Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

Ryan Roberts
2023-12-05T11:06:17Z

On 05/12/2023 03:28, Barry Song wrote:
> On Mon, Dec 4, 2023 at 11:20 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> 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?
>>
>
> Hi Ryan,
>
> A question but i don't think it should block this series, do we have any plan
> to extend /proc/meminfo, /proc/pid/smaps, /proc/vmstat to present some
> information regarding the new multi-size THP.
>
> e.g how many folios in each-size for the system, how many multi-size folios LRU,
> how many large folios in each VMA etc.
>
> In products and labs, we need some health monitors to make sure the system
> status is visible and works as expected. right now, i feel i am like
> blindly exploring
> the system without those statistics.

Yes it's definitely on the list. I had a patch in v6 that added various stats.
But after discussion with David, it became clear there were a few issues with
the implementation and I ripped it out. We also decided that since the page
cache already uses large folios and we don't have counters for those, we could
probably live (initially at least) without counters for multi-size THP too. But
you are the second person to raise this in as many weeks, so clearly this should
be at the top of the list for enhancements after this initial merge.

For now, you can parse /proc/<pid>/pagemap to see how well multi-size THP is
being utilized. That's not a simple interface though. Yu Zhou shared a Python
script a while back. I wonder if there is value in tidying that up and putting
it in tools/mm in the short term?

There were 2 main issues with the previous implementation:

1) What should the semantic of a counter be? Because a PTE-mapped THP can be
partially unmapped or mremapped. So should we count number of pages from a folio
of a given size that are mapped (easy) or should we only count when the whole
folio is contiguously mapped? (I'm sure there are many other semantics we could
consider). The latter is not easy to spot at the moment - perhaps the work David
has been doing on tidying up the rmap functions might help?

2) How should we expose the info? There has been pushback for extending files in
sysfs that expose multiple pieces of data, so David has suggested that in the
long term it might be good to completely redesign the stats interface.

It's certainly something that needs a lot more discussion - input encouraged!

Thanks,
Ryan

>
>>
>> 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
>>
>
> Thanks
> Barry