linux-kernel - Re: [RFC 3/4] ARM: topology: Update cpu

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Date:	Wed, 13 Jun 2012 18:06:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linaro-dev@...ts.linaro.org, devicetree-discuss@...ts.ozlabs.org,
	linux@....linux.org.uk, grant.likely@...retlab.ca,
	rob.herring@...xeda.com
Subject: Re: [RFC 3/4] ARM: topology: Update cpu_power according to DT
 information

On Wed, 2012-06-13 at 16:54 +0200, Vincent Guittot wrote:
> On 13 June 2012 15:07, Peter Zijlstra <peterz@...radead.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.
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