Message-ID: <CAA1CXcDRx94xsqcAHnkxNUB6u2Z_T3tRp4uDeZsMShJgHcLmyA@mail.gmail.com>
Date: Fri, 16 May 2025 05:54:27 -0600
From: Nico Pache <npache@...hat.com>
To: Dev Jain <dev.jain@....com>
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Subject: Re: [PATCH v6 00/12] khugepaged: mTHP support

On Thu, May 15, 2025 at 12:45 AM Dev Jain <dev.jain@....com> wrote:
>
>
>
> On 15/05/25 8:51 am, Nico Pache wrote:
> > Ugh... So sorry, I forgot to turn off the chain-reply-to.
> >
> > resending V7 *facepalm*
>
> In the future you can just send the same version again with [RESEND]
> prefixed in the subject, that prevents confusion.
Thanks I'll do that next time.
>
> >
> > 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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