.e. the total energy is > > unchanged. > > It is a counter-intuitive comment, now I get it, thanks for the > clarification. It is a good point. No problem. > Taking into consideration the dynamic, in the case #1, the break even is > not reached, the idle duration is smaller and that leads the governor to > choose shallower idle states after and consequently CPU0 will be used in > priority. We end up with CPU0 in a shallow state and CPU1 in a deep state. Indeed. I was speculating earlier if the opposite could happen too. If we extended the second case to form a repeating pattern, could we prevent somehow prevent CPU1 from reaching a deeper state? Could we have pattern that would keep both CPUs in shallow state where it would have been more efficient to consolidate the wake-ups on CPU0 and let CPU1 enter deeper states? > > With the case #2, we can have the CPUs in both deep state and the > governor should be keeping choosing the same idle state. Ideally yes. However it depends on the break-even times of the deeper states and when the next wake-ups happen. > I don't know what is more energy/perf efficient. IMO this is very > workload dependent. The only way to check is to test. Hopefully I can > find a platform for that. Moving the wake-up shouldn't impact energy directly, although it have a positive latency impact as you are more likely to avoid waking up CPUs that haven't finished the idle entry sequence. However, changing the wake-up pattern could have an indirect energy impact, positive or negative. It isn't clear to me either what outcome to expect. Morten