Messages in this thread | | | Date | Tue, 2 Oct 2018 13:51:17 +0100 | From | Quentin Perret <> | Subject | Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework |
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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
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