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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 4A8E7C07E9B for ; Wed, 7 Jul 2021 09:48:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 32B6561A13 for ; Wed, 7 Jul 2021 09:48:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231320AbhGGJux (ORCPT ); Wed, 7 Jul 2021 05:50:53 -0400 Received: from foss.arm.com ([217.140.110.172]:33092 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230498AbhGGJuv (ORCPT ); Wed, 7 Jul 2021 05:50:51 -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 12945ED1; Wed, 7 Jul 2021 02:48:11 -0700 (PDT) Received: from [10.57.1.129] (unknown [10.57.1.129]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 35E7F3F694; Wed, 7 Jul 2021 02:48:08 -0700 (PDT) Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy To: Vincent Guittot Cc: linux-kernel , Chris Redpath , Dietmar Eggemann , Morten Rasmussen , Quentin Perret , "open list:THERMAL" , Peter Zijlstra , "Rafael J. Wysocki" , Viresh Kumar , Ingo Molnar , Juri Lelli , Steven Rostedt , segall@google.com, Mel Gorman , Daniel Bristot de Oliveira , CCj.Yeh@mediatek.com References: <20210625152603.25960-1-lukasz.luba@arm.com> <20210625152603.25960-2-lukasz.luba@arm.com> <2f43b211-da86-9d48-4e41-1c63359865bb@arm.com> From: Lukasz Luba Message-ID: <297df159-1681-f0a7-843d-f34d86e51d4c@arm.com> Date: Wed, 7 Jul 2021 10:48:06 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/7/21 10:37 AM, Vincent Guittot wrote: > On Wed, 7 Jul 2021 at 10:23, Lukasz Luba wrote: >> >> >> >> On 7/7/21 9:00 AM, Vincent Guittot wrote: >>> On Wed, 7 Jul 2021 at 09:49, Lukasz Luba wrote: >>>> >>>> >>>> >>>> On 7/7/21 8:07 AM, Vincent Guittot wrote: >>>>> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba wrote: >>>>>> >>>>>> The Energy Aware Scheduler (EAS) tries to find best CPU for a waking up >>>>>> task. It probes many possibilities and compares the estimated energy values >>>>>> for different scenarios. For calculating those energy values it relies on >>>>>> Energy Model (EM) data and em_cpu_energy(). The precision which is used in >>>>>> EM data is in milli-Watts (or abstract scale), which sometimes is not >>>>>> sufficient. In some cases it might happen that two CPUs from different >>>>>> Performance Domains (PDs) get the same calculated value for a given task >>>>>> placement, but in more precised scale, they might differ. This rounding >>>>>> error has to be addressed. This patch prepares EAS code for better >>>>>> precision in the coming EM improvements. >>>>> >>>>> Could you explain why 32bits results are not enough and you need to >>>>> move to 64bits ? >>>>> >>>>> Right now the result is in the range [0..2^32[ mW. If you need more >>>>> precision and you want to return uW instead, you will have a result in >>>>> the range [0..4kW[ which seems to be still enough >>>>> >>>> >>>> Currently we have the max value limit for 'power' in EM which is >>>> EM_MAX_POWER 0xffff (64k - 1). We allow to register such big power >>>> values ~64k mW (~64Watts) for an OPP. Then based on 'power' we >>>> pre-calculate 'cost' fields: >>>> cost[i] = power[i] * freq_max / freq[i] >>>> So, for max freq the cost == power. Let's use that in the example. >>>> >>>> Then the em_cpu_energy() calculates as follow: >>>> cost * sum_util / scale_cpu >>>> We are interested in the first part - the value of multiplication. >>> >>> But all these are internal computations of the energy model. At the >>> end, the computed energy that is returned by compute_energy() and >>> em_cpu_energy(), fits in a long >> >> Let's take a look at existing *10000 precision for x CPUs: >> cost * sum_util / scale_cpu = >> (64k *10000) * (x * 800) / 1024 >> which is: >> x * ~500mln >> >> So to be close to overflowing u32 the 'x' has to be > (?=) 8 >> (depends on sum_util). > > Sorry but I don't get your point. > This patch is about the return type of compute_energy() and > em_cpu_energy(). And even if we decide to return uW instead of mW, > there is still a lot of margin. > > It's not because you need u64 for computing intermediate value that > you must returns u64 The example above shows the need of u64 return value for platforms which are: - 32bit - have e.g. 16 CPUs - has big power value e.g. ~64k mW Then let's to the calc: (64k * 10000) * (16 * 800) / 1024 = ~8000mln = ~8bln The returned value after applying the whole patch set won't fit in u32 for such cluster. We might make *assumption* that the 32bit platforms will not have bigger number of CPUs in the cluster or won't report big power values. But I didn't wanted to make such assumption.