Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation helper function

[Quentin Perret <quentin.perret@arm.com>]

On Wednesday 18 Apr 2018 at 17:23:16 (+0800), Leo Yan wrote:
> On Fri, Apr 06, 2018 at 04:36:05PM +0100, Dietmar Eggemann wrote:
> 
> [...]
> 
> > +/*
> > + * Estimates the system level energy assuming that p wakes-up on dst_cpu.
> > + *
> > + * compute_energy() is safe to call only if an energy model is available for
> > + * the platform, which is when sched_energy_enabled() is true.
> > + */
> > +static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
> > +{
> > +	unsigned long util, max_util, sum_util;
> > +	struct capacity_state *cs;
> > +	unsigned long energy = 0;
> > +	struct freq_domain *fd;
> > +	int cpu;
> > +
> > +	for_each_freq_domain(fd) {
> > +		max_util = sum_util = 0;
> > +		for_each_cpu_and(cpu, freq_domain_span(fd), cpu_online_mask) {
> > +			util = cpu_util_next(cpu, p, dst_cpu);
> > +			util += cpu_util_dl(cpu_rq(cpu));
> > +			max_util = max(util, max_util);
> > +			sum_util += util;
> > +		}
> > +
> > +		/*
> > +		 * Here we assume that the capacity states of CPUs belonging to
> > +		 * the same frequency domains are shared. Hence, we look at the
> > +		 * capacity state of the first CPU and re-use it for all.
> > +		 */
> > +		cpu = cpumask_first(freq_domain_span(fd));
> > +		cs = find_cap_state(cpu, max_util);
> > +		energy += cs->power * sum_util / cs->cap;
> 
> I am a bit worry about the resolution issue, especially when the
> capacity value is a quite high value and sum_util is a minor value.

Good point. As of now, the cs->power values happen to be in micro-watts
for the platforms we've been testing with, so they're typically high
enough to avoid significant resolution problem I guess.

Now, the energy model code has to be reworked significantly as we have
to remove the dependency on PM_OPP, so I'll try to make sure to keep
this issue in mind for the next version.

Thanks,
Quentin