Messages in this thread | | | Date | Thu, 28 Mar 2024 22:18:28 +0800 | Subject | Re: [PATCH 2/2] mm: support multi-size THP numa balancing | From | Baolin Wang <> |
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On 2024/3/28 20:25, David Hildenbrand wrote: > 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.
IIUC, the protnone PTEs will not set cont-ptes bit.
Another concern is that the protnone PTEs of the large folio might not be contiguous. For example, if a middle section of the large folio has been zapped, we would still like to restore all the protnone PTE mapping for the entire folio. However, folio_pte_batch() seems to only help identify the initial contiguous protnone PTEs.
> Are you sure about that?
Sorry for noise, I am wrong. Folio validation is needed for some corner cases, but I may optimize the code with a simple pfn validation.
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