Re: [PATCH v5 12/14] sched/fair: Select an energy-efficient CPU on task wake-up

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

On Thursday 02 Aug 2018 at 15:54:26 (+0200), Peter Zijlstra wrote:
> On Tue, Jul 24, 2018 at 01:25:19PM +0100, Quentin Perret wrote:
> > @@ -6385,18 +6492,26 @@ static int
> >  select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
> >  {
> >  	struct sched_domain *tmp, *sd = NULL;
> > +	struct freq_domain *fd;
> >  	int cpu = smp_processor_id();
> >  	int new_cpu = prev_cpu;
> > -	int want_affine = 0;
> > +	int want_affine = 0, want_energy = 0;
> >  	int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> >  
> > +	rcu_read_lock();
> >  	if (sd_flag & SD_BALANCE_WAKE) {
> >  		record_wakee(p);
> > -		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> > -			      && cpumask_test_cpu(cpu, &p->cpus_allowed);
> > +		fd = rd_freq_domain(cpu_rq(cpu)->rd);
> > +		want_energy = fd && !READ_ONCE(cpu_rq(cpu)->rd->overutilized);
> > +		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) &&
> > +			      cpumask_test_cpu(cpu, &p->cpus_allowed);
> > +	}
> > +
> > +	if (want_energy) {
> > +		new_cpu = find_energy_efficient_cpu(p, prev_cpu, fd);
> > +		goto unlock;
> >  	}
> >  
> 
> And I suppose you rely on the compiler to optimize that for the static
> key inside rd_freq_domain()... Does it do a good job of that?

I does for sure when CONFIG_ENERGY_MODEL=n since rd_freq_domain() is
stubbed to false, but that's an easy one ;-)

> 
> That is, would not something like:
> 
> 
> 	rcu_read_lock();
> 	if (sd_flag & SD_BALANCE_WAKE) {
> 		record_wakee(p);
> 
> 		if (static_branch_unlikely(&sched_energy_present)) {
> 			struct root_domain *rd = cpu_rq(cpu)->rd;
> 			struct freq_domain *fd = rd_freq_domain(rd);
> 
> 			if (fd && !READ_ONCE(rd->overutilized)) {
> 				new_cpu = find_energy_efficient_cpu(p, prev_cpu, fd);
> 				goto unlock;
> 			}
> 		}
> 
> 		/* ... */
> 	}
> 
> 
> Be far more clear ?

It is clearer. Having the static key check in rd_freq_domain() makes
the change to find_busiest_group() smaller, but I can totally change it
with something like the above.

I'll do that in v6.

Thanks,
Quentin