Messages in this thread | | | Subject | Re: [QUESTIONS] THP allocation in NUMA fault migration path | From | Yang Shi <> | Date | Fri, 19 Apr 2019 09:28:21 -0700 |
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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), >> HPAGE_PMD_ORDER); >> 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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