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

[Mel Gorman <mgorman@techsingularity.net>]

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.

-- 
Mel Gorman
SUSE Labs