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References: <20201020173409.1266576-1-vanshikonda@os.amperecomputing.com>
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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
Cc:     linux-arm-kernel@lists.infradead.org, patches@amperecomputing.com,
        linux-kernel@vger.kernel.org, Anshuman.Khandual@arm.com
Subject: Re: [PATCH] arm64: NUMA: Kconfig: Increase max number of nodes
In-reply-to: <20201020173409.1266576-1-vanshikonda@os.amperecomputing.com>
Date:   Tue, 20 Oct 2020 19:09:36 +0100
Message-ID: <jhj7drkrcpr.mognet@arm.com>
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Hi,

Nit on the subject: this only increases the default, the max is still 2¹⁰.

On 20/10/20 18:34, Vanshidhar Konda wrote:
> The current arm64 max NUMA nodes default to 4. Today's arm64 systems can
> reach or exceed 16. Increase the number to 64 (matching x86_64).
>
> Signed-off-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 893130ce1626..3e69d3c981be 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -980,7 +980,7 @@ config NUMA
>  config NODES_SHIFT
>       int "Maximum NUMA Nodes (as a power of 2)"
>       range 1 10
> -	default "2"
> +	default "6"

This leads to more statically allocated memory for things like node to CPU
maps (see uses of MAX_NUMNODES), but that shouldn't be too much of an
issue.

AIUI this also directly correlates to how many more page->flags bits are
required: are we sure the max 10 works on any aarch64 platform? I'm
genuinely asking here, given that I'm mostly a stranger to the mm
world. The default should be something we're somewhat confident works
everywhere.

>       depends on NEED_MULTIPLE_NODES
>       help
>         Specify the maximum number of NUMA Nodes available on the target