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

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Date:	Wed, 14 Aug 2013 12:27:20 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Patch Tracking <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linaro-acpi <linaro-acpi@...ts.linaro.org>,
	Al Stone <al.stone@...aro.org>,
	Graeme Gregory <graeme.gregory@...aro.org>,
	Naresh Bhat <naresh.bhat@...aro.org>,
	Tomasz Nowicki <tomasz.nowicki@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>
Subject: Re: [RFC][PATCH 1/2] ARM64: add cpu topology definition

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@...aro.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
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