linux-kernel - Re: [PATCH 09/10] percpu: implement new dynamic percpu allocator

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Date:	Thu, 19 Feb 2009 12:01:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, rusty@...tcorp.com.au,
	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, jeremy@...p.org, cpw@....com
Subject: Re: [PATCH 09/10] percpu: implement new dynamic percpu allocator


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> > + * To use this allocator, arch code should do the followings.
> > + *
> > + * - define CONFIG_HAVE_DYNAMIC_PER_CPU_AREA
> > + *
> > + * - define __addr_to_pcpu_ptr() and __pcpu_ptr_to_addr() to translate
> > + *   regular address to percpu pointer and back
> > + *
> > + * - use pcpu_setup_static() during percpu area initialization to
> > + *   setup kernel static percpu area
> > + */
> 
> afacit nobody has answered your "is num_possible_cpus() ever a 
> lot larger than num_online_cpus()" question.
> 
> It is fairly important.

yeah.

On x86 we limit num_possible_cpus() at boot time from NR_CPUS to 
the BIOS-enumerated set of possible CPUs - i.e. the two will 
always be either equal, or be very close to each other.

( there used to be broken early BIOSes that enumerated more CPUs 
  than needed but it's very rare and because it also wastes BIOS 
  RAM/ROM it's something they'll usually avoid even if they dont 
  care about Linux. )

So this should be a pretty OK assumption.

	Ingo
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