Message-Id: <20231204102027.57185-1-ryan.roberts@arm.com>
Date:   Mon,  4 Dec 2023 10:20:17 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Barry Song <21cnbao@...il.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     Ryan Roberts <ryan.roberts@....com>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH v8 00/10] Multi-size THP for anonymous memory

Hi All,

A new week, a new version, a new name... This is v8 of a series to implement
multi-size THP (mTHP) for anonymous memory (previously called "small-sized THP"
and "large anonymous folios"). Matthew objected to "small huge" so hopefully
this fares better.

The objective of this is to improve performance by allocating larger chunks of
memory during anonymous page faults:

1) Since SW (the kernel) is dealing with larger chunks of memory than base
   pages, there are efficiency savings to be had; fewer page faults, batched PTE
   and RMAP manipulation, reduced lru list, etc. In short, we reduce kernel
   overhead. This should benefit all architectures.
2) Since we are now mapping physically contiguous chunks of memory, we can take
   advantage of HW TLB compression techniques. A reduction in TLB pressure
   speeds up kernel and user space. arm64 systems have 2 mechanisms to coalesce
   TLB entries; "the contiguous bit" (architectural) and HPA (uarch).

This version changes the name and tidies up some of the kernel code and test
code, based on feedback against v7 (see change log for details).

By default, the existing behaviour (and performance) is maintained. The user
must explicitly enable multi-size THP to see the performance benefit. This is
done via a new sysfs interface (as recommended by David Hildenbrand - thanks to
David for the suggestion)! This interface is inspired by the existing
per-hugepage-size sysfs interface used by hugetlb, provides full backwards
compatibility with the existing PMD-size THP interface, and provides a base for
future extensibility. See [8] for detailed discussion of the interface.

This series is based on mm-unstable (715b67adf4c8).


Prerequisites
=============

Some work items identified as being prerequisites are listed on page 3 at [9].
The summary is:

| item                          | status                  |
|:------------------------------|:------------------------|
| mlock                         | In mainline (v6.7)      |
| madvise                       | In mainline (v6.6)      |
| compaction                    | v1 posted [10]          |
| numa balancing                | Investigated: see below |
| user-triggered page migration | In mainline (v6.7)      |
| khugepaged collapse           | In mainline (NOP)       |

On NUMA balancing, which currently ignores any PTE-mapped THPs it encounters,
John Hubbard has investigated this and concluded that it is A) not clear at the
moment what a better policy might be for PTE-mapped THP and B) questions whether
this should really be considered a prerequisite given no regression is caused
for the default "multi-size THP disabled" case, and there is no correctness
issue when it is enabled - its just a potential for non-optimal performance.

If there are no disagreements about removing numa balancing from the list (none
were raised when I first posted this comment against v7), then that just leaves
compaction which is in review on list at the moment.

I really would like to get this series (and its remaining comapction
prerequisite) in for v6.8. I accept that it may be a bit optimistic at this
point, but lets see where we get to with review?


Testing
=======

The series includes patches for mm selftests to enlighten the cow and khugepaged
tests to explicitly test with multi-size THP, in the same way that PMD-sized
THP is tested. The new tests all pass, and no regressions are observed in the mm
selftest suite. I've also run my usual kernel compilation and java script
benchmarks without any issues.

Refer to my performance numbers posted with v6 [6]. (These are for multi-size
THP only - they do not include the arm64 contpte follow-on series).

John Hubbard at Nvidia has indicated dramatic 10x performance improvements for
some workloads at [11]. (Observed using v6 of this series as well as the arm64
contpte series).

Kefeng Wang at Huawei has also indicated he sees improvements at [12] although
there are some latency regressions also.


Changes since v7 [7]
====================

  - Renamed "small-sized THP" -> "multi-size THP" in commit logs
  - Added various Reviewed-by/Tested-by tags (Barry, David, Alistair)
  - Patch 3:
      - Fine-tuned transhuge documentation multi-size THP (JohnH)
      - Converted hugepage_global_enabled() and hugepage_global_always() macros
        to static inline functions (JohnH)
      - Renamed hugepage_vma_check() to thp_vma_allowable_orders() (JohnH)
      - Renamed transhuge_vma_suitable() to thp_vma_suitable_orders() (JohnH)
      - Renamed "global" enabled sysfs file option to "inherit" (JohnH)
  - Patch 9:
      - cow selftest: Renamed param size -> thpsize (David)
      - cow selftest: Changed test fail to assert() (David)
      - cow selftest: Log PMD size separately from all the supported THP sizes
        (David)
  - Patch 10:
      - cow selftest: No longer special case pmdsize; keep all THP sizes in
        thpsizes[]


Changes since v6 [6]
====================

  - Refactored vmf_pte_range_changed() to remove uffd special-case (suggested by
    JohnH)
  - Dropped accounting patch (#3 in v6) (suggested by DavidH)
      - Continue to account *PMD-sized* THP only for now
      - Can add more counters in future if needed
      - Page cache large folios haven't needed any new counters yet
  - Pivot to sysfs ABI proposed by DavidH
      - per-size directories in a similar shape to that used by hugetlb
  - Dropped "recommend" keyword patch (#6 in v6) (suggested by DavidH, Yu Zhou)
      - For now, users need to understand implicitly which sizes are beneficial
        to their HW/SW
  - Dropped arch_wants_pte_order() patch (#7 in v6)
      - No longer needed due to dropping patch "recommend" keyword patch
  - Enlightened khugepaged mm selftest to explicitly test with small-size THP
  - Scrubbed commit logs to use "small-sized THP" consistently (suggested by
    DavidH)


Changes since v5 [5]
====================

  - Added accounting for PTE-mapped THPs (patch 3)
  - Added runtime control mechanism via sysfs as extension to THP (patch 4)
  - Minor refactoring of alloc_anon_folio() to integrate with runtime controls
  - Stripped out hardcoded policy for allocation order; its now all user space
    controlled (although user space can request "recommend" which will configure
    the HW-preferred order)


Changes since v4 [4]
====================

  - Removed "arm64: mm: Override arch_wants_pte_order()" patch; arm64
    now uses the default order-3 size. I have moved this patch over to
    the contpte series.
  - Added "mm: Allow deferred splitting of arbitrary large anon folios" back
    into series. I originally removed this at v2 to add to a separate series,
    but that series has transformed significantly and it no longer fits, so
    bringing it back here.
  - Reintroduced dependency on set_ptes(); Originally dropped this at v2, but
    set_ptes() is in mm-unstable now.
  - Updated policy for when to allocate LAF; only fallback to order-0 if
    MADV_NOHUGEPAGE is present or if THP disabled via prctl; no longer rely on
    sysfs's never/madvise/always knob.
  - Fallback to order-0 whenever uffd is armed for the vma, not just when
    uffd-wp is set on the pte.
  - alloc_anon_folio() now returns `struct folio *`, where errors are encoded
    with ERR_PTR().

  The last 3 changes were proposed by Yu Zhao - thanks!


Changes since v3 [3]
====================

  - Renamed feature from FLEXIBLE_THP to LARGE_ANON_FOLIO.
  - Removed `flexthp_unhinted_max` boot parameter. Discussion concluded that a
    sysctl is preferable but we will wait until real workload needs it.
  - Fixed uninitialized `addr` on read fault path in do_anonymous_page().
  - Added mm selftests for large anon folios in cow test suite.


Changes since v2 [2]
====================

  - Dropped commit "Allow deferred splitting of arbitrary large anon folios"
      - Huang, Ying suggested the "batch zap" work (which I dropped from this
        series after v1) is a prerequisite for merging FLXEIBLE_THP, so I've
        moved the deferred split patch to a separate series along with the batch
        zap changes. I plan to submit this series early next week.
  - Changed folio order fallback policy
      - We no longer iterate from preferred to 0 looking for acceptable policy
      - Instead we iterate through preferred, PAGE_ALLOC_COSTLY_ORDER and 0 only
  - Removed vma parameter from arch_wants_pte_order()
  - Added command line parameter `flexthp_unhinted_max`
      - clamps preferred order when vma hasn't explicitly opted-in to THP
  - Never allocate large folio for MADV_NOHUGEPAGE vma (or when THP is disabled
    for process or system).
  - Simplified implementation and integration with do_anonymous_page()
  - Removed dependency on set_ptes()


Changes since v1 [1]
====================

  - removed changes to arch-dependent vma_alloc_zeroed_movable_folio()
  - replaced with arch-independent alloc_anon_folio()
      - follows THP allocation approach
  - no longer retry with intermediate orders if allocation fails
      - fallback directly to order-0
  - remove folio_add_new_anon_rmap_range() patch
      - instead add its new functionality to folio_add_new_anon_rmap()
  - remove batch-zap pte mappings optimization patch
      - remove enabler folio_remove_rmap_range() patch too
      - These offer real perf improvement so will submit separately
  - simplify Kconfig
      - single FLEXIBLE_THP option, which is independent of arch
      - depends on TRANSPARENT_HUGEPAGE
      - when enabled default to max anon folio size of 64K unless arch
        explicitly overrides
  - simplify changes to do_anonymous_page():
      - no more retry loop


[1] https://lore.kernel.org/linux-mm/20230626171430.3167004-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/linux-mm/20230703135330.1865927-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/linux-mm/20230714160407.4142030-1-ryan.roberts@arm.com/
[4] https://lore.kernel.org/linux-mm/20230726095146.2826796-1-ryan.roberts@arm.com/
[5] https://lore.kernel.org/linux-mm/20230810142942.3169679-1-ryan.roberts@arm.com/
[6] https://lore.kernel.org/linux-mm/20230929114421.3761121-1-ryan.roberts@arm.com/
[7] https://lore.kernel.org/linux-mm/20231122162950.3854897-1-ryan.roberts@arm.com/
[8] https://lore.kernel.org/linux-mm/6d89fdc9-ef55-d44e-bf12-fafff318aef8@redhat.com/
[9] https://drive.google.com/file/d/1GnfYFpr7_c1kA41liRUW5YtCb8Cj18Ud/view?usp=sharing&resourcekey=0-U1Mj3-RhLD1JV6EThpyPyA
[10] https://lore.kernel.org/linux-mm/20231113170157.280181-1-zi.yan@sent.com/
[11] https://lore.kernel.org/linux-mm/c507308d-bdd4-5f9e-d4ff-e96e4520be85@nvidia.com/
[12] https://lore.kernel.org/linux-mm/479b3e2b-456d-46c1-9677-38f6c95a0be8@huawei.com/


Thanks,
Ryan

Ryan Roberts (10):
  mm: Allow deferred splitting of arbitrary anon large folios
  mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()
  mm: thp: Introduce multi-size THP sysfs interface
  mm: thp: Support allocation of anonymous multi-size THP
  selftests/mm/kugepaged: Restore thp settings at exit
  selftests/mm: Factor out thp settings management
  selftests/mm: Support multi-size THP interface in thp_settings
  selftests/mm/khugepaged: Enlighten for multi-size THP
  selftests/mm/cow: Generalize do_run_with_thp() helper
  selftests/mm/cow: Add tests for anonymous multi-size THP

 Documentation/admin-guide/mm/transhuge.rst |  97 ++++-
 Documentation/filesystems/proc.rst         |   6 +-
 fs/proc/task_mmu.c                         |   3 +-
 include/linux/huge_mm.h                    | 116 ++++--
 mm/huge_memory.c                           | 268 ++++++++++++--
 mm/khugepaged.c                            |  20 +-
 mm/memory.c                                | 114 +++++-
 mm/page_vma_mapped.c                       |   3 +-
 mm/rmap.c                                  |  32 +-
 tools/testing/selftests/mm/Makefile        |   4 +-
 tools/testing/selftests/mm/cow.c           | 185 +++++++---
 tools/testing/selftests/mm/khugepaged.c    | 410 ++++-----------------
 tools/testing/selftests/mm/run_vmtests.sh  |   2 +
 tools/testing/selftests/mm/thp_settings.c  | 349 ++++++++++++++++++
 tools/testing/selftests/mm/thp_settings.h  |  80 ++++
 15 files changed, 1177 insertions(+), 512 deletions(-)
 create mode 100644 tools/testing/selftests/mm/thp_settings.c
 create mode 100644 tools/testing/selftests/mm/thp_settings.h

--
2.25.1

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