Re: [PATCH 2/2] mm: support multi-size THP numa balancing

[Baolin Wang]

On 2024/3/28 17:25, David Hildenbrand wrote:
> On 26.03.24 12:51, Baolin Wang wrote:
> > Now the anonymous page allocation already supports multi-size THP (mTHP),
> > but the numa balancing still prohibits mTHP migration even though it 
> > is an
> > exclusive mapping, which is unreasonable.
> >
> > Allow scanning mTHP:
> > Commit 859d4adc3415 ("mm: numa: do not trap faults on shared data section
> > pages") skips shared CoW pages' NUMA page migration to avoid shared data
> > segment migration. In addition, commit 80d47f5de5e3 ("mm: don't try to
> > NUMA-migrate COW pages that have other uses") change to use page_count()
> > to avoid GUP pages migration, that will also skip the mTHP numa scaning.
> > Theoretically, we can use folio_maybe_dma_pinned() to detect the GUP
> > issue, although there is still a GUP race, the issue seems to have been
> > resolved by commit 80d47f5de5e3. Meanwhile, use the 
> > folio_likely_mapped_shared()
> > to skip shared CoW pages though this is not a precise sharers count. To
> > check if the folio is shared, ideally we want to make sure every page is
> > mapped to the same process, but doing that seems expensive and using
> > the estimated mapcount seems can work when running autonuma benchmark.
> >
> > Allow migrating mTHP:
> > As mentioned in the previous thread[1], large folios (including THP) are
> > more susceptible to false sharing issues among threads than 4K base page,
> > leading to pages ping-pong back and forth during numa balancing, which is
> > currently not easy to resolve. Therefore, as a start to support mTHP numa
> > balancing, we can follow the PMD mapped THP's strategy, that means we can
> > reuse the 2-stage filter in should_numa_migrate_memory() to check if the
> > mTHP is being heavily contended among threads (through checking the 
> > CPU id
> > and pid of the last access) to avoid false sharing at some degree. Thus,
> > we can restore all PTE maps upon the first hint page fault of a large 
> > folio
> > to follow the PMD mapped THP's strategy. In the future, we can 
> > continue to
> > optimize the NUMA balancing algorithm to avoid the false sharing issue 
> > with
> > large folios as much as possible.
> >
> > Performance data:
> > Machine environment: 2 nodes, 128 cores Intel(R) Xeon(R) Platinum
> > Base: 2024-03-25 mm-unstable branch
> > Enable mTHP to run autonuma-benchmark
> >
> > mTHP:16K
> > Base                Patched
> > numa01                numa01
> > 224.70                137.23
> > numa01_THREAD_ALLOC        numa01_THREAD_ALLOC
> > 118.05                50.57
> > numa02                numa02
> > 13.45                9.30
> > numa02_SMT            numa02_SMT
> > 14.80                7.43
> >
> > mTHP:64K
> > Base                Patched
> > numa01                numa01
> > 216.15                135.20
> > numa01_THREAD_ALLOC        numa01_THREAD_ALLOC
> > 115.35                46.93
> > numa02                numa02
> > 13.24                9.24
> > numa02_SMT            numa02_SMT
> > 14.67                7.31
> >
> > mTHP:128K
> > Base                Patched
> > numa01                numa01
> > 205.13                140.41
> > numa01_THREAD_ALLOC        numa01_THREAD_ALLOC
> > 112.93                44.78
> > numa02                numa02
> > 13.16                9.19
> > numa02_SMT            numa02_SMT
> > 14.81                7.39
> >
> > [1] 
> > https://lore.kernel.org/all/20231117100745.fnpijbk4xgmals3k@xxxxxxxxxxxxxxxxxxx/
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/memory.c   | 56 +++++++++++++++++++++++++++++++++++++++++++--------
> >   mm/mprotect.c |  3 ++-
> >   2 files changed, 50 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c30fb4b95e15..36191a9c799c 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5068,16 +5068,55 @@ static void numa_rebuild_single_mapping(struct 
> > vm_fault *vmf, struct vm_area_str
> >       update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> >   }
> > +static void numa_rebuild_large_mapping(struct vm_fault *vmf, struct 
> > vm_area_struct *vma,
> > +                       struct folio *folio, pte_t fault_pte, bool 
> > ignore_writable)
> > +{
> > +    int nr = pte_pfn(fault_pte) - folio_pfn(folio);
> > +    unsigned long start = max(vmf->address - nr * PAGE_SIZE, 
> > vma->vm_start);
> > +    unsigned long end = min(start + folio_nr_pages(folio) * 
> > PAGE_SIZE, vma->vm_end);
> > +    pte_t *start_ptep = vmf->pte - (vmf->address - start) / PAGE_SIZE;
> > +    bool pte_write_upgrade = vma_wants_manual_pte_write_upgrade(vma);
> > +    unsigned long addr;
> > +
> > +    /* Restore all PTEs' mapping of the large folio */
> > +    for (addr = start; addr != end; start_ptep++, addr += PAGE_SIZE) {
> > +        pte_t pte, old_pte;
> > +        pte_t ptent = ptep_get(start_ptep);
> > +        bool writable = false;
> > +
> > +        if (!pte_present(ptent) || !pte_protnone(ptent))
> > +            continue;
> > +
> > +        if (vm_normal_folio(vma, addr, ptent) != folio)
> > +            continue;
> > +
>
> Should you be using folio_pte_batch() in the caller to collect all 
> applicable PTEs and then only have function that batch-changes a given 
> nr of PTEs?
>
> (just like we are now batching other stuff)

Seems folio_pte_batch() is not suitable for numa balancing, since we did 
not care about other PTE bits, only care about the protnone bits. And 
after more thinking, I think I can drop the vm_normal_folio() 
validation, since all PTEs are ensured to be within the range of the 
folio size.