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=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,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 7100AC282C6 for ; Thu, 24 Jan 2019 16:00:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 498C121872 for ; Thu, 24 Jan 2019 16:00:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728764AbfAXQAP (ORCPT ); Thu, 24 Jan 2019 11:00:15 -0500 Received: from foss.arm.com ([217.140.101.70]:59710 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727848AbfAXQAP (ORCPT ); Thu, 24 Jan 2019 11:00:15 -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 E975BEBD; Thu, 24 Jan 2019 08:00:13 -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 0183B3F5AF; Thu, 24 Jan 2019 08:00:10 -0800 (PST) Date: Thu, 24 Jan 2019 16:00:08 +0000 From: Patrick Bellasi To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, linux-api@vger.kernel.org, Ingo Molnar , Tejun Heo , "Rafael J . Wysocki" , Vincent Guittot , Viresh Kumar , Paul Turner , Quentin Perret , Dietmar Eggemann , Morten Rasmussen , Juri Lelli , Todd Kjos , Joel Fernandes , Steve Muckle , Suren Baghdasaryan Subject: Re: [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks Message-ID: <20190124160008.x43eucgtyc62w3l3@e110439-lin> References: <20190115101513.2822-1-patrick.bellasi@arm.com> <20190115101513.2822-10-patrick.bellasi@arm.com> <20190123104944.GX27931@hirez.programming.kicks-ass.net> <20190123144011.iid3avb63r5v4r2c@e110439-lin> <20190123201146.GH17749@hirez.programming.kicks-ass.net> <20190124123009.2yulcf25ld66popd@e110439-lin> <20190124123835.nliwk2onvrhe5aqf@e110439-lin> <20190124151212.GO13777@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190124151212.GO13777@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24-Jan 16:12, Peter Zijlstra wrote: > On Thu, Jan 24, 2019 at 12:38:35PM +0000, Patrick Bellasi wrote: > > On 24-Jan 12:30, Patrick Bellasi wrote: > > > On 23-Jan 21:11, Peter Zijlstra wrote: > > > > On Wed, Jan 23, 2019 at 02:40:11PM +0000, Patrick Bellasi wrote: > > > > > On 23-Jan 11:49, Peter Zijlstra wrote: > > > > > > On Tue, Jan 15, 2019 at 10:15:06AM +0000, Patrick Bellasi wrote: > > > > [...] > > > > > > I'm thikning that if we haz a single bit, say: > > > > > > > > struct uclamp_se { > > > > ... > > > > unsigned int changed : 1; > > > > }; > > > > > > > > We can update uclamp_se::value and set uclamp_se::changed, and then the > > > > next enqueue will (unlikely) test-and-clear changed and recompute the > > > > bucket_id. > > > > > > This mean will lazy update the "requested" bucket_id by deferring its > > > computation at enqueue time. Which saves us a copy of the bucket_id, > > > i.e. we will have only the "effective" value updated at enqueue time. > > > > > > But... > > > > > > > Would that not be simpler? > > > > > > ... although being simpler it does not fully exploit the slow-path, > > > a syscall which is usually running from a different process context > > > (system management software). > > > > > > It also fits better for lazy updates but, in the cgroup case, where we > > > wanna enforce an update ASAP for RUNNABLE tasks, we will still have to > > > do the updates from the slow-path. > > > > > > Will look better into this simplification while working on v7, perhaps > > > the linear mapping can really help in that too. > > > > Actually, I forgot to mention that: > > > > uclamp_se::effective::{ > > value, bucket_id > > } > > > > will be still required to proper support the cgroup delegation model, > > where a child group could be restricted by the parent but we want to > > keep track of the original "requested" value for when the parent > > should relax the restriction. > > > > Thus, since effective values are already there, why not using them > > also to pre-compute the new requested bucket_id from the slow path? > > Well, we need the orig_value; but I'm still not sure why you need more > bucket_id's. Also, retaining orig_value is already required for the > system limits, there's nothing cgroup-y about this afaict. Sure, the "effective" values are just a very convenient way (IMHO) to know exactly which value/bucket_id is currently in use by a task while keeping them well separated from the "requested" values. So, you propose to add "orig_value"... but the end effect will be the same... it's just that if we look at uclamp_se you have two dual information: A) whatever a task or cgroup "request" is always in: uclamp_se::value uclamp_se::bucket_id B) whatever a task or cgroup "gets" is always in: uclamp_se::effective::value uclamp_se::effective::bucket_id I find this partitioning useful and easy to use: 1) the slow-path updates only data in A 2) the fast-path updates only data in B by composing A data in uclamp_effective_get() @enqueue time. -- #include Patrick Bellasi