Re: [RFC][PATCH v0.1 3/6] PM: EM: Add special case to em_dev_register_perf_domain()

[Rafael J. Wysocki]

On Mon, Nov 18, 2024 at 4:25 PM Hongyan Xia <hongyan.xia2@xxxxxxx> wrote:
>
> On 08/11/2024 16:38, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > Allow em_dev_register_perf_domain() to register a cost-only stub
> > perf domain with one-element states table if the .active_power()
> > callback is not provided.
> >
> > Subsequently, this will be used by the intel_pstate driver to register
> > stub perf domains for CPUs on hybrid systems.
> >
> > No intentional functional impact.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> >   kernel/power/energy_model.c |   26 +++++++++++++++++++++++---
> >   1 file changed, 23 insertions(+), 3 deletions(-)
> >
> > Index: linux-pm/kernel/power/energy_model.c
> > ===================================================================
> > --- linux-pm.orig/kernel/power/energy_model.c
> > +++ linux-pm/kernel/power/energy_model.c
> > @@ -426,9 +426,11 @@ static int em_create_pd(struct device *d
> >       if (!em_table)
> >               goto free_pd;
> >
> > -     ret = em_create_perf_table(dev, pd, em_table->state, cb, flags);
> > -     if (ret)
> > -             goto free_pd_table;
> > +     if (cb->active_power) {
> > +             ret = em_create_perf_table(dev, pd, em_table->state, cb, flags);
> > +             if (ret)
> > +                     goto free_pd_table;
> > +     }
> >
> >       ret = em_compute_costs(dev, em_table->state, cb, nr_states, flags);
> >       if (ret)
> > @@ -561,11 +563,20 @@ int em_dev_register_perf_domain(struct d
> >   {
> >       unsigned long cap, prev_cap = 0;
> >       unsigned long flags = 0;
> > +     bool stub_pd = false;
> >       int cpu, ret;
> >
> >       if (!dev || !nr_states || !cb)
> >               return -EINVAL;
> >
> > +     if (!cb->active_power) {
> > +             if (!cb->get_cost || nr_states > 1 || microwatts)
> > +                     return -EINVAL;
> > +
> > +             /* Special case: a stub perf domain. */
> > +             stub_pd = true;
> > +     }
> > +
>
> I wonder if the only purpose of stub_pd is to just skip the capacity
> check below, which doesn't look very nice.

It is.

I guess I could just skip it if nr_states == 1 because that case means
the same cost for all frequency values.

>
> I may be echoing Dietmar's comments here. What's the problem of just
> having 3 domains?

The energy-efficiency of a CPU is not strictly related to its capacity.

It's about the cases when there are some special CPUs that can
turbo-up higher, but there's no other difference between them and the
other CPUs in the domain.

> Or, could you just specify the same capacities so that the same-capacity
> check won't fail, but just to use hardware load or CPU pressure to model
> the slight difference in real capacities? This way you'd re-use a lot of
> existing infrastructure.

That would have been confusing though, so thanks, but no thanks.

> >       /*
> >        * Use a mutex to serialize the registration of performance domains and
> >        * let the driver-defined callback functions sleep.
> > @@ -590,6 +601,15 @@ int em_dev_register_perf_domain(struct d
> >                               ret = -EEXIST;
> >                               goto unlock;
> >                       }
> > +
> > +                     /*
> > +                      * The capacity need not be the same for all CPUs in a
> > +                      * stub perf domain, so long as the average cost of
> > +                      * running on each of them is approximately the same.
> > +                      */
> > +                     if (stub_pd)
> > +                             continue;
> > +
> >                       /*
> >                        * All CPUs of a domain must have the same
> >                        * micro-architecture since they all share the same
> >
> >
> >
>
>