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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/3/27 10:04, Huang, Ying wrote:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> 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.
> 
> Because now we can deal with shared mTHP, it appears even possible to
> remove folio_likely_mapped_shared() check?

IMO, the issue solved by commit 859d4adc3415 is about shared CoW 
mapping, and I prefer to measure it in another patch:)

>> 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);
> 
> If start is in the middle of folio, it's possible for end to go beyond
> the end of folio.  So, should be something like below?

Yes, good catch, even though below iteration can skip over the parts 
that exceed the size of that folio.

> 	unsigned long end = min(vmf->address + (folio_nr_pages(folio) - nr) * PAGE_SIZE, vma->vm_end);

Yes, this looks good to me. Will do in next version. Thanks.

>> +	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;
>> +
>> +		if (!ignore_writable) {
>> +			writable = pte_write(pte);
>> +			if (!writable && pte_write_upgrade &&
>> +			    can_change_pte_writable(vma, addr, pte))
>> +				writable = true;
>> +		}
>> +
>> +		old_pte = ptep_modify_prot_start(vma, addr, start_ptep);
>> +		pte = pte_modify(old_pte, vma->vm_page_prot);
>> +		pte = pte_mkyoung(pte);
>> +		if (writable)
>> +			pte = pte_mkwrite(pte, vma);
>> +		ptep_modify_prot_commit(vma, addr, start_ptep, old_pte, pte);
>> +		update_mmu_cache_range(vmf, vma, addr, start_ptep, 1);
> 
> Can this be batched for the whole folio?

I thought about it, but things are a little tricky. The folio may not 
contain continuous protnone PTEs, should skip non-present or 
non-protnone PTEs.

Moreover, it is necessary to define architecture-specified 
ptep_modify_prot_start*_nr and ptep_modify_prot_commit*_nr that can 
handle multiple PTEs, which is in my TODO list including batch numa 
scanning in change_pte_range().