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SubjectRe: [PATCH 8/8] migrate_pages: batch flushing TLB
Date
Zi Yan <ziy@nvidia.com> writes:

> On 26 Dec 2022, at 19:28, Huang Ying wrote:
>
>> The TLB flushing will cost quite some CPU cycles during the folio
>> migration in some situations. For example, when migrate a folio of a
>> process with multiple active threads that run on multiple CPUs. After
>> batching the _unmap and _move in migrate_pages(), the TLB flushing can
>> be batched easily with the existing TLB flush batching mechanism.
>> This patch implements that.
>>
>> We use the following test case to test the patch.
>>
>> On a 2-socket Intel server,
>>
>> - Run pmbench memory accessing benchmark
>>
>> - Run `migratepages` to migrate pages of pmbench between node 0 and
>> node 1 back and forth.
>>
>> With the patch, the TLB flushing IPI reduces 99.1% during the test and
>> the number of pages migrated successfully per second increases 291.7%.
>>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Yang Shi <shy828301@gmail.com>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Bharata B Rao <bharata@amd.com>
>> Cc: Alistair Popple <apopple@nvidia.com>
>> Cc: haoxin <xhao@linux.alibaba.com>
>> ---
>> mm/migrate.c | 4 +++-
>> mm/rmap.c | 20 +++++++++++++++++---
>> 2 files changed, 20 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 70a40b8fee1f..d7413164e748 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1215,7 +1215,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>> /* Establish migration ptes */
>> VM_BUG_ON_FOLIO(folio_test_anon(src) &&
>> !folio_test_ksm(src) && !anon_vma, src);
>> - try_to_migrate(src, 0);
>> + try_to_migrate(src, TTU_BATCH_FLUSH);
>> page_was_mapped = 1;
>> }
>>
>> @@ -1732,6 +1732,8 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>> stats->nr_thp_failed += thp_retry;
>> stats->nr_failed_pages += nr_retry_pages;
>> move:
>> + try_to_unmap_flush();
>> +
>> retry = 1;
>> for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
>> retry = 0;
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index b616870a09be..2e125f3e462e 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1976,7 +1976,21 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>> } else {
>> flush_cache_page(vma, address, pte_pfn(*pvmw.pte));
>> /* Nuke the page table entry. */
>> - pteval = ptep_clear_flush(vma, address, pvmw.pte);
>> + if (should_defer_flush(mm, flags)) {
>> + /*
>> + * We clear the PTE but do not flush so potentially
>> + * a remote CPU could still be writing to the folio.
>> + * If the entry was previously clean then the
>> + * architecture must guarantee that a clear->dirty
>> + * transition on a cached TLB entry is written through
>> + * and traps if the PTE is unmapped.
>> + */
>> + pteval = ptep_get_and_clear(mm, address, pvmw.pte);
>> +
>> + set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
>> + } else {
>> + pteval = ptep_clear_flush(vma, address, pvmw.pte);
>> + }
>> }
>>
>
> This is only for PTE mapped pages, right? We also need something similar
> in set_pmd_migration_entry() in mm/huge_memory.c for PMD-mapped THPs.
> Oh, since you limit NR_MAX_BATCHED_MIGRATION to HPAGE_PMD_NR and count
> nr_pages with folio_nr_pages(), THPs will only be migrated one by one.
> This is not obvious from the cover letter.
>
> Are you planning to support batched THP migration? If not, it might be
> better to update cover letter to be explicit about it and add comments
> in migrate_pages(). It would be nice to also note that we need to
> increase NR_MAX_BATCHED_MIGRATION beyond HPAGE_PMD_NR and make similar
> changes in set_pmd_migration_entry() to get batched THP migration support.

For now, I have no plan to support batching THP migration. Because the
overhead of THP TLB shootdown is only 1/512 of that of the 4KB normal
page. I will add some words in patch description for that.

Best Regards,
Huang, Ying

>> /* Set the dirty flag on the folio now the pte is gone. */
>> @@ -2148,10 +2162,10 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags)
>>
>> /*
>> * Migration always ignores mlock and only supports TTU_RMAP_LOCKED and
>> - * TTU_SPLIT_HUGE_PMD and TTU_SYNC flags.
>> + * TTU_SPLIT_HUGE_PMD, TTU_SYNC, and TTU_BATCH_FLUSH flags.
>> */
>> if (WARN_ON_ONCE(flags & ~(TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD |
>> - TTU_SYNC)))
>> + TTU_SYNC | TTU_BATCH_FLUSH)))
>> return;
>>
>> if (folio_is_zone_device(folio) &&
>> --
>> 2.35.1

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