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SubjectRe: [QUESTIONS] THP allocation in NUMA fault migration path

On 4/19/19 4:13 AM, Mel Gorman wrote:
> On Thu, Apr 18, 2019 at 09:18:15AM -0700, Yang Shi wrote:
>> On 4/17/19 11:32 PM, Michal Hocko wrote:
>>> On Wed 17-04-19 21:15:41, Yang Shi wrote:
>>>> Hi folks,
>>>> I noticed that there might be new THP allocation in NUMA fault migration
>>>> path (migrate_misplaced_transhuge_page()) even when THP is disabled (set to
>>>> "never"). When THP is set to "never", there should be not any new THP
>>>> allocation, but the migration path is kind of special. So I'm not quite sure
>>>> if this is the expected behavior or not?
>>>> And, it looks this allocation disregards defrag setting too, is this
>>>> expected behavior too?H
>>> Could you point to the specific code? But in general the miTgration path
>> Yes. The code is in migrate_misplaced_transhuge_page() called by
>> do_huge_pmd_numa_page().
>> It would just do:
>> alloc_pages_node(node, (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
>> without checking if transparent_hugepage is enabled or not.
>> THP may be disabled before calling into do_huge_pmd_numa_page(). The
>> do_huge_pmd_wp_page() does check if THP is disabled or not. If THP is
>> disabled, it just tries to allocate 512 base pages.
>>> should allocate the memory matching the migration origin. If the origin
>>> was a THP then I find it quite natural if the target was a huge page as
>> Yes, this is what I would like to confirm. Migration allocates a new THP to
>> replace the old one.
>>> well. How hard the allocation should try is another question and I
>>> suspect we do want to obedy the defrag setting.
>> Yes, I thought so too. However, THP NUMA migration was added in 3.8 by
>> commit b32967f ("mm: numa: Add THP migration for the NUMA working set
>> scanning fault case."). It disregarded defrag setting at the very beginning.
>> So, I'm not quite sure if it was done on purpose or just forgot it.
> It was on purpose as migration due to NUMA misplacement was not intended
> to change the type of page used. It would be impossible to tell in advance
> if locality was more important than the page size from a performance point
> of view. This is particularly relevant if the workload is virtualised and
> there is an expectation that huge pages are preserved. I'm not aware of
> any bugs whereby there was a complaint that the THP migration caused an
> excessive stall. It could be altered of course, but it would be preferred
> to have an example workload demonstrating the problem before making a
> decision.

Thanks a lot for elaborating the idea. I didn't run into any problem at
the moment, just didn't get the thinking behind the choice since other
page fault paths (i.e. wp) do allocate hugepages more aggressively.


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