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 0D995C07E95 for ; Wed, 7 Jul 2021 09:45:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DA09961C82 for ; Wed, 7 Jul 2021 09:45:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231175AbhGGJrp (ORCPT ); Wed, 7 Jul 2021 05:47:45 -0400 Received: from foss.arm.com ([217.140.110.172]:33016 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230498AbhGGJro (ORCPT ); Wed, 7 Jul 2021 05:47:44 -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 8CEDAED1; Wed, 7 Jul 2021 02:45:04 -0700 (PDT) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 04D773F694; Wed, 7 Jul 2021 02:45:01 -0700 (PDT) Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy To: Lukasz Luba , Vincent Guittot Cc: linux-kernel , Chris Redpath , 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: Dietmar Eggemann Message-ID: <9b0ea7bc-934a-43bd-7dd8-9fe33dec97bc@arm.com> Date: Wed, 7 Jul 2021 11:45:00 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <2f43b211-da86-9d48-4e41-1c63359865bb@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org On 07/07/2021 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: [...] >>>> 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). I assume the worst case is `x * 1024` (max return value of effective_cpu_util = effective_cpu_util()) so x ~ 6.7. I'm not aware of any arm32 b.L. systems with > 4 CPUs in a PD.