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 B4C05C07E9C for ; Wed, 7 Jul 2021 08:24:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 999C961CB0 for ; Wed, 7 Jul 2021 08:24:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230181AbhGGI0k (ORCPT ); Wed, 7 Jul 2021 04:26:40 -0400 Received: from foss.arm.com ([217.140.110.172]:59560 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230086AbhGGI0j (ORCPT ); Wed, 7 Jul 2021 04:26:39 -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 15C98ED1; Wed, 7 Jul 2021 01:23:59 -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 3CA3B3F694; Wed, 7 Jul 2021 01:23:56 -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: <2f43b211-da86-9d48-4e41-1c63359865bb@arm.com> Date: Wed, 7 Jul 2021 09:23:54 +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 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). > >> >> 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.