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

[Usama Arif]

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;