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

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Date:	Mon, 29 Jul 2013 14:36:30 +0100
From:	Dave Martin <Dave.Martin@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Graeme Gregory <graeme.gregory@...aro.org>,
	Al Stone <al.stone@...aro.org>,
	Patch Tracking <patches@...aro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	linaro-acpi <linaro-acpi@...ts.linaro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tomasz Nowicki <tomasz.nowicki@...aro.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Naresh Bhat <naresh.bhat@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>
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

In fact, the ARM Architecture doesn't place any requirements on MPIDRs to
force the aff fields to exist _at all_.  It's just a recommendation.
Instead, you have a 24 or 32-bit number which is unique per CPU, and which
is _probably_ assigned in a way resembling the aff fields.

> 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.

+1

Also, we should align arm and arm64.  The problem is basically exactly
the same, and the solution needs to be the same.  struct cputopo_arm is
already being abused  -- for example, TC2 describes the A15 and A7
clusters on a single die as having different "socket_id" values, even
though this is obviously nonsense.  But there's no other way to describe
that system today.

> Can't we just use the device-tree to represent this topological data for
> arm64? Lorenzo has been working on bindings in this area.

This may become more important as we start to see things like asymmetric
topologies appearing (different numbers of nodes and different
interdependence characteristics in adjacent branches of the topology
etc.)

Cheers
---Dave
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