Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework

Peter Zijlstra
2018-10-02T13:48:57Z

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.