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Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1g7L2p-00062D-HC; Tue, 02 Oct 2018 13:48:59 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 2D877202549E7; Tue, 2 Oct 2018 15:48:57 +0200 (CEST) Date: Tue, 2 Oct 2018 15:48:57 +0200 From: Peter Zijlstra To: Quentin Perret 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: <20181002134857.GE26858@hirez.programming.kicks-ass.net> References: <20180912091309.7551-1-quentin.perret@arm.com> <20180912091309.7551-4-quentin.perret@arm.com> <20181002123031.GZ3439@hirez.programming.kicks-ass.net> <20181002125115.245r3ocusvyiexno@queper01-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181002125115.245r3ocusvyiexno@queper01-lin> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 02, 2018 at 01:51:17PM +0100, Quentin Perret wrote: > 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. +/** + * em_cpu_get() - Return the performance domain for a CPU + * @cpu : CPU to find the performance domain for + * + * Return: the performance domain to which 'cpu' belongs, or NULL if it doesn't + * exist. + */ +struct em_perf_domain *em_cpu_get(int cpu) +{ + return READ_ONCE(per_cpu(em_data, cpu)); +} +EXPORT_SYMBOL_GPL(em_cpu_get); But your read side doesn't take, not is required to take em_pd_mutex. At that point, the mutex_unlock() doesn't guarantee anything. A CPU observing the em_data store, doesn't need to observe the store that filled the data structure it points to.