linux-kernel - Re: [PATCH ARM64]: Introduce CONFIG

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Date:   Mon, 27 Nov 2023 11:58:22 -0800 (PST)
From:   "Christoph Lameter (Ampere)" <cl@...ux.com>
To:     Catalin Marinas <catalin.marinas@....com>
cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anshuman.Khandual@....com, Valentin.Schneider@....com,
        Vanshidhar Konda <vanshikonda@...amperecomputing.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Robin Murphy <robin.murphy@....com>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        Matteo Carlini <Matteo.Carlini@....com>
Subject: Re: [PATCH ARM64]: Introduce CONFIG_MAXSMP to allow up to 512 cpus

On Thu, 23 Nov 2023, Catalin Marinas wrote:

>> +config NR_CPUS_RANGE_BEGIN
>> +	int
>> +	default NR_CPUS_RANGE_END if MAXSMP
>> +	default    1 if !SMP
>> +	default    2
>
> We don't support !SMP on arm64.

Ok we can drop that.

>> +	  This is purely to save memory: each supported CPU adds about 8KB
>> +	  to the kernel image.
>
> Is this all needed just to select CPUMASK_OFFSTACK if larger NR_CPUS?
> Would something like this do:
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7b071a00425d..697d5700bad1 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -119,6 +119,7 @@ config ARM64
> 	select CLONE_BACKWARDS
> 	select COMMON_CLK
> 	select CPU_PM if (SUSPEND || CPU_IDLE)
> +	select CPUMASK_OFFSTACK if NR_CPUS > 512
> 	select CRC32
> 	select DCACHE_WORD_ACCESS
> 	select DYNAMIC_FTRACE if FUNCTION_TRACER
>
> togehther with a larger NR_CPUS in defconfig?

Well that is certainly better because it does not introduce an additional 
kernel config option.