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

[Usama Arif]

On 30/07/2024 17:11, David Hildenbrand wrote:
> 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@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.
> 
> 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 :/
> 
> 

Thanks for the example!

Just to understand the issue better, as I am not very familiar with live-migration code, the problem is only for zero-filled pages that were migrated, right? If a THP is created and a subpage of it was a zero-page that was migrated and its split before VMA is armed with uffd, userfaultfd_armed(pvmw->vma) will return false when splitting and it will become pte_none. And afterwards when the destination faults on it, uffd will see that its pte_clear and will request the zero-page back from source. Uffd will then have to get the page again from source.

If I understand the example correctly, the below diff over patch 6 should be good? i.e. just point to the empty_zero_page instead of doing pte_clear. This should still use the same amount of memory, although ptep_clear_flush means it might be slighly more expensive.

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;