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

David Hildenbrand
2024-07-30T16:11:56Z

On 30.07.24 17:19, Usama Arif wrote:
>
>
> 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@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);
>> }
>
>
> 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.

I remember one ugly case in QEMU with postcopy live-migration where we
must not zap zero-filled pages. I am not 100% regarding THP (if it could
be enabled at that point), but imagine the following

1) mmap(), enable THP
2) Migrate a bunch of pages from the source during precopy (writing to
the memory). Might end up creating THPs (during fault/khugepaged)
3) Register UFFD on the VMA
4) Disable new THPs from forming via MADV_NOHUGEPAGE on the VMA
5) Discard any pages that have been re-dirtied or not migrated yet
6) Migrate-on-demand any holes using uffd

If we discard zero-filled pages between 2) and 3) we might get wrong
uffd notifications in 6 for pages that have already been migrated).

I'll have to check if that actually happens in that sequence in QEMU: if
QEMU would disable THP right before 2) we would be safe. But I recall
that it is not the case :/

--
Cheers,

David / dhildenb