Re: [PATCH 0/6] mm: split underutilized THPs

Usama Arif
2024-07-30T15:14:09Z

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@fb.com/.
>> 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);
}