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=-2.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 44D61C31E51 for ; Tue, 18 Jun 2019 09:02:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1E6D52080A for ; Tue, 18 Jun 2019 09:02:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729178AbfFRJCp (ORCPT ); Tue, 18 Jun 2019 05:02:45 -0400 Received: from foss.arm.com ([217.140.110.172]:58088 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728754AbfFRJCp (ORCPT ); Tue, 18 Jun 2019 05:02:45 -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 64697360; Tue, 18 Jun 2019 02:02:44 -0700 (PDT) Received: from queper01-lin (queper01-lin.cambridge.arm.com [10.1.195.48]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7BA453F246; Tue, 18 Jun 2019 02:02:43 -0700 (PDT) Date: Tue, 18 Jun 2019 10:02:42 +0100 From: Quentin Perret To: "Rafael J. Wysocki" Cc: Viresh Kumar , Peter Zijlstra , Ingo Molnar , Linux Kernel Mailing List , Vincent Guittot Subject: Re: [PATCH] sched/fair: Introduce fits_capacity() Message-ID: <20190618090239.a4prlefzpc27gwve@queper01-lin> References: <20190605091644.w3g7hc7r3eiscz4f@queper01-lin> <20190606025204.qe5v7j6fysjkgxc6@vireshk-i7> <20190617150204.GG3436@hirez.programming.kicks-ass.net> <20190618031217.63md32da5pzydqia@vireshk-i7> <20190618074728.gf6wugkbndhhcqql@vireshk-i7> <20190618082553.tonhmthyy6mo3iui@queper01-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20171215 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tuesday 18 Jun 2019 at 10:36:56 (+0200), Rafael J. Wysocki wrote: > On Tue, Jun 18, 2019 at 10:25 AM Quentin Perret wrote: > > > > On Tuesday 18 Jun 2019 at 10:10:48 (+0200), Rafael J. Wysocki wrote: > > > On Tue, Jun 18, 2019 at 9:47 AM Viresh Kumar wrote: > > > > > > > > On 18-06-19, 09:26, Rafael J. Wysocki wrote: > > > > > On Tue, Jun 18, 2019 at 5:12 AM Viresh Kumar wrote: > > > > > > > > > > > > +Rafael > > > > > > > > > > > > On 17-06-19, 17:02, Peter Zijlstra wrote: > > > > > > > On Thu, Jun 06, 2019 at 08:22:04AM +0530, Viresh Kumar wrote: > > > > > > > > Hmm, even if the values are same currently I am not sure if we want > > > > > > > > the same for ever. I will write a patch for it though, if Peter/Rafael > > > > > > > > feel the same as you. > > > > > > > > > > > > > > Is it really the same variable or just two numbers that happen to be the > > > > > > > same? > > > > > > > > > > > > In both cases we are trying to keep the load under 80% of what can be supported. > > > > > > But I am not sure of the answer to your question. > > > > > > > > > > > > Maybe Rafael knows :) > > > > > > > > > > Which variable? > > > > > > > > Schedutil multiplies the target frequency by 1.25 (20% more capacity eventually) > > > > to get enough room for more load and similar thing is done in fair.c at several > > > > places to see if the new task can fit in a runqueue without overloading it. > > > > > > For the schedutil part, see the changelog of the commit that introduced it: > > > > > > 9bdcb44e391d cpufreq: schedutil: New governor based on scheduler > > > utilization data > > > > > > As for the other places, I don't know about the exact reasoning. > > > > > > > Quentin suggested to use common code for this calculation and that is what is > > > > getting discussed here. > > > > > > I guess if the rationale for the formula is the same in all cases, it > > > would be good to consolidate that code and document the rationale > > > while at it. > > > > I _think_ it is, but I guess others could correct me if this is > > incorrect. > > > > When choosing a CPU or a frequency using a util value, we look for a > > capacity that will provide us with 20% of idle time. And in both case we > > use the same threshold, just hardcoded in different places. Hence the > > suggestion to unify things. > > > > I hope that makes sense :-) > > Well, for schedutil, the 1.25 value comes from the case when > utilization is not frequency-invariant the next-frequency formula is > recursive (the next frequency is proportional to the current one). It > is chosen to get the new frequency equal to the old one if (util / > max) is .8. That translates to the "capacity that will provide 20% > more of idle time" in the frequency-invariant utilization case, but > the original rationale was different. OK, thanks, I wasn't aware of this. I understood it the other way around, but re-reading the commit message you shared earlier this makes sense. I guess it is also worth mentioning that, to the best of my knowledge, the vast majority of real-world sugov users are in fact frequency invariant. So perhaps there is still a case for the code factorization suggested earlier. But in the end it is really just a cleanup to help maintainability, so if you guys don't buy in there is no point pushing further :-) Thanks, Quentin