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

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Date: Thu, 28 Mar 2024 10:25:05 +0100
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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 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)


-- 
Cheers,

David / dhildenb