linux-kernel - Re: [PATCH v2 2/2] mm: add docs for per-order mTHP split counters

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Date: Mon, 1 Jul 2024 12:46:57 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, 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,
 Mingzhe Yang <mingzhe.yang@...com>
Subject: Re: [PATCH v2 2/2] mm: add docs for per-order mTHP split counters

On 01/07/2024 11:50, Lance Yang wrote:
> On Mon, Jul 1, 2024 at 4:31 PM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> On 28/06/2024 14:07, Lance Yang wrote:
>>> This commit introduces documentation for mTHP split counters in
>>> transhuge.rst.
>>>
>>> Signed-off-by: Mingzhe Yang <mingzhe.yang@...com>
>>> Signed-off-by: Lance Yang <ioworker0@...il.com>
>>> ---
>>>  Documentation/admin-guide/mm/transhuge.rst | 16 ++++++++++++++++
>>>  1 file changed, 16 insertions(+)
>>>
>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>>> index 1f72b00af5d3..709fe10b60f4 100644
>>> --- a/Documentation/admin-guide/mm/transhuge.rst
>>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>>> @@ -514,6 +514,22 @@ file_fallback_charge
>>>       falls back to using small pages even though the allocation was
>>>       successful.
>>
>>
>> I note at the top of this section there is a note:
>>
>> Monitoring usage
>> ================
>>
>> .. note::
>>    Currently the below counters only record events relating to
>>    PMD-sized THP. Events relating to other THP sizes are not included.
>>
>> Which is out of date, now that we support mTHP stats. Perhaps it should be removed?
> 
> Good catch! Let's remove that in this patch ;)
> 
>>
>>>
>>> +split
>>> +     is incremented every time a huge page is successfully split into
>>> +     base pages. 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.
>>
>> Now that I'm reading this, I'm reminded that Yang Shi suggested at LSFMM that a
>> potential aid so solving the swap-out fragmentation problem is to split high
>> orders to lower (but not 0) orders. I don't know if we would take that route,
>> but in principle it sounds like splitting mTHP to smaller mTHP might be
>> something we want some day. I wonder if we should spec this counter to also
>> include splits to smaller orders and not just splits to base pages?
>>
>> Actually looking at the code, I think split_huge_page_to_list_to_order(order>0)
>> would already increment this counter without actually splitting to base pages.
>> So the documantation should probably just reflect that.
> 
> Yep, you're right.
> 
> It’s important that the documentation reflects that to ensure consistency.
> 
> How about "...  is successfully split into smaller orders. This can..."?

fine by me.

> 
> Thanks,
> Lance
> 
>>
>>> +
>>> +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
>>