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

[David Hildenbrand <david@redhat.com>]

On 28.03.24 13:07, David Hildenbrand wrote:
> On 28.03.24 12:34, Baolin Wang wrote:
>>
>>
>> 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@techsingularity.net/
>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>> ---
>>>>     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
> 
> You should be able to ignore most bits we care about, which case are you
> concerned about folio_pte_batch() would miss. Hand crafting own
> functions to cover some corner cases nobody cares about is likely a bad
> idea.

Note that the reason why I am asking is that folio_pte_batch() can 
optimize-out repeated ptep_get() with cont-ptes.

-- 
Cheers,

David / dhildenb