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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 80AFAC43441 for ; Wed, 28 Nov 2018 14:40:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 51FB42086B for ; Wed, 28 Nov 2018 14:40:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 51FB42086B 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 S1728642AbeK2Bmi (ORCPT ); Wed, 28 Nov 2018 20:42:38 -0500 Received: from foss.arm.com ([217.140.101.70]:42050 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728443AbeK2Bmh (ORCPT ); Wed, 28 Nov 2018 20:42:37 -0500 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 2109B2D97; Wed, 28 Nov 2018 06:40:44 -0800 (PST) Received: from e110439-lin (e110439-lin.cambridge.arm.com [10.1.194.43]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DDD543F5AF; Wed, 28 Nov 2018 06:40:41 -0800 (PST) Date: Wed, 28 Nov 2018 14:40:39 +0000 From: Patrick Bellasi To: Vincent Guittot Cc: Peter Zijlstra , Ingo Molnar , linux-kernel , "Rafael J. Wysocki" , Dietmar Eggemann , Morten Rasmussen , Paul Turner , Ben Segall , Thara Gopinath , pkondeti@codeaurora.org, Quentin Perret , Srinivas Pandruvada Subject: Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT Message-ID: <20181128144039.GC23094@e110439-lin> References: <1542711308-25256-1-git-send-email-vincent.guittot@linaro.org> <1542711308-25256-3-git-send-email-vincent.guittot@linaro.org> <20181128100241.GA2131@hirez.programming.kicks-ass.net> <20181128115336.GB23094@e110439-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28-Nov 14:33, Vincent Guittot wrote: > On Wed, 28 Nov 2018 at 12:53, Patrick Bellasi wrote: > > > > On 28-Nov 11:02, Peter Zijlstra wrote: > > > On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote: > > > > > > > Is there anything else that I should do for these patches ? > > > > > > IIRC, Morten mention they break util_est; Patrick was going to explain. > > > > I guess the problem is that, once we cross the current capacity, > > strictly speaking util_avg does not represent anymore a utilization. > > > > With the new signal this could happen and we end up storing estimated > > utilization samples which will overestimate the task requirements. > > > > We will have a spike in estimated utilization at next wakeup, since we > > use MAX(util_avg@dequeue_time, ewma). Potentially we also inflate the EWMA in > > case we collect multiple samples above the current capacity. > > TBH I don't see how it's different from current implementation with a > task that was scheduled on big core and now wakes up on little core. > The util_est is overestimated as well. While running below the capacity of a CPU, either big or LITTLE, we can still measure the actual used bandwidth as long as we have idle time. If the task is then moved into a lower capacity core, I think it's still safe to assume that, likely, it would need more capacity. Why do you say it's the same ? With your new signal instead, once we cross the current capacity, utilization is just not anymore utilization. Thus, IMHO it make sense avoid to accumulate a sample for what we call "estimated utilization". I would also say that, with the current implementation which caps utilization to the current capacity, we get better estimation in general. At least we can say with absolute precision: "the task needs _at least_ that amount of capacity". Potentially we can also flag the task as being under-provisioned, in case there was not idle time, and _let a policy_ decide what to do with it and the granted information we have. While, with your new signal, once we are over the current capacity, the "utilization" is just a sort of "random" number at best useful to drive some conclusions about how long the task has been delayed. IOW, I fear that we are embedding a policy within a signal which is currently representing something very well defined: how much cpu bandwidth a task used. While, latency/under-provisioning policies perhaps should be better placed somewhere else. Perhaps I've missed it in some of the previous discussions: have we have considered/discussed this signal-vs-policy aspect ? -- #include Patrick Bellasi