Re: [RFC PATCH v4 03/12] PM: Introduce an Energy Model management framework

[Quentin Perret]

Hi Vincent,

On Friday 06 Jul 2018 at 11:57:37 (+0200), Vincent Guittot wrote:
> On Thu, 28 Jun 2018 at 13:41, Quentin Perret <quentin.perret@xxxxxxx> wrote:
> > +static inline unsigned long em_fd_energy(struct em_freq_domain *fd,
> > +                               unsigned long max_util, unsigned long sum_util)
> > +{
> > +       struct em_cs_table *cs_table;
> > +       struct em_cap_state *cs;
> > +       unsigned long freq;
> > +       int i;
> > +
> > +       cs_table = rcu_dereference(fd->cs_table);
> > +       if (!cs_table)
> > +               return 0;
> > +
> > +       /* Map the utilization value to a frequency */
> > +       cs = &cs_table->state[cs_table->nr_cap_states - 1];
> > +       freq = map_util_freq(max_util, cs->frequency, cs->capacity);
> 
> The 2 lines above deserve more explanation:
> 1st, you get the max capacity of the freq domain
> Then, you estimate what will be the selected frequency according to
> the max_utilization.
> Might worth to mention that we must keep sync how sched_util and EM
> select a freq for a given capacity which is the reason of patch 02

Agreed, this could benefit from more explanations. I'll comment that
better in the next version.

> 
> > +
> > +       /* Find the lowest capacity state above this frequency */
> > +       for (i = 0; i < cs_table->nr_cap_states; i++) {
> > +               cs = &cs_table->state[i];
> > +               if (cs->frequency >= freq)
> > +                       break;
> > +       }
> > +
> > +       return cs->power * sum_util / cs->capacity;
> 
> IIUC the formula above, you consider that all CPUs in a frequency
> domain has the same capacity. This sounds a reasonable assumption but
> it would be good to write that somewhere

That's correct, we agreed on the assumption that CPUs in the same freq
domain must have the same micro-arch, and consequently the same capacity.

But you're right, that needs at the very least to be documented. Or even
better, I should check that when the EM is created and bail out with an
error message if that's not the case. That shouldn't be too hard to
implement, I think.

Thanks,
Quentin