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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 62E14C07E95 for ; Wed, 7 Jul 2021 08:00:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4304C61C7C for ; Wed, 7 Jul 2021 08:00:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230414AbhGGIDH (ORCPT ); Wed, 7 Jul 2021 04:03:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37768 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230398AbhGGIDG (ORCPT ); Wed, 7 Jul 2021 04:03:06 -0400 Received: from mail-lj1-x232.google.com (mail-lj1-x232.google.com [IPv6:2a00:1450:4864:20::232]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3F80BC061574 for ; Wed, 7 Jul 2021 01:00:26 -0700 (PDT) Received: by mail-lj1-x232.google.com with SMTP id q184so1059556ljq.0 for ; Wed, 07 Jul 2021 01:00:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=haGVx5xtTBF+VtzwL/lf0Kn8JJj/cDxNCUCB/6GBJpg=; b=Pbd+/aMolsKOQG+lwZY8mgF04Ayl5UHoly0ldhMkisOAGIhNt5qwhhQ/P3EcyhLNas glu4XAf1X2q8XsK5AbHYivdZfc+gEy8MtYEFkyxi8qrLiCPabCblemNaQ70uBOzShmjk KLnt3PM5wrzFmPQgsV6bbHaFX+wAi7ZVMxcYBeaXMr3BcBL0E79YtKSjySrOkOLqEYBc h0LJNdg+RN50S9Hyt1l9vbkwQdHemPeiluOoSUIzegZdDnaa9EN/htD46iUErWF+G1vr 7qBvUrJG/qtKQ516HgeWpx5tNgEU5nrsgg4dIYNVLh3X9Lxa10eDTbjIadSUFUvlQLzR Mdzg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=haGVx5xtTBF+VtzwL/lf0Kn8JJj/cDxNCUCB/6GBJpg=; b=fEb4Mo5AFPbkscVjUHICSPm0BXJceBa+0Sqmrcw5UL+Bfdj5dn0BWCOj2i4qFItzUf 2AJrkVUD3Nsm2XzcStbPtutjuEP1zlM7mDwG8jBhejtaMq6bf4z2N8Cgk9OsZXfNoxRe KEtxI6zsExDTLMSMalQdxxhZYpCnLmA8wYehNcA4+sBTlq+Pas4NlJQVKAU5DFp9/3re VluEF1FCcK29/qwiba0JIH5HPGm/dkqDvovdhrX8BgcuvGoNhweie5s123iCkSlxOSwc YiOYXuKB1KUGxNWt1KLif4G82qruxcrQXXQJByoWmW07rWk51OVjCFQBsOfpgF1SODMv FD6w== X-Gm-Message-State: AOAM533Rk39yMP0YFrxeqmG5jTWvTS4lh8BYLvn6jrf+tQay1nCqeaTP RhGWzpnNLB2PunXoN6D3hCH24aC4yne3zzMog0FirA== X-Google-Smtp-Source: ABdhPJz0lm5Be97IeqSsNKXdCQo7E3elpVDfj98ujSXkZ5RvypuKhI42qylz+ZcbGgwbPiBFBCxchCyO37/GwC3bkaY= X-Received: by 2002:a2e:3a05:: with SMTP id h5mr6279612lja.209.1625644824533; Wed, 07 Jul 2021 01:00:24 -0700 (PDT) MIME-Version: 1.0 References: <20210625152603.25960-1-lukasz.luba@arm.com> <20210625152603.25960-2-lukasz.luba@arm.com> In-Reply-To: From: Vincent Guittot Date: Wed, 7 Jul 2021 10:00:13 +0200 Message-ID: Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy To: Lukasz Luba 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org 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 > > 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.