Re: [RFC 00/11] khugepaged: mTHP support

Dev Jain
2025-01-12T14:12:16Z

On 11/01/25 3:31 am, Nico Pache wrote:
> On Thu, Jan 9, 2025 at 9:56 PM Dev Jain <dev.jain@arm.com> wrote:
>>
>>
>>
>> On 10/01/25 7:57 am, Nico Pache wrote:
>>> On Wed, Jan 8, 2025 at 11:22 PM Dev Jain <dev.jain@arm.com> wrote:
>>>>
>>>>
>>>> On 09/01/25 5:01 am, Nico Pache wrote:
>>>>> The following series provides khugepaged and madvise collapse with the
>>>>> capability to collapse regions to mTHPs.
>>>>>
>>>>> To achieve this we generalize the khugepaged functions to no longer depend
>>>>> on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages
>>>>> (defined by MTHP_MIN_ORDER) that are fully utilized. This info is tracked
>>>>> using a bitmap. After the PMD scan is done, we do binary recursion on the
>>>>> bitmap to find the optimal mTHP sizes for the PMD range. The restriction
>>>>> on max_ptes_none is removed during the scan, to make sure we account for
>>>>> the whole PMD range. max_ptes_none is mapped to a 0-100 range to
>>>>> determine how full a mTHP order needs to be before collapsing it.
>>>>>
>>>>> Some design choices to note:
>>>>> - bitmap structures are allocated dynamically because on some arch's
>>>>> (like PowerPC) the value of MTHP_BITMAP_SIZE cannot be computed at
>>>>> compile time leading to warnings.
>>>>> - The recursion is masked through a stack structure.
>>>>> - A MTHP_MIN_ORDER was added to compress the bitmap, and ensure it was
>>>>> 64bit on x86. This provides some optimization on the bitmap operations.
>>>>> if other arches/configs that have larger than 512 PTEs per PMD want to
>>>>> compress their bitmap further we can change this value per arch.
>>>>>
>>>>> Patch 1-2: Some refactoring to combine madvise_collapse and khugepaged
>>>>> Patch 3: A minor "fix"/optimization
>>>>> Patch 4: Refactor/rename hpage_collapse
>>>>> Patch 5-7: Generalize khugepaged functions for arbitrary orders
>>>>> Patch 8-11: The mTHP patches
>>>>>
>>>>> This series acts as an alternative to Dev Jain's approach [1]. The two
>>>>> series differ in a few ways:
>>>>> - My approach uses a bitmap to store the state of the linear scan_pmd to
>>>>> then determine potential mTHP batches. Devs incorporates his directly
>>>>> into the scan, and will try each available order.
>>>>> - Dev is attempting to optimize the locking, while my approach keeps the
>>>>> locking changes to a minimum. I believe his changes are not safe for
>>>>> uffd.
>>>>> - Dev's changes only work for khugepaged not madvise_collapse (although
>>>>> i think that was by choice and it could easily support madvise)
>>>>> - Dev scales all khugepaged sysfs tunables by order, while im removing
>>>>> the restriction of max_ptes_none and converting it to a scale to
>>>>> determine a (m)THP threshold.
>>>>> - Dev turns on khugepaged if any order is available while mine still
>>>>> only runs if PMDs are enabled. I like Dev's approach and will most
>>>>> likely do the same in my PATCH posting.
>>>>> - mTHPs need their ref count updated to 1<<order, which Dev is missing.
>>>>>
>>>>> Patch 11 was inspired by one of Dev's changes.
>>>>>
>>>>> [1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@arm.com/
>>>>>
>>>>> Nico Pache (11):
>>>>> introduce khugepaged_collapse_single_pmd to collapse a single pmd
>>>>> khugepaged: refactor madvise_collapse and khugepaged_scan_mm_slot
>>>>> khugepaged: Don't allocate khugepaged mm_slot early
>>>>> khugepaged: rename hpage_collapse_* to khugepaged_*
>>>>> khugepaged: generalize hugepage_vma_revalidate for mTHP support
>>>>> khugepaged: generalize alloc_charge_folio for mTHP support
>>>>> khugepaged: generalize __collapse_huge_page_* for mTHP support
>>>>> khugepaged: introduce khugepaged_scan_bitmap for mTHP support
>>>>> khugepaged: add mTHP support
>>>>> khugepaged: remove max_ptes_none restriction on the pmd scan
>>>>> khugepaged: skip collapsing mTHP to smaller orders
>>>>>
>>>>> include/linux/khugepaged.h | 4 +-
>>>>> mm/huge_memory.c | 3 +-
>>>>> mm/khugepaged.c | 436 +++++++++++++++++++++++++------------
>>>>> 3 files changed, 306 insertions(+), 137 deletions(-)
>>>>
>>>> Before I take a proper look at your series, can you please include any testing
>>>> you may have done?
>>>
>>> I Built these changes for the following arches: x86_64, arm64,
>>> arm64-64k, ppc64le, s390x
>>>
>>> x86 testing:
>>> - Selftests mm
>>> - some stress-ng tests
>>> - compile kernel
>>> - I did some tests with my defer [1] set on top. This pushes all the
>>> work to khugepaged, which removes the noise of all the PF allocations.
>>>
>>> I recently got an ARM64 machine and did some simple sanity tests (on
>>> both 4k and 64k) like selftests, stress-ng, and playing around with
>>> the tunables, etc.
>>>
>>> I will also be running all the builds through our CI, and perf testing
>>> environments before posting.
>>>
>>> [1] https://lore.kernel.org/lkml/20240729222727.64319-1-npache@redhat.com/
>>>
>>>>
>>>
>> I tested your series with the program I was using and it is not working;
>> can you please confirm it.
>
> Yes, this is expected because you are not fully filling any 32K chunk
> (MIN_MTHP_ORDER) so no bit is ever set.

That is weird, because if this is the case, then PMD-collapse should
have also failed, but that succeeded. Do you have some userspace program
I can test with?