Date: Thu, 28 Mar 2024 19:34:34 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc: mgorman@...hsingularity.net, wangkefeng.wang@...wei.com,
 jhubbard@...dia.com, ying.huang@...el.com, 21cnbao@...il.com,
 ryan.roberts@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: support multi-size THP numa balancing



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@...ux.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 
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.

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