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 88F72C07E9B for ; Wed, 7 Jul 2021 07:49:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6C91761C71 for ; Wed, 7 Jul 2021 07:49:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230475AbhGGHwb (ORCPT ); Wed, 7 Jul 2021 03:52:31 -0400 Received: from foss.arm.com ([217.140.110.172]:58998 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230453AbhGGHwa (ORCPT ); Wed, 7 Jul 2021 03:52:30 -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 DDC0EED1; Wed, 7 Jul 2021 00:49:50 -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 BA9DF3F694; Wed, 7 Jul 2021 00:49:47 -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> From: Lukasz Luba Message-ID: Date: Wed, 7 Jul 2021 08:49:46 +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 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. The sum_util values that we can see for x CPUs which have scale_cap=1024 can be close to 800, let's use it in the example: cost * sum_util = 64k * (x * 800), where x=4: ~200mln x=8: ~400mln x=16: ~800mln x=64: ~3200mln (last one which would fit in u32) When we increase the precision by even 100, then the above values won't fit in the u32. Even a max cost of e.g. 10k mW and 100 precision has issues: cost * sum_util = (10k *100) * (x * 800), where x=4: ~3200mln x=8: ~6400mln For *1000 precision even a power of 1Watt becomes an issue: cost * sum_util = (1k *1000) * (x * 800), where x=4: ~3200mln x=8: ~6400mln That's why to make the code safe for bigger power values, I had to use the u64 on 32bit machines.