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Date:   Mon, 27 Nov 2023 11:58:22 -0800 (PST)
From:   "Christoph Lameter (Ampere)" <cl@linux.com>
To:     Catalin Marinas <catalin.marinas@arm.com>
cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Anshuman.Khandual@arm.com, Valentin.Schneider@arm.com,
        Vanshidhar Konda <vanshikonda@os.amperecomputing.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Dave Kleikamp <dave.kleikamp@oracle.com>,
        Matteo Carlini <Matteo.Carlini@arm.com>
Subject: Re: [PATCH ARM64]: Introduce CONFIG_MAXSMP to allow up to 512 cpus
In-Reply-To: <ZV-o9ORrq7OKWOry@arm.com>
Message-ID: <a32a30d9-f352-cecd-c6f9-16b25963fac1@linux.com>
References: <6a854175-5f89-c754-17b8-deda18447f1f@gentwo.org> <ZV-o9ORrq7OKWOry@arm.com>
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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.