linux-kernel - Re: [RFC PATCH 1/5] use nr_cpus= to set nr_cpu

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Date:	Tue, 12 Jan 2010 12:57:15 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Yinghai Lu <yinghai@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"ananth@...ibm.com" <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] use nr_cpus= to set nr_cpu_ids early

On Tue, 12 Jan 2010, Christoph Lameter wrote:
>
> Trouble is that NR_CPUS is used all over the place. If nr_cpu_ids < 
> NR_CPUS at boot then there is a danger of for loops to NR_CPUS going out 
> of bounds.
> 
> CONFIG_NR_CPUS is used for various bitmaps so that seems to be okay.
> 
> drivers/acpi/numa.c:acpi_numa_init probably should use nr_cpu_ids 
> instead now.
> 
> There is an octeon driver in staging that has some issues with NR_CPUS 
> as well.

I suspect that this is the real reason for the current behavior of 
'maxcpus=', and that if all of those issues get fixed we could probably 
make maxcpus do what Yinghai's new 'nr_cpus=' does.

So in a perfect world, CONFIG_NR_CPUS's would never really be used, except 
for some fundamental static allocations/limits that are too painful to try 
to make dynamic.

I doubt that anybody really _cares_ about the "you can add them later" 
behavior of the current maxcpus thing, and I suspect that the nr_cpus 
semantics is what people generally would have expected.

			Linus
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