On 30/07/2024 16:14, Usama Arif wrote: > > > On 30/07/2024 15:35, David Hildenbrand wrote: >> On 30.07.24 14:45, Usama Arif wrote: >>> The current upstream default policy for THP is always. However, Meta >>> uses madvise in production as the current THP=always policy vastly >>> overprovisions THPs in sparsely accessed memory areas, resulting in >>> excessive memory pressure and premature OOM killing. >>> Using madvise + relying on khugepaged has certain drawbacks over >>> THP=always. Using madvise hints mean THPs aren't "transparent" and >>> require userspace changes. Waiting for khugepaged to scan memory and >>> collapse pages into THP can be slow and unpredictable in terms of performance >>> (i.e. you dont know when the collapse will happen), while production >>> environments require predictable performance. If there is enough memory >>> available, its better for both performance and predictability to have >>> a THP from fault time, i.e. THP=always rather than wait for khugepaged >>> to collapse it, and deal with sparsely populated THPs when the system is >>> running out of memory. >>> >>> This patch-series is an attempt to mitigate the issue of running out of >>> memory when THP is always enabled. During runtime whenever a THP is being >>> faulted in or collapsed by khugepaged, the THP is added to a list. >>> Whenever memory reclaim happens, the kernel runs the deferred_split >>> shrinker which goes through the list and checks if the THP was underutilized, >>> i.e. how many of the base 4K pages of the entire THP were zero-filled. >>> If this number goes above a certain threshold, the shrinker will attempt >>> to split that THP. Then at remap time, the pages that were zero-filled are >>> not remapped, hence saving memory. This method avoids the downside of >>> wasting memory in areas where THP is sparsely filled when THP is always >>> enabled, while still providing the upside THPs like reduced TLB misses without >>> having to use madvise. >>> >>> Meta production workloads that were CPU bound (>99% CPU utilzation) were >>> tested with THP shrinker. The results after 2 hours are as follows: >>> >>> | THP=madvise | THP=always | THP=always >>> | | | + shrinker series >>> | | | + max_ptes_none=409 >>> ----------------------------------------------------------------------------- >>> Performance improvement | - | +1.8% | +1.7% >>> (over THP=madvise) | | | >>> ----------------------------------------------------------------------------- >>> Memory usage | 54.6G | 58.8G (+7.7%) | 55.9G (+2.4%) >>> ----------------------------------------------------------------------------- >>> max_ptes_none=409 means that any THP that has more than 409 out of 512 >>> (80%) zero filled filled pages will be split. >>> >>> To test out the patches, the below commands without the shrinker will >>> invoke OOM killer immediately and kill stress, but will not fail with >>> the shrinker: >>> >>> echo 450 > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none >>> mkdir /sys/fs/cgroup/test >>> echo $$ > /sys/fs/cgroup/test/cgroup.procs >>> echo 20M > /sys/fs/cgroup/test/memory.max >>> echo 0 > /sys/fs/cgroup/test/memory.swap.max >>> # allocate twice memory.max for each stress worker and touch 40/512 of >>> # each THP, i.e. vm-stride 50K. >>> # With the shrinker, max_ptes_none of 470 and below won't invoke OOM >>> # killer. >>> # Without the shrinker, OOM killer is invoked immediately irrespective >>> # of max_ptes_none value and kill stress. >>> stress --vm 1 --vm-bytes 40M --vm-stride 50K >>> >>> Patches 1-2 add back helper functions that were previously removed >>> to operate on page lists (needed by patch 3). >>> Patch 3 is an optimization to free zapped tail pages rather than >>> waiting for page reclaim or migration. >>> Patch 4 is a prerequisite for THP shrinker to not remap zero-filled >>> subpages when splitting THP. >>> Patches 6 adds support for THP shrinker. >>> >>> (This patch-series restarts the work on having a THP shrinker in kernel >>> originally done in >>> https://lore.kernel.org/all/cover.1667454613.git.alexlzhu@xxxxxx/. >>> The THP shrinker in this series is significantly different than the >>> original one, hence its labelled v1 (although the prerequisite to not >>> remap clean subpages is the same).) >> >> As shared previously, there is one issue with uffd (even when currently not active for a VMA!), where we must not zap present page table entries. >> >> Something that is always possible (assuming no GUP pins of course, which) is replacing the zero-filled subpages by shared zeropages. >> >> Is that being done in this patch set already, or are we creating pte_none() entries? >> > > I think thats done in Patch 4/6. In function try_to_unmap_unused, we have below which I think does what you are suggesting? i.e. point to shared zeropage and not clear pte for uffd armed vma. > > if (userfaultfd_armed(pvmw->vma)) { > newpte = pte_mkspecial(pfn_pte(page_to_pfn(ZERO_PAGE(pvmw->address)), > pvmw->vma->vm_page_prot)); > ptep_clear_flush(pvmw->vma, pvmw->address, pvmw->pte); > set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); > } Ah are you suggesting userfaultfd_armed(pvmw->vma) will evaluate to false even if its uffd? I think something like below would work in that case. diff --git a/mm/migrate.c b/mm/migrate.c index 2731ac20ff33..52aa4770fbed 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -206,14 +206,10 @@ static bool try_to_unmap_unused(struct page_vma_mapped_walk *pvmw, if (dirty) return false; - pte_clear_not_present_full(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, false); - - if (userfaultfd_armed(pvmw->vma)) { - newpte = pte_mkspecial(pfn_pte(page_to_pfn(ZERO_PAGE(pvmw->address)), - pvmw->vma->vm_page_prot)); - ptep_clear_flush(pvmw->vma, pvmw->address, pvmw->pte); - set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); - } + newpte = pte_mkspecial(pfn_pte(page_to_pfn(ZERO_PAGE(pvmw->address)), + pvmw->vma->vm_page_prot)); + ptep_clear_flush(pvmw->vma, pvmw->address, pvmw->pte); + set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio)); return true;
