Re: [RFC 3/4] ARM: topology: Update cpu_power according to DT information

Peter Zijlstra
2012-06-13T16:07:06Z

On Wed, 2012-06-13 at 16:54 +0200, Vincent Guittot wrote:
> On 13 June 2012 15:07, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Tue, 2012-06-12 at 14:02 +0200, Vincent Guittot wrote:
> >> +struct cpu_capacity cpu_capacity[NR_CPUS];
> >
> > I know ARM isn't likely to suffer from the 4k cpu issue, but is there a
> > reason to use a NR_CPUS array over a per-cpu variable?
>
> At this stage, we don't know which logical CPU will match which hwid.
> During the boot of each CPU, we parse the table to found an efficiency
> value for the booting CPU. The whole table is used by one CPU at a
> time.

Its not so much the usage as the dynamic sizing that I was after.
NR_CPUS will always be the max size, whereas per-cpu data will only use
the amount of storage required to back the number of cpus present.

For this reason we've spend a great deal of effort to remove NR_CPUS
sized arrays all over the core (and x86/ia64 arch) code, since distros
now build with NR_CPUS=4096 but hardly anybody has that many cpus, so
arrays sized that way waste tons of resources.