[PATCH v3 00/11] Perf improvements for hugetlb and vmalloc on arm64

[Ryan Roberts]

Hi Catalin,


On 04/03/2025 15:04, Ryan Roberts wrote:
> Hi All,
> 
> This is v3 of a series to improve performance for hugetlb and vmalloc on arm64.
> Although some of these patches are core-mm, advice from Andrew was to go via the
> arm64 tree. Hopefully I can get some ACKs from mm folks.
> 
> The 2 key performance improvements are 1) enabling the use of contpte-mapped
> blocks in the vmalloc space when appropriate (which reduces TLB pressure). There
> were already hooks for this (used by powerpc) but they required some tidying and
> extending for arm64. And 2) batching up barriers when modifying the vmalloc
> address space for upto 30% reduction in time taken in vmalloc().

Thanks for your reviews - I'm just trying to get my ducks lined up for when I'm
back in the office next week...

The last remaining 2 patches without R-b are patch #4 and #11 (both arm64). Do
you have any feedback on these? I have the series ready to repost with some
minor nits and build warnings addressed. I'm hoping we can get these last to
patches squared away then I'll repost next week against -rc3.

Thanks,
Ryan


> 
> vmalloc() performance was measured using the test_vmalloc.ko module. Tested on
> Apple M2 and Ampere Altra. Each test had loop count set to 500000 and the whole
> test was repeated 10 times.
> 
> legend:
>   - p: nr_pages (pages to allocate)
>   - h: use_huge (vmalloc() vs vmalloc_huge())
>   - (I): statistically significant improvement (95% CI does not overlap)
>   - (R): statistically significant regression (95% CI does not overlap)
>   - measurements are times; smaller is better
> 
> +--------------------------------------------------+-------------+-------------+
> | Benchmark                                        |             |             |
> |   Result Class                                   |    Apple M2 | Ampere Alta |
> +==================================================+=============+=============+
> | micromm/vmalloc                                  |             |             |
> |   fix_align_alloc_test: p:1, h:0 (usec)          | (I) -11.53% |      -2.57% |
> |   fix_size_alloc_test: p:1, h:0 (usec)           |       2.14% |       1.79% |
> |   fix_size_alloc_test: p:4, h:0 (usec)           |  (I) -9.93% |  (I) -4.80% |
> |   fix_size_alloc_test: p:16, h:0 (usec)          | (I) -25.07% | (I) -14.24% |
> |   fix_size_alloc_test: p:16, h:1 (usec)          | (I) -14.07% |   (R) 7.93% |
> |   fix_size_alloc_test: p:64, h:0 (usec)          | (I) -29.43% | (I) -19.30% |
> |   fix_size_alloc_test: p:64, h:1 (usec)          | (I) -16.39% |   (R) 6.71% |
> |   fix_size_alloc_test: p:256, h:0 (usec)         | (I) -31.46% | (I) -20.60% |
> |   fix_size_alloc_test: p:256, h:1 (usec)         | (I) -16.58% |   (R) 6.70% |
> |   fix_size_alloc_test: p:512, h:0 (usec)         | (I) -31.96% | (I) -20.04% |
> |   fix_size_alloc_test: p:512, h:1 (usec)         |       2.30% |       0.71% |
> |   full_fit_alloc_test: p:1, h:0 (usec)           |      -2.94% |       1.77% |
> |   kvfree_rcu_1_arg_vmalloc_test: p:1, h:0 (usec) |      -7.75% |       1.71% |
> |   kvfree_rcu_2_arg_vmalloc_test: p:1, h:0 (usec) |      -9.07% |   (R) 2.34% |
> |   long_busy_list_alloc_test: p:1, h:0 (usec)     | (I) -29.18% | (I) -17.91% |
> |   pcpu_alloc_test: p:1, h:0 (usec)               |     -14.71% |      -3.14% |
> |   random_size_align_alloc_test: p:1, h:0 (usec)  | (I) -11.08% |  (I) -4.62% |
> |   random_size_alloc_test: p:1, h:0 (usec)        | (I) -30.25% | (I) -17.95% |
> |   vm_map_ram_test: p:1, h:0 (usec)               |       5.06% |   (R) 6.63% |
> +--------------------------------------------------+-------------+-------------+
> 
> So there are some nice improvements but also some regressions to explain:
> 
> fix_size_alloc_test with h:1 and p:16,64,256 regress by ~6% on Altra. The
> regression is actually introduced by enabling contpte-mapped 64K blocks in these
> tests, and that regression is reduced (from about 8% if memory serves) by doing
> the barrier batching. I don't have a definite conclusion on the root cause, but
> I've ruled out the differences in the mapping paths in vmalloc. I strongly
> believe this is likely due to the difference in the allocation path; 64K blocks
> are not cached per-cpu so we have to go all the way to the buddy. I'm not sure
> why this doesn't show up on M2 though. Regardless, I'm going to assert that it's
> better to choose 16x reduction in TLB pressure vs 6% on the vmalloc allocation
> call duration.
> 
> Changes since v2 [2]
> ====================
> - Removed the new arch_update_kernel_mappings_[begin|end]() API
> - Switches to arch_[enter|leave]_lazy_mmu_mode() instead for barrier batching
> - Removed clean up to avoid barriers for invalid or user mappings
> 
> Changes since v1 [1]
> ====================
> - Split out the fixes into their own series
> - Added Rbs from Anshuman - Thanks!
> - Added patch to clean up the methods by which huge_pte size is determined
> - Added "#ifndef __PAGETABLE_PMD_FOLDED" around PUD_SIZE in
>   flush_hugetlb_tlb_range()
> - Renamed ___set_ptes() -> set_ptes_anysz()
> - Renamed ___ptep_get_and_clear() -> ptep_get_and_clear_anysz()
> - Fixed typos in commit logs
> - Refactored pXd_valid_not_user() for better reuse
> - Removed TIF_KMAP_UPDATE_PENDING after concluding that single flag is sufficent
> - Concluded the extra isb() in __switch_to() is not required
> - Only call arch_update_kernel_mappings_[begin|end]() for kernel mappings
> 
> Applies on top of v6.14-rc5, which already contains the fixes from [3]. All
> mm selftests run and pass.
> 
> NOTE: Its possible that the changes in patch #10 may cause bugs I found in other
> archs' lazy mmu implementations to become more likely to trigger. I've fixed all
> those bugs in the series at [4], which is now in mm-unstable. But some
> coordination when merging this may be required.
> 
> [1] https://lore.kernel.org/all/20250205151003.88959-1-ryan.roberts@arm.com/
> [2] https://lore.kernel.org/all/20250217140809.1702789-1-ryan.roberts@arm.com/
> [3] https://lore.kernel.org/all/20250217140419.1702389-1-ryan.roberts@arm.com/
> [4] https://lore.kernel.org/all/20250303141542.3371656-1-ryan.roberts@arm.com/
> 
> Thanks,
> Ryan
> 
> Ryan Roberts (11):
>   arm64: hugetlb: Cleanup huge_pte size discovery mechanisms
>   arm64: hugetlb: Refine tlb maintenance scope
>   mm/page_table_check: Batch-check pmds/puds just like ptes
>   arm64/mm: Refactor __set_ptes() and __ptep_get_and_clear()
>   arm64: hugetlb: Use set_ptes_anysz() and ptep_get_and_clear_anysz()
>   arm64/mm: Hoist barriers out of set_ptes_anysz() loop
>   mm/vmalloc: Warn on improper use of vunmap_range()
>   mm/vmalloc: Gracefully unmap huge ptes
>   arm64/mm: Support huge pte-mapped pages in vmap
>   mm/vmalloc: Enter lazy mmu mode while manipulating vmalloc ptes
>   arm64/mm: Batch barriers when updating kernel mappings
> 
>  arch/arm64/include/asm/hugetlb.h     |  29 ++--
>  arch/arm64/include/asm/pgtable.h     | 195 ++++++++++++++++++---------
>  arch/arm64/include/asm/thread_info.h |   2 +
>  arch/arm64/include/asm/vmalloc.h     |  45 +++++++
>  arch/arm64/kernel/process.c          |   9 +-
>  arch/arm64/mm/hugetlbpage.c          |  72 ++++------
>  include/linux/page_table_check.h     |  30 +++--
>  include/linux/vmalloc.h              |   8 ++
>  mm/page_table_check.c                |  34 +++--
>  mm/vmalloc.c                         |  40 +++++-
>  10 files changed, 315 insertions(+), 149 deletions(-)
> 
> --
> 2.43.0
>