Re: [RFCv7 PATCH 01/10] sched: Compute cpu capacity available at current frequency

[Rafael J. Wysocki]

On Tuesday, February 23, 2016 10:19:16 AM Peter Zijlstra wrote:
> On Tue, Feb 23, 2016 at 02:41:20AM +0100, Rafael J. Wysocki wrote:
> > >  /*
> > > + * Returns the current capacity of cpu after applying both
> > > + * cpu and freq scaling.
> > > + */
> > > +static unsigned long capacity_curr_of(int cpu)
> > > +{
> > > +       return cpu_rq(cpu)->cpu_capacity_orig *
> > > +              arch_scale_freq_capacity(NULL, cpu)
> > 
> > What about architectures that don't have this?
> 
> They get the 'default' which is a constant SCHED_CAPACITY_SCALE unit.
> 
> > Why is that an architecture feature?
> 
> Because not all archs can tell the frequency the same way. Some you
> program the DVFS state and they really run at this speed, for those you
> can simply report back.
> 
> For others, x86 for example, you program a DVFS 'hint' and the hardware
> does whatever, we'd have to do APERF/MPERF samples to get an idea of the
> actual frequency we ran at.
> 
> Also, the having of this makes the load tracking slightly more
> expensive, instead of compile time constants we get function calls and
> actual multiplications. Its not _too_ bad, but still.

That's all correct, but my question should rather be: is arch the right
granularity?

In theory, there may be ARM64-based platforms using ACPI and behaving
like x86 in that respect in the future.

Thanks,
Rafael