linux-kernel - Re: [PATCH v2] mm: Override mTHP "enabled" defaults at kernel cmdline

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Message-ID: <99b78488-c524-4269-b1fb-a13eaa4f274c@redhat.com>
Date: Fri, 9 Aug 2024 11:19:13 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>,
 Barry Song <baohua@...nel.org>, Lance Yang <ioworker0@...il.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: Override mTHP "enabled" defaults at kernel cmdline

On 08.08.24 12:16, Ryan Roberts wrote:
> Add thp_anon= cmdline parameter to allow specifying the default
> enablement of each supported anon THP size. The parameter accepts the
> following format and can be provided multiple times to configure each
> size:
> 
> thp_anon=<size>[KMG]:<value>
> 
> See Documentation/admin-guide/mm/transhuge.rst for more details.
> 
> Configuring the defaults at boot time is useful to allow early user
> space to take advantage of mTHP before its been configured through
> sysfs.

I suspect a khugeapged enhancement and/or kernel-config-dependant 
defaults and/or early system settings will also be able to mitigate that 
without getting kernel cmdlines involved in the future.

> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
> 
> Hi All,
> 
> I've split this off from my RFC at [1] because Barry highlighted that he would
> benefit from it immediately [2]. There are no changes vs the version in that
> series.
> 
> It applies against today's mm-unstable (275d686abcb59). (although I had to fix a
> minor build bug in stackdepot.c due to MIN() not being defined in this tree).
> 
> Thanks,
> Ryan
> 
> 
>   .../admin-guide/kernel-parameters.txt         |  8 +++
>   Documentation/admin-guide/mm/transhuge.rst    | 26 +++++++--
>   mm/huge_memory.c                              | 55 ++++++++++++++++++-
>   3 files changed, 82 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index bcdee8984e1f0..5c79b58c108ec 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6631,6 +6631,14 @@
>   			<deci-seconds>: poll all this frequency
>   			0: no polling (default)
> 
> +	thp_anon=	[KNL]
> +			Format: <size>[KMG]:always|madvise|never|inherit
> +			Can be used to control the default behavior of the
> +			system with respect to anonymous transparent hugepages.
> +			Can be used multiple times for multiple anon THP sizes.
> +			See Documentation/admin-guide/mm/transhuge.rst for more
> +			details.
> +
>   	threadirqs	[KNL,EARLY]
>   			Force threading of all interrupt handlers except those
>   			marked explicitly IRQF_NO_THREAD.
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 24eec1c03ad88..f63b0717366c6 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -284,13 +284,27 @@ that THP is shared. Exceeding the number would block the collapse::
> 
>   A higher value may increase memory footprint for some workloads.
> 
> -Boot parameter
> -==============
> +Boot parameters
> +===============
> 
> -You can change the sysfs boot time defaults of Transparent Hugepage
> -Support by passing the parameter ``transparent_hugepage=always`` or
> -``transparent_hugepage=madvise`` or ``transparent_hugepage=never``
> -to the kernel command line.
> +You can change the sysfs boot time default for the top-level "enabled"
> +control by passing the parameter ``transparent_hugepage=always`` or
> +``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the
> +kernel command line.
> +
> +Alternatively, each supported anonymous THP size can be controlled by
> +passing ``thp_anon=<size>[KMG]:<state>``, where ``<size>`` is the THP size
> +and ``<state>`` is one of ``always``, ``madvise``, ``never`` or
> +``inherit``.
> +
> +For example, the following will set 64K THP to ``always``::
> +
> +	thp_anon=64K:always
> +
> +``thp_anon=`` may be specified multiple times to configure all THP sizes as
> +required. If ``thp_anon=`` is specified at least once, any anon THP sizes
> +not explicitly configured on the command line are implicitly set to
> +``never``.

I suggest documenting that "thp_anon=" will not effect the value of 
"transparent_hugepage=", or any configured default.

Wondering if a syntax like

thp_anon=16K,32K,64K:always;1048K,2048K:madvise

(one could also support ranges, like "16K-64K")

Would be even better. Then, maybe only allow a single instance.

Maybe consider it if it's not too crazy to parse ;)

-- 
Cheers,

David / dhildenb