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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 6D6F2C433E0 for ; Fri, 26 Jun 2020 23:17:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4DB552073E for ; Fri, 26 Jun 2020 23:17:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726415AbgFZXRZ (ORCPT ); Fri, 26 Jun 2020 19:17:25 -0400 Received: from foss.arm.com ([217.140.110.172]:35008 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725883AbgFZXRY (ORCPT ); Fri, 26 Jun 2020 19:17:24 -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 0098830E; Fri, 26 Jun 2020 16:17:24 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 526713F73C; Fri, 26 Jun 2020 16:17:22 -0700 (PDT) References: <20200625154352.24767-1-qais.yousef@arm.com> <20200625154352.24767-2-qais.yousef@arm.com> <87eeq2nh1k.derkling@matbug.net> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Patrick Bellasi Cc: Qais Yousef , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Chris Redpath , Lukasz Luba , linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 1/2] sched/uclamp: Fix initialization of struct uclamp_rq In-reply-to: <87eeq2nh1k.derkling@matbug.net> Date: Sat, 27 Jun 2020 00:17:20 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26/06/20 13:32, Patrick Bellasi wrote: > On Thu, Jun 25, 2020 at 17:43:51 +0200, Qais Yousef wrote... > >> struct uclamp_rq was zeroed out entirely in assumption that in the first >> call to uclamp_rq_inc() they'd be initialized correctly in accordance to >> default settings. > > Perhaps I was not clear in my previous comment: > > https://lore.kernel.org/lkml/87sgekorfq.derkling@matbug.net/ > > when I did say: > > Does not this means the problem is more likely with > uclamp_rq_util_with(), which should be guarded? > > I did not mean that we have to guard the calls to that function but > instead that we should just make that function aware of uclamp being > opted in or not. > >> But when next patch introduces a static key to skip >> uclamp_rq_{inc,dec}() until userspace opts in to use uclamp, schedutil >> will fail to perform any frequency changes because the >> rq->uclamp[UCLAMP_MAX].value is zeroed at init and stays as such. Which >> means all rqs are capped to 0 by default. > > The initialization you wants to do here it's needed because with the > current approach you keep calling the same uclamp_rq_util_with() and > keep doing min/max aggregations even when uclamp is not opted in. > But this means also that we have min/max aggregation _when not really > required_. > >> Fix it by making sure we do proper initialization at init without >> relying on uclamp_rq_inc() doing it later. > > My proposal was as simple as: > > ---8<--- > static __always_inline > unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util, > struct task_struct *p) > { > unsigned long min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value); > unsigned long max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value); > > + if (!static_branch_unlikely(&sched_uclamp_used)) > + return rt_task(p) ? uclamp_none(UCLAMP_MAX) : util > > if (p) { > min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN)); > max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX)); > } > > /* > * Since CPU's {min,max}_util clamps are MAX aggregated considering > * RUNNABLE tasks with _different_ clamps, we can end up with an > * inversion. Fix it now when the clamps are applied. > */ > if (unlikely(min_util >= max_util)) > return min_util; > > return clamp(util, min_util, max_util); > } > ---8<--- > > Such small change is more self-contained IMHO and does not remove > an existing optimizations like this lazy RQ's initialization at first > usage. > > Moreover, it can folded in the following patch, with all the other > static keys shortcuts. I'd have to think some more over it, but I like this in that we wouldn't have to molest schedutil anymore.