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Message-ID: <3f465638-f96f-4d81-87b2-779897c03b21@redhat.com>
Date: Thu, 28 Mar 2024 13:25:04 +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 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@...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
>
> 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
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