Re: [RFC][PATCH 1/2] ARM64: add cpu topology definition

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Jul 29, 2013 at 10:54:01AM +0100, Will Deacon wrote:
> On Mon, Jul 29, 2013 at 10:46:06AM +0100, Vincent Guittot wrote:
> > On 27 July 2013 12:42, Hanjun Guo <hanjun.guo@linaro.org> wrote:
> > > Power aware scheduling needs the cpu topology information to improve the
> > > cpu scheduler decision making.
> > 
> > It's not only power aware scheduling. The scheduler already uses
> > topology and cache sharing when  CONFIG_SCHED_MC and/or
> > CONFIG_SCHED_SMT are enable. So you should also add these configs for
> > arm64 so the scheduler can use it
> 
> ... except that the architecture doesn't define what the AFF fields in MPIDR
> really represent. Using them to make key scheduling decisions relating to
> cache proximity seems pretty risky to me, especially given the track record
> we've seen already on AArch32 silicon. It's a convenient register if it
> contains the data we want it to contain, but we need to force ourselves to
> come to terms with reality here and simply use it as an identifier for a
> CPU.
> 
> Can't we just use the device-tree to represent this topological data for
> arm64? Lorenzo has been working on bindings in this area.

Catching up on email after holiday - I agree with Will here, we should
use DT for representing the topology (or ACPI) and not rely on the MPIDR
value.

-- 
Catalin