Re: [RFC PATCH v2 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

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

Hi Leo,

On Tuesday 17 Apr 2018 at 23:39:44 (+0800), Leo Yan wrote:
> > +	for_each_freq_domain(fd) {
> > +		unsigned long spare_cap, max_spare_cap = 0;
> > +		int max_spare_cap_cpu = -1;
> > +		unsigned long util;
> > +
> > +		/* Find the CPU with the max spare cap in the freq. dom. */
> > +		for_each_cpu_and(cpu, freq_domain_span(fd), sched_domain_span(sd)) {
> > +			if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> > +				continue;
> > +
> > +			if (cpu == prev_cpu)
> > +				continue;
> > +
> > +			util = cpu_util_wake(cpu, p);
> > +			cpu_cap = capacity_of(cpu);
> > +			if (!util_fits_capacity(util + task_util, cpu_cap))
> > +				continue;
> > +
> > +			spare_cap = cpu_cap - util;
> > +			if (spare_cap > max_spare_cap) {
> > +				max_spare_cap = spare_cap;
> > +				max_spare_cap_cpu = cpu;
> > +			}
> > +		}
> 
> If have two clusters, and if firstly iterate the big cluster, then
> max_spare_cap is a big value for big cluster and later LITTLE cluster
> has no chance to have higher value for spare_cap.  For this case, the
> LITTLE CPU will be skipped for energy computation?

max_spare_cap is reset to 0 at the top of the for_each_freq_domain()
loop above so that shouldn't happen.

Thanks,
Quentin