On Wed, 7 Nov 2018 at 18:02, Quentin Perret <quentin.per...@arm.com> wrote:
>
> Hi Vincent,
>
> On Wednesday 07 Nov 2018 at 17:32:32 (+0100), Vincent Guittot wrote:
> > Hi Quentin,
> >
> > On Tue, 16 Oct 2018 at 12:15, Quentin Perret <quentin.per...@arm.com> wrote:
> > >
> >
> > > +
> > > +/**
> > > + * em_pd_energy() - Estimates the energy consumed by the CPUs of a perf.
> > > domain
> > > + * @pd : performance domain for which energy has to be estimated
> > > + * @max_util : highest utilization among CPUs of the domain
> > > + * @sum_util : sum of the utilization of all CPUs in the domain
> > > + *
> > > + * Return: the sum of the energy consumed by the CPUs of the domain
> > > assuming
> > > + * a capacity state satisfying the max utilization of the domain.
> > > + */
> > > +static inline unsigned long em_pd_energy(struct em_perf_domain *pd,
> > > + unsigned long max_util, unsigned long
> > > sum_util)
> > > +{
> > > + unsigned long freq, scale_cpu;
> > > + struct em_cap_state *cs;
> > > + int i, cpu;
> > > +
> > > + /*
> > > + * In order to predict the capacity state, map the utilization of
> > > the
> > > + * most utilized CPU of the performance domain to a requested
> > > frequency,
> > > + * like schedutil.
> > > + */
> > > + cpu = cpumask_first(to_cpumask(pd->cpus));
> > > + scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> > > + cs = &pd->table[pd->nr_cap_states - 1];
> > > + freq = map_util_freq(max_util, cs->frequency, scale_cpu);
> > > +
> > > + /*
> > > + * Find the lowest capacity state of the Energy Model above the
> > > + * requested frequency.
> > > + */
> > > + for (i = 0; i < pd->nr_cap_states; i++) {
> > > + cs = &pd->table[i];
> > > + if (cs->frequency >= freq)
> > > + break;
> > > + }
> > > +
> > > + /*
> > > + * The capacity of a CPU in the domain at that capacity state (cs)
> > > + * can be computed as:
> > > + *
> > > + * cs->freq * scale_cpu
> > > + * cs->cap = -------------------- (1)
> > > + * cpu_max_freq
> > > + *
> > > + * So, ignoring the costs of idle states (which are not available
> > > in
> > > + * the EM), the energy consumed by this CPU at that capacity
> > > state is
> > > + * estimated as:
> > > + *
> > > + * cs->power * cpu_util
> > > + * cpu_nrg = -------------------- (2)
> > > + * cs->cap
> > > + *
> > > + * since 'cpu_util / cs->cap' represents its percentage of busy
> > > time.
> > > + *
> > > + * NOTE: Although the result of this computation actually is in
> > > + * units of power, it can be manipulated as an energy
> > > value
> > > + * over a scheduling period, since it is assumed to be
> > > + * constant during that interval.
> > > + *
> > > + * By injecting (1) in (2), 'cpu_nrg' can be re-expressed as a
> > > product
> > > + * of two terms:
> > > + *
> > > + * cs->power * cpu_max_freq cpu_util
> > > + * cpu_nrg = ------------------------ * --------- (3)
> > > + * cs->freq scale_cpu
> > > + *
> > > + * The first term is static, and is stored in the em_cap_state
> > > struct
> > > + * as 'cs->cost'.
> > > + *
> > > + * Since all CPUs of the domain have the same micro-architecture,
> > > they
> > > + * share the same 'cs->cost', and the same CPU capacity. Hence,
> > > the
> > > + * total energy of the domain (which is the simple sum of the
> > > energy of
> > > + * all of its CPUs) can be factorized as:
> > > + *
> > > + * cs->cost * \Sum cpu_util
> > > + * pd_nrg = ------------------------ (4)
> > > + * scale_cpu
> > > + */
> > > + return cs->cost * sum_util / scale_cpu;
> >
> > Why do you need to keep scale_cpu outside the cs->cost ? do you expect
> > arch_scale_cpu_capacity() to change at runtime ?
>
> Unfortunately yes, it can. It'll change at least during boot on arm64,
> for example (see drivers/base/arch_topology.c). And also, userspace can
> actually set that value via sysfs ...
yes. I had this in mind too but we are also rebuilding sched_domain in
this case and thought that everything could be changed at the same
time
>
> > If the returned value of arch_scale_cpu_capacity() changes, we will
> > have to rebuild several others things and we can include the update of
> > cs->cost
>
> Yeah, that was the original approach I had actually. Some of the older
> versions of this patch set were doing just that. The only issue is that,
> in order to make the cs->cost updatable are run time, you need to
> introduce some level of protection around that data structure (RCU or
> something). And that would make it a bit harder for IPA (for example) to
> access the data -- it doesn't need any kind of RCU to access it's EM at
> the moment.
>
> We can probably do something a bit smarter and introduce RCU protection
> only for the 'cost' field or something, but I was hoping that we could
> keep things simple for now and do that kind of small optimization a bit
> later :-)
Thanks for the explanation
>
> Thanks,
> Quentin
