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Date:   Mon, 4 Dec 2023 11:13:01 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Dmytro Maluka <dmaluka@chromium.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/thp: add CONFIG_TRANSPARENT_HUGEPAGE_NEVER option
Message-Id: <20231204111301.7e087b2f851b30121561e8fc@linux-foundation.org>
In-Reply-To: <20231204163254.2636289-1-dmaluka@chromium.org>
References: <20231204163254.2636289-1-dmaluka@chromium.org>
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On Mon,  4 Dec 2023 17:32:54 +0100 Dmytro Maluka <dmaluka@chromium.org> wrote:

> Add an option to disable transparent hugepages by default, in line with
> the existing transparent_hugepage=never command line setting.
> 
> Rationale: khugepaged has its own non-negligible memory cost even if it
> is not used by any applications, since it bumps up vm.min_free_kbytes to
> its own required minimum in set_recommended_min_free_kbytes(). For
> example, on a machine with 4GB RAM, with 3 mm zones and pageblock_order
> == MAX_ORDER, starting khugepaged causes vm.min_free_kbytes increase
> from 8MB to 132MB.
> 
> So if we use THP on machines with e.g. >=8GB of memory for better
> performance, but avoid using it on lower-memory machines to avoid its
> memory overhead, then for the same reason we also want to avoid even
> starting khugepaged on those <8GB machines. So with
> CONFIG_TRANSPARENT_HUGEPAGE_NEVER we can use the same kernel image on
> both >=8GB and <8GB machines, with THP support enabled but khugepaged
> not started by default. The userspace can then decide to enable THP
> (i.e. start khugepaged) via sysfs if needed, based on the total amount
> of memory.
> 
> This could also be achieved with the existing transparent_hugepage=never
> setting in the kernel command line instead. But it seems cleaner to
> avoid tweaking the command line for such a basic setting.
> 
> P.S. I see that CONFIG_TRANSPARENT_HUGEPAGE_NEVER was already proposed
> in the past [1] but without an explanation of the purpose.
> 
> ...
>
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -859,6 +859,12 @@ choice
>  	  madvise(MADV_HUGEPAGE) but it won't risk to increase the
>  	  memory footprint of applications without a guaranteed
>  	  benefit.
> +
> +	config TRANSPARENT_HUGEPAGE_NEVER
> +		bool "never"
> +	help
> +	  Disabling Transparent Hugepage by default. It can still be

s/Disabling/Disable/

> +	  enabled at runtime via sysfs.
>  endchoice

The patch adds the config option but doesn't use it?