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); }
