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_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 D21E7C43143 for ; Tue, 2 Oct 2018 12:51:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A367120684 for ; Tue, 2 Oct 2018 12:51:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A367120684 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 S1727822AbeJBTeg (ORCPT ); Tue, 2 Oct 2018 15:34:36 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:36642 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727707AbeJBTef (ORCPT ); Tue, 2 Oct 2018 15:34:35 -0400 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 7E4DA7A9; Tue, 2 Oct 2018 05:51:23 -0700 (PDT) Received: from queper01-lin (queper01-lin.emea.arm.com [10.4.13.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 607C13F5B7; Tue, 2 Oct 2018 05:51:19 -0700 (PDT) Date: Tue, 2 Oct 2018 13:51:17 +0100 From: Quentin Perret To: Peter Zijlstra Cc: rjw@rjwysocki.net, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, gregkh@linuxfoundation.org, mingo@redhat.com, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, chris.redpath@arm.com, patrick.bellasi@arm.com, valentin.schneider@arm.com, vincent.guittot@linaro.org, thara.gopinath@linaro.org, viresh.kumar@linaro.org, tkjos@google.com, joel@joelfernandes.org, smuckle@google.com, adharmap@codeaurora.org, skannan@codeaurora.org, pkondeti@codeaurora.org, juri.lelli@redhat.com, edubezval@gmail.com, srinivas.pandruvada@linux.intel.com, currojerez@riseup.net, javi.merino@kernel.org Subject: Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework Message-ID: <20181002125115.245r3ocusvyiexno@queper01-lin> References: <20180912091309.7551-1-quentin.perret@arm.com> <20180912091309.7551-4-quentin.perret@arm.com> <20181002123031.GZ3439@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181002123031.GZ3439@hirez.programming.kicks-ass.net> 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 02 Oct 2018 at 14:30:31 (+0200), Peter Zijlstra wrote: > On Wed, Sep 12, 2018 at 10:12:58AM +0100, Quentin Perret wrote: > > +/** > > + * em_register_perf_domain() - Register the Energy Model of a performance domain > > + * @span : Mask of CPUs in the performance domain > > + * @nr_states : Number of capacity states to register > > + * @cb : Callback functions providing the data of the Energy Model > > + * > > + * Create Energy Model tables for a performance domain using the callbacks > > + * defined in cb. > > + * > > + * If multiple clients register the same performance domain, all but the first > > + * registration will be ignored. > > + * > > + * Return 0 on success > > + */ > > +int em_register_perf_domain(cpumask_t *span, unsigned int nr_states, > > + struct em_data_callback *cb) > > +{ > > + unsigned long cap, prev_cap = 0; > > + struct em_perf_domain *pd; > > + int cpu, ret = 0; > > + > > + if (!span || !nr_states || !cb) > > + return -EINVAL; > > + > > + /* > > + * Use a mutex to serialize the registration of performance domains and > > + * let the driver-defined callback functions sleep. > > + */ > > + mutex_lock(&em_pd_mutex); > > + > > + for_each_cpu(cpu, span) { > > + /* Make sure we don't register again an existing domain. */ > > + if (READ_ONCE(per_cpu(em_data, cpu))) { > > + ret = -EEXIST; > > + goto unlock; > > + } > > + > > + /* > > + * All CPUs of a domain must have the same micro-architecture > > + * since they all share the same table. > > + */ > > + cap = arch_scale_cpu_capacity(NULL, cpu); > > + if (prev_cap && prev_cap != cap) { > > + pr_err("CPUs of %*pbl must have the same capacity\n", > > + cpumask_pr_args(span)); > > + ret = -EINVAL; > > + goto unlock; > > + } > > + prev_cap = cap; > > + } > > + > > + /* Create the performance domain and add it to the Energy Model. */ > > + pd = em_create_pd(span, nr_states, cb); > > + if (!pd) { > > + ret = -EINVAL; > > + goto unlock; > > + } > > + > > + for_each_cpu(cpu, span) > > + WRITE_ONCE(per_cpu(em_data, cpu), pd); > > It's not immediately obvious to me why this doesn't need to be > smp_store_release(). The moment you publish that pointer, it can be > read, right? > > Even if you never again change the pointer value, you want to ensure the > content of pd is stable before pd itself is observable, right? So, I figured the mutex already gives me some of that. I mean, AFAIU it should guarantee that concurrent callers to em_register_perf_domain are serialized correctly. For example, if I have two concurrent calls (let's name them A and B) to em_register_perf_domain(), and say A takes the mutex first, then B should be guaranteed to always see the totality of the update that A made to the per_cpu table. Is that right ? If the above is correct, then it's pretty much all I can do, I think ... In the case of concurrent readers and writers to em_data, the smp_store_release() call still doesn't give me the guarantee that the per_cpu table is stable since em_cpu_get() is lock-free ... If I want to be sure the per_cpu thing is stable from em_cpu_get() then I can add a mutex_lock/unlock there too, but even then I won't need the smp_store_release(), I think. Or maybe I got confused again ? Thanks, Quentin