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mm: huge_memory: batch tlb flush when splitting a pte-mapped THP
I can observe an obvious tlb flush hotspot when splitting a pte-mapped THP on my ARM64 server, and the distribution of this hotspot is as follows: - 16.85% split_huge_page_to_list + 7.80% down_write - 7.49% try_to_migrate - 7.48% rmap_walk_anon 7.23% ptep_clear_flush + 1.52% __split_huge_page The reason is that the split_huge_page_to_list() will build migration entries for each subpage of a pte-mapped Anon THP by try_to_migrate(), or unmap for file THP, and it will clear and tlb flush for each subpage's pte. Moreover, the split_huge_page_to_list() will set TTU_SPLIT_HUGE_PMD flag to ensure the THP is already a pte-mapped THP before splitting it to some normal pages. Actually, there is no need to flush tlb for each subpage immediately, instead we can batch tlb flush for the pte-mapped THP to improve the performance. After this patch, we can see the batch tlb flush can improve the latency obviously when running thpscale. k6.5-base patched Amean fault-both-1 1071.17 ( 0.00%) 901.83 * 15.81%* Amean fault-both-3 2386.08 ( 0.00%) 1865.32 * 21.82%* Amean fault-both-5 2851.10 ( 0.00%) 2273.84 * 20.25%* Amean fault-both-7 3679.91 ( 0.00%) 2881.66 * 21.69%* Amean fault-both-12 5916.66 ( 0.00%) 4369.55 * 26.15%* Amean fault-both-18 7981.36 ( 0.00%) 6303.57 * 21.02%* Amean fault-both-24 10950.79 ( 0.00%) 8752.56 * 20.07%* Amean fault-both-30 14077.35 ( 0.00%) 10170.01 * 27.76%* Amean fault-both-32 13061.57 ( 0.00%) 11630.08 * 10.96%* Link: https://lkml.kernel.org/r/431d9fb6823036369dcb1d3b2f63732f01df21a7.1698488264.git.baolin.wang@linux.alibaba.com Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> Reviewed-by: "Huang, Ying" <ying.huang@intel.com> Reviewed-by: Yang Shi <shy828301@gmail.com> Reviewed-by: Alistair Popple <apopple@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@ -2379,7 +2379,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
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static void unmap_folio(struct folio *folio)
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{
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enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
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TTU_SYNC;
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TTU_SYNC | TTU_BATCH_FLUSH;
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VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
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@ -2392,6 +2392,8 @@ static void unmap_folio(struct folio *folio)
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try_to_migrate(folio, ttu_flags);
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else
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try_to_unmap(folio, ttu_flags | TTU_IGNORE_MLOCK);
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try_to_unmap_flush();
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}
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static void remap_page(struct folio *folio, unsigned long nr)
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