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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 B1550C65C22 for ; Fri, 2 Nov 2018 15:36:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 872C92081B for ; Fri, 2 Nov 2018 15:36:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 872C92081B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728104AbeKCAoQ (ORCPT ); Fri, 2 Nov 2018 20:44:16 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:43134 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727848AbeKCAoP (ORCPT ); Fri, 2 Nov 2018 20:44:15 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 1FC8D1596; Fri, 2 Nov 2018 08:36:47 -0700 (PDT) Received: from [0.0.0.0] (e107985-lin.cambridge.arm.com [10.1.194.38]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0E6693F718; Fri, 2 Nov 2018 08:36:40 -0700 (PDT) Subject: Re: [PATCH v5 2/2] sched/fair: update scale invariance of PELT To: Vincent Guittot , peterz@infradead.org, mingo@kernel.org, linux-kernel@vger.kernel.org Cc: rjw@rjwysocki.net, Morten.Rasmussen@arm.com, patrick.bellasi@arm.com, pjt@google.com, bsegall@google.com, thara.gopinath@linaro.org, pkondeti@codeaurora.org References: <1540570303-6097-1-git-send-email-vincent.guittot@linaro.org> <1540570303-6097-3-git-send-email-vincent.guittot@linaro.org> From: Dietmar Eggemann Message-ID: Date: Fri, 2 Nov 2018 16:36:35 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <1540570303-6097-3-git-send-email-vincent.guittot@linaro.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/26/18 6:11 PM, Vincent Guittot wrote: > The current implementation of load tracking invariance scales the > contribution with current frequency and uarch performance (only for > utilization) of the CPU. One main result of this formula is that the > figures are capped by current capacity of CPU. Another one is that the > load_avg is not invariant because not scaled with uarch. > > The util_avg of a periodic task that runs r time slots every p time slots > varies in the range : > > U * (1-y^r)/(1-y^p) * y^i < Utilization < U * (1-y^r)/(1-y^p) > > with U is the max util_avg value = SCHED_CAPACITY_SCALE > > At a lower capacity, the range becomes: > > U * C * (1-y^r')/(1-y^p) * y^i' < Utilization < U * C * (1-y^r')/(1-y^p) > > with C reflecting the compute capacity ratio between current capacity and > max capacity. > > so C tries to compensate changes in (1-y^r') but it can't be accurate. > > Instead of scaling the contribution value of PELT algo, we should scale the > running time. The PELT signal aims to track the amount of computation of > tasks and/or rq so it seems more correct to scale the running time to > reflect the effective amount of computation done since the last update. > > In order to be fully invariant, we need to apply the same amount of > running time and idle time whatever the current capacity. Because running > at lower capacity implies that the task will run longer, we have to ensure > that the same amount of idle time will be apply when system becomes idle > and no idle time has been "stolen". But reaching the maximum utilization > value (SCHED_CAPACITY_SCALE) means that the task is seen as an > always-running task whatever the capacity of the CPU (even at max compute > capacity). In this case, we can discard this "stolen" idle times which > becomes meaningless. > > In order to achieve this time scaling, a new clock_pelt is created per rq. > The increase of this clock scales with current capacity when something > is running on rq and synchronizes with clock_task when rq is idle. With > this mecanism, we ensure the same running and idle time whatever the > current capacity. Thinking about this new approach on a big.LITTLE platform: CPU Capacities big: 1024 LITTLE: 512, performance CPUfreq governor A 50% (runtime/period) task on a big CPU will become an always running task on the little CPU. The utilization signal of the task and the cfs_rq of the little CPU converges to 1024. With contrib scaling the utilization signal of the 50% task converges to 512 on the little CPU, even it is always running on it, and so does the one of the cfs_rq. Two 25% tasks on a big CPU will become two 50% tasks on a little CPU. The utilization signal of the tasks converges to 512 and the one of the cfs_rq of the little CPU converges to 1024. With contrib scaling the utilization signal of the 25% tasks converges to 256 on the little CPU, even they run each 50% on it, and the one of the cfs_rq converges to 512. So what do we consider system-wide invariance? I thought that e.g. a 25% task should have a utilization value of 256 no matter on which CPU it is running? In both cases, the little CPU is not going idle whereas the big CPU does.