Message-ID: <CAA1CXcAKk5z+gTrQDe-eLsxkrsNrf8G9HD5KBPTJv_EECei_Mg@mail.gmail.com>
Date: Wed, 14 May 2025 21:21:14 -0600
From: Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v6 00/12] khugepaged: mTHP support

Ugh... So sorry, I forgot to turn off the chain-reply-to.

resending V7 *facepalm*

On Wed, May 14, 2025 at 9:03 PM Nico Pache <npache@...hat.com> wrote:
>
> The following series provides khugepaged and madvise collapse with the
> capability to collapse anonymous memory 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 KHUGEPAGED_MTHP_MIN_ORDER) that are 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. When no mTHP size is enabled, the legacy
> behavior of khugepaged is maintained. max_ptes_none will be scaled by the
> attempted collapse order to determine how full a THP must be to be
> eligible. If a mTHP collapse is attempted, but contains swapped out, or
> shared pages, we dont perform the collapse.
>
> With the default max_ptes_none=511, the code should keep its most of its
> original behavior. To exercise mTHP collapse we need to set
> max_ptes_none<=255. With max_ptes_none > HPAGE_PMD_NR/2 you will
> experience collapse "creep" and constantly promote mTHPs to the next
> available size. This is due the fact that it will introduce at least 2x
> the number of pages, and on a future scan will satisfy that condition once
> again.
>
> Patch 1:     Refactor/rename hpage_collapse
> Patch 2:     Some refactoring to combine madvise_collapse and khugepaged
> Patch 3-5:   Generalize khugepaged functions for arbitrary orders
> Patch 6-9:   The mTHP patches
> Patch 10-11: Tracing/stats
> Patch 12:    Documentation
>
> ---------
>  Testing
> ---------
> - Built for x86_64, aarch64, ppc64le, and s390x
> - selftests mm
> - I created a test script that I used to push khugepaged to its limits
>    while monitoring a number of stats and tracepoints. The code is
>    available here[1] (Run in legacy mode for these changes and set mthp
>    sizes to inherit)
>    The summary from my testings was that there was no significant
>    regression noticed through this test. In some cases my changes had
>    better collapse latencies, and was able to scan more pages in the same
>    amount of time/work, but for the most part the results were consistent.
> - redis testing. I tested these changes along with my defer changes
>   (see followup post for more details).
> - some basic testing on 64k page size.
> - lots of general use.
>
> V6 Changes:
> - Dont release the anon_vma_lock early (like in the PMD case), as not all
>   pages are isolated.
> - Define the PTE as null to avoid a uninitilized condition
> - minor nits and newline cleanup
> - make sure to unmap and unlock the pte for the swapin case
> - change the revalidation to always check the PMD order (as this will make
>   sure that no other VMA spans it)
>
> V5 Changes [2]:
> - switched the order of patches 1 and 2
> - fixed some edge cases on the unified madvise_collapse and khugepaged
> - Explained the "creep" some more in the docs
> - fix EXCEED_SHARED vs EXCEED_SWAP accounting issue
> - fix potential highmem issue caused by a early unmap of the PTE
>
> V4 Changes:
> - Rebased onto mm-unstable
> - small changes to Documentation
>
> V3 Changes:
> - corrected legacy behavior for khugepaged and madvise_collapse
> - added proper mTHP stat tracking
> - Minor changes to prevent a nested lock on non-split-lock arches
> - Took Devs version of alloc_charge_folio as it has the proper stats
> - Skip cases were trying to collapse to a lower order would still fail
> - Fixed cases were the bitmap was not being updated properly
> - Moved Documentation update to this series instead of the defer set
> - Minor bugs discovered during testing and review
> - Minor "nit" cleanup
>
> V2 Changes:
> - Minor bug fixes discovered during review and testing
> - removed dynamic allocations for bitmaps, and made them stack based
> - Adjusted bitmap offset from u8 to u16 to support 64k pagesize.
> - Updated trace events to include collapsing order info.
> - Scaled max_ptes_none by order rather than scaling to a 0-100 scale.
> - No longer require a chunk to be fully utilized before setting the bit.
>    Use the same max_ptes_none scaling principle to achieve this.
> - Skip mTHP collapse that requires swapin or shared handling. This helps
>    prevent some of the "creep" that was discovered in v1.
>
> [1] - https://gitlab.com/npache/khugepaged_mthp_test
> [2] - https://lore.kernel.org/all/20250428181218.85925-1-npache@redhat.com/
>
> Dev Jain (1):
>   khugepaged: generalize alloc_charge_folio()
>
> Nico Pache (11):
>   khugepaged: rename hpage_collapse_* to khugepaged_*
>   introduce khugepaged_collapse_single_pmd to unify khugepaged and
>     madvise_collapse
>   khugepaged: generalize hugepage_vma_revalidate for mTHP support
>   khugepaged: generalize __collapse_huge_page_* for mTHP support
>   khugepaged: introduce khugepaged_scan_bitmap for mTHP support
>   khugepaged: add mTHP support
>   khugepaged: skip collapsing mTHP to smaller orders
>   khugepaged: avoid unnecessary mTHP collapse attempts
>   khugepaged: improve tracepoints for mTHP orders
>   khugepaged: add per-order mTHP khugepaged stats
>   Documentation: mm: update the admin guide for mTHP collapse
>
>  Documentation/admin-guide/mm/transhuge.rst |  14 +-
>  include/linux/huge_mm.h                    |   5 +
>  include/linux/khugepaged.h                 |   4 +
>  include/trace/events/huge_memory.h         |  34 +-
>  mm/huge_memory.c                           |  11 +
>  mm/khugepaged.c                            | 472 ++++++++++++++-------
>  6 files changed, 382 insertions(+), 158 deletions(-)
>
> --
> 2.49.0
>

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