linux-kernel - Re: [PATCH v5 3/4] mm: add docs for per-order mTHP counters and transhuge

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Date: Fri, 12 Apr 2024 11:19:56 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: cerasuolodomenico@...il.com, chrisl@...nel.org, david@...hat.com,
 kasong@...cent.com, linux-kernel@...r.kernel.org, peterx@...hat.com,
 surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org,
 yosryahmed@...gle.com, yuzhao@...gle.com, corbet@....net
Subject: Re: [PATCH v5 3/4] mm: add docs for per-order mTHP counters and
 transhuge_page ABI

On 12/04/2024 08:37, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> This patch includes documentation for mTHP counters and an ABI file
> for sys-kernel-mm-transparent-hugepage, which appears to have been
> missing for some time.
> 
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> Cc: Chris Li <chrisl@...nel.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Domenico Cerasuolo <cerasuolodomenico@...il.com>
> Cc: Kairui Song <kasong@...cent.com>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Yosry Ahmed <yosryahmed@...gle.com>
> Cc: Yu Zhao <yuzhao@...gle.com>
> Cc: Jonathan Corbet <corbet@....net>

A few nits, but regardless:

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

> ---
>  .../sys-kernel-mm-transparent-hugepage        | 17 +++++++++++
>  Documentation/admin-guide/mm/transhuge.rst    | 28 +++++++++++++++++++
>  2 files changed, 45 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sys-kernel-mm-transparent-hugepage
> 
> diff --git a/Documentation/ABI/testing/sys-kernel-mm-transparent-hugepage b/Documentation/ABI/testing/sys-kernel-mm-transparent-hugepage
> new file mode 100644
> index 000000000000..80dde0fd576c
> --- /dev/null
> +++ b/Documentation/ABI/testing/sys-kernel-mm-transparent-hugepage
> @@ -0,0 +1,17 @@
> +What:		/sys/kernel/mm/hugepages/

Err, transparent_hugepage, right? copy/paste error?

> +Date:		April 2024
> +Contact:	Barry Song <baohua@...nel.org>

Looks like a bunch of mm sysfs interfaces use:

Contact:	Linux memory management mailing list <linux-mm@...ck.org>

I'll leave that up to you!

> +Description:
> +		/sys/kernel/mm/transparent_hugepage/ contains a number of files and
> +		subdirectories,
> +			- defrag
> +			- enabled
> +			- hpage_pmd_size
> +			- khugepaged
> +			- shmem_enabled
> +			- use_zero_page
> +			- subdirectories of the form hugepages-<size>kB, where <size>
> +			  is the page size of the hugepages supported by the kernel/CPU
> +			  combination.
> +
> +		See Documentation/admin-guide/mm/transhuge.rst for details.> diff --git a/Documentation/admin-guide/mm/transhuge.rst
b/Documentation/admin-guide/mm/transhuge.rst
> index 04eb45a2f940..f436ff982f22 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -447,6 +447,34 @@ thp_swpout_fallback
>  	Usually because failed to allocate some continuous swap space
>  	for the huge page.
>  
> +In /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats, There are
> +also individual counters for each huge page size, which can be utilized to
> +monitor the system's effectiveness in providing huge pages for usage. Each
> +counter has its own corresponding file.
> +
> +anon_fault_alloc
> +	is incremented every time a huge page is successfully
> +	allocated and charged to handle a page fault.
> +
> +anon_fault_fallback
> +	is incremented if a page fault fails to allocate or charge
> +	a huge page and instead falls back to using huge pages with
> +	lower orders or small pages.
> +
> +anon_fault_fallback_charge
> +	is incremented if a page fault fails to charge a huge page and
> +	instead falls back to using huge pages with lower orders or
> +	small pages even though the allocation was successful.
> +
> +anon_swpout
> +	is incremented every time a huge page is swapout in one

nit: swapout -> "swapped out"? Although I see this is just a copy/paste of the
description of the existing counter...

> +	piece without splitting.
> +
> +anon_swpout_fallback
> +	is incremented if a huge page has to be split before swapout.
> +	Usually because failed to allocate some continuous swap space
> +	for the huge page.
> +
>  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