linux-kernel - Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

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Message-ID: <b99ba664-982d-4b8b-b923-c3cefae8e021@arm.com>
Date: Fri, 5 Jul 2024 10:16:55 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: dj456119@...il.com, 21cnbao@...il.com, david@...hat.com,
 shy828301@...il.com, ziy@...dia.com, libang.li@...group.com,
 baolin.wang@...ux.alibaba.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Barry Song <baohua@...nel.org>,
 Mingzhe Yang <mingzhe.yang@...com>
Subject: Re: [PATCH v3 2/2] mm: add docs for per-order mTHP split counters

On 04/07/2024 02:29, Lance Yang wrote:
> This commit introduces documentation for mTHP split counters in
> transhuge.rst.
> 
> Reviewed-by: Barry Song <baohua@...nel.org>
> Signed-off-by: Mingzhe Yang <mingzhe.yang@...com>
> Signed-off-by: Lance Yang <ioworker0@...il.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 1f72b00af5d3..0830aa173a8b 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -369,10 +369,6 @@ also applies to the regions registered in khugepaged.
>  Monitoring usage
>  ================
>  
> -.. note::
> -   Currently the below counters only record events relating to
> -   PMD-sized THP. Events relating to other THP sizes are not included.
> -
>  The number of PMD-sized anonymous transparent huge pages currently used by the
>  system is available by reading the AnonHugePages field in ``/proc/meminfo``.
>  To identify what applications are using PMD-sized anonymous transparent huge
> @@ -514,6 +510,22 @@ file_fallback_charge
>  	falls back to using small pages even though the allocation was
>  	successful.
>  
> +split
> +	is incremented every time a huge page is successfully split into
> +	smaller orders. This can happen for a variety of reasons but a
> +	common reason is that a huge page is old and is being reclaimed.
> +	This action implies splitting any block mappings into PTEs.

nit: the block mappings will already be PTEs if starting with mTHP?

regardless:

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> +
> +split_failed
> +	is incremented if kernel fails to split huge
> +	page. This can happen if the page was pinned by somebody.
> +
> +split_deferred
> +	is incremented when a huge page is put onto split
> +	queue. This happens when a huge page is partially unmapped and
> +	splitting it would free up some memory. Pages on split queue are
> +	going to be split under memory pressure.
> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help