Re: [RFCv3 PATCH 33/48] sched: Energy-aware wake-up task placement

[Morten Rasmussen]

On Tue, Mar 24, 2015 at 01:00:58PM +0000, Peter Zijlstra wrote:
> On Wed, Feb 04, 2015 at 06:31:10PM +0000, Morten Rasmussen wrote:
> > @@ -5138,6 +5224,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> >  		prev_cpu = cpu;
> >  
> >  	if (sd_flag & SD_BALANCE_WAKE) {
> > +		if (energy_aware()) {
> > +			new_cpu = energy_aware_wake_cpu(p);
> > +			goto unlock;
> > +		}
> >  		new_cpu = select_idle_sibling(p, prev_cpu);
> >  		goto unlock;
> >  	}
> 
> So that is fundamentally wrong I think. We only care about power aware
> scheduling when U < 1, after that we should do the normal thing. This
> setup does not allow for that.

Right, I agree that we should preferably do the normal thing for U ~= 1.
We can restructure the wake-up path to follow that pattern, but we need
to know U beforehand to choose the right path. U isn't just
get_cpu_usage(prev_cpu) but some broader view of the of the cpu
utilizations. For example, prev_cpu might be full, but everyone else is
idle so we still want to try to do an energy aware wake-up on some other
cpu. U could be the minium utilization of all cpus in prev_cpu's
sd_llc, which is somewhat similar to what energy_aware_wake_cpu() does.

I guess energy_aware_wake_cpu() could be refactored to call
select_idle_sibling() if it find U ~= 1?
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