[PATCH REPOST v2] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512

[Russell King (Oracle)]

On Tue, Feb 06, 2024 at 10:09:59AM -0800, Christoph Lameter (Ampere) wrote:
> Can we get this merged for 6.9? The patch has been around for awhile now.
> 
> 
> 
> Ampere Computing develops high end ARM processor that support an ever
> increasing number of processors. The default 256 processors are
> not enough for our newer products. The default is used by
> distros and therefore our customers cannot use distro kernels because
> the number of processors is not supported.
> 
> One of the objections against earlier patches to increase the limit
> was that the memory use becomes too high. There is a feature called
> CPUMASK_OFFSTACK that configures the cpumasks in the kernel to be
> dynamically allocated. This was used in the X86 architecture in the
> past to enable support for larger CPU configurations up to 8k cpus.
> 
> With that is becomes possible to dynamically size the allocation of
> the cpu bitmaps depending on the quantity of processors detected on
> bootup.
> 
> This patch enables that logic if more than 256 processors
> are configured and increases the default to 512 processors.
> 
> Further increases may be needed if ARM processor vendors start
> supporting more processors. Given the current inflationary trends
> in core counts from multiple processor manufacturers this may occur.
> 
> Tested-by: Eric Mackay <eric.mackay at oracle.com>

Reviewed-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>

Thanks!

> Signed-off-by: Christoph Lameter (Ampere) <cl at linux.com>

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