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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 D823FC388F7 for ; Fri, 13 Nov 2020 12:54:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D5202224B for ; Fri, 13 Nov 2020 12:54:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726456AbgKMMyi (ORCPT ); Fri, 13 Nov 2020 07:54:38 -0500 Received: from mail.kernel.org ([198.145.29.99]:58992 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726176AbgKMMyi (ORCPT ); Fri, 13 Nov 2020 07:54:38 -0500 Received: from gaia (unknown [2.26.170.190]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6713E206D9; Fri, 13 Nov 2020 12:54:36 +0000 (UTC) Date: Fri, 13 Nov 2020 12:54:33 +0000 From: Catalin Marinas To: Ionela Voinescu Cc: mark.rutland@arm.com, sudeep.holla@arm.com, will@kernel.org, morten.rasmussen@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 3/3] arm64: implement CPPC FFH support using AMUs Message-ID: <20201113125432.GE3212@gaia> References: <20201106125334.21570-1-ionela.voinescu@arm.com> <20201106125334.21570-4-ionela.voinescu@arm.com> <20201112180045.GO29613@gaia> <20201113122846.GA31212@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201113122846.GA31212@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 13, 2020 at 12:28:46PM +0000, Ionela Voinescu wrote: > On Thursday 12 Nov 2020 at 18:00:46 (+0000), Catalin Marinas wrote: > > On Fri, Nov 06, 2020 at 12:53:34PM +0000, Ionela Voinescu wrote: > > > +static inline > > > +int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val) > > > +{ > > > + if (!cpu_has_amu_feat(cpu)) > > > + return -EOPNOTSUPP; > > > + > > > + smp_call_function_single(cpu, func, val, 1); > > > + > > > + return 0; > > > +} > > > > I got lost in the cpufreq call chains. Can this function ever be called > > with interrupts disabled? > > The short answer is: not with the current implementation of its only > user, the cppc_cpufreq driver (given the current cpufreq implementation). > > The long answer is: there is a case where the cpufreq .get function is > called with local interrupts disabled - cpufreq_quick_get(), but there > are a few "if"s in between this becoming a problem: > > 1. If cppc_cpufreq ever implements .setpolicy or, > 1.1 Current implementation of cpufreq_quick_get() changes. > 2. If one of the few users of cpufreq_quick_get() is used: cppc_cpufreq > ends up being used as CPU cooling device(+IPA governor) or > devfreq/tegra30 is used. > > In this potential case, smp_call_function_single() will warn us of call > with irqs_disable() and if the stars align there could be a potential > deadlock if two CPUs try to IPI (get counter reads of) each other. > > So it could be called with irqs disabled, but a few code changes are > needed before that happens. > > I can bail out of counters_read_on_cpu() if irqs_disabled() to be on the > safe side. Thanks for the explanation. In case we forget two years from now and the core code changes, I think it's safe to bail out early with an error. You can add a patch on top of this series, no need to resend the whole. -- Catalin