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.per...@arm.com> 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
