Re: [RFC] mm: khugepaged: use largest enabled hugepage order for min_free_kbytes

[Usama Arif <usamaarif642@gmail.com>]

On 06/06/2025 16:18, Zi Yan wrote:
> On 6 Jun 2025, at 10:37, Usama Arif wrote:
> 
>> On arm64 machines with 64K PAGE_SIZE, the min_free_kbytes and hence the
>> watermarks are evaluated to extremely high values, for e.g. a server with
>> 480G of memory, only 2M mTHP hugepage size set to madvise, with the rest
>> of the sizes set to never, the min, low and high watermarks evaluate to
>> 11.2G, 14G and 16.8G respectively.
>> In contrast for 4K PAGE_SIZE of the same machine, with only 2M THP hugepage
>> size set to madvise, the min, low and high watermarks evaluate to 86M, 566M
>> and 1G respectively.
>> This is because set_recommended_min_free_kbytes is designed for PMD
>> hugepages (pageblock_order = min(HPAGE_PMD_ORDER, PAGE_BLOCK_ORDER)).
>> Such high watermark values can cause performance and latency issues in
>> memory bound applications on arm servers that use 64K PAGE_SIZE, eventhough
>> most of them would never actually use a 512M PMD THP.
>>
>> Instead of using HPAGE_PMD_ORDER for pageblock_order use the highest large
>> folio order enabled in set_recommended_min_free_kbytes.
>> With this patch, when only 2M THP hugepage size is set to madvise for the
>> same machine with 64K page size, with the rest of the sizes set to never,
>> the min, low and high watermarks evaluate to 2.08G, 2.6G and 3.1G
>> respectively. When 512M THP hugepage size is set to madvise for the same
>> machine with 64K page size, the min, low and high watermarks evaluate to
>> 11.2G, 14G and 16.8G respectively, the same as without this patch.
> 
> Getting pageblock_order involved here might be confusing. I think you just
> want to adjust min, low and high watermarks to reasonable values.
> Is it OK to rename min_thp_pageblock_nr_pages to min_nr_free_pages_per_zone
> and move MIGRATE_PCPTYPES * MIGRATE_PCPTYPES inside? Otherwise, the changes
> look reasonable to me.

Hi Zi,

Thanks for the review!

I forgot to change it in another place, sorry about that! So can't move
MIGRATE_PCPTYPES * MIGRATE_PCPTYPES into the combined function.
Have added the additional place where min_thp_pageblock_nr_pages() is called
as a fixlet here:
https://lore.kernel.org/all/a179fd65-dc3f-4769-9916-3033497188ba@gmail.com/

I think atleast in this context the orginal name pageblock_nr_pages isn't
correct as its min(HPAGE_PMD_ORDER, PAGE_BLOCK_ORDER).
The new name min_thp_pageblock_nr_pages is also not really good, so happy
to change it to something appropriate.
> 
> Another concern on tying watermarks to highest THP order is that if
> user enables PMD THP on such systems with 2MB mTHP enabled initially,
> it could trigger unexpected memory reclaim and compaction, right?
> That might surprise user, since they just want to adjust availability
> of THP sizes, but the whole system suddenly begins to be busy.
> Have you experimented with it?
> 

Yes I would imagine it would trigger reclaim and compaction if the system memory
is too low, but that should hopefully be expected? If the user is enabling 512M
THP, they should expect changes by kernel to allow them to give hugepage of
that size.
Also hopefully, no one is enabling PMD THPs when the system is so low on
memory that it triggers reclaim! There would be an OOM after just a few
of those are faulted in.

Thanks!