linux-kernel - Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512

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Date: Mon, 18 Mar 2024 18:17:22 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Rutland <mark.rutland@....com>,
	"Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Will Deacon <will@...nel.org>, Jonathan.Cameron@...wei.com,
	Matteo.Carlini@....com, akpm@...ux-foundation.org,
	anshuman.khandual@....com, Eric Mackay <eric.mackay@...cle.com>,
	dave.kleikamp@...cle.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux@...linux.org.uk, robin.murphy@....com,
	vanshikonda@...amperecomputing.com, yang@...amperecomputing.com,
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase
 supported CPUs to 512

On Thu, Mar 07, 2024 at 07:07:07PM +0000, Catalin Marinas wrote:
> On Wed, 06 Mar 2024 17:45:04 -0800, Christoph Lameter (Ampere) wrote:
> > Currently defconfig selects NR_CPUS=256, but some vendors (e.g. Ampere
> > Computing) are planning to ship systems with 512 CPUs. So that all CPUs on
> > these systems can be used with defconfig, we'd like to bump NR_CPUS to 512.
> > Therefore this patch increases the default NR_CPUS from 256 to 512.
> > 
> > As increasing NR_CPUS will increase the size of cpumasks, there's a fear that
> > this might have a significant impact on stack usage due to code which places
> > cpumasks on the stack. To mitigate that concern, we can select
> > CPUMASK_OFFSTACK. As that doesn't seem to be a problem today with
> > NR_CPUS=256, we only select this when NR_CPUS > 256.
> > 
> > [...]
> 
> Applied to arm64 (for-next/misc), thanks!
> 
> I dropped the config entry and comment, replaced it with a select as per
> Mark's suggestion.
> 
> [1/1] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512
>       https://git.kernel.org/arm64/c/0499a78369ad

I re-instated this patch in arm64 for-next/core as:

https://git.kernel.org/arm64/c/3fbd56f0e7c1

-- 
Catalin