Re: [BUG] mm, thp: khugepaged can't allocate on requested node when confined to a cpuset

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Oct 08, 2014 at 02:10:50PM -0500, Alex Thorlton wrote:
> Hey everyone,
> 
> I've run into a some frustrating behavior from the khugepaged thread,
> that I'm hoping to get sorted out.  It appears that if you pin
> khugepaged to a cpuset (i.e. node 0),

Why whould you want to pin khugpeaged? Is there a valid use-case?
Looks like userspace shoots to its leg.

> and it begins scanning/collapsing pages for a process on a cpuset that
> doesn't have any memory nodes in common with kugepaged (i.e. node 1),
> then the collapsed pages will all be allocated khugepaged's node (in
> this case node 0), clearly breaking the cpuset boundary set up for the
> process in question.
> 
> I'm aware that there are some known issues with khugepaged performing
> off-node allocations in certain situations, but I believe this is a bit
> of a special circumstance since, in this situation, there's no way for
> khugepaged to perform an allocation on the desired node.
> 
> The problem really stems from the way that we determine the allowed
> memory nodes in get_page_from_freelist.  When we call down to
> cpuset_zone_allowed_softwall, we check current->mems_allowed to
> determine what nodes we're allowed on.  In the case of khugepaged, we'll
> be making allocations for the mm of the process we're collapsing for,
> but we'll be checking the mems_allowed of khugepaged, which can
> obviously cause some problems.

Is there a reason why we should respect cpuset limitation for kernel
threads?

Should we bypass cpuset for PF_KTHREAD completely?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 736d8e1b6381..03a74878ad46 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1960,6 +1960,9 @@ get_page_from_freelist(gfp_t gfp_mask, nodemask_t *nodemask, unsigned int order,
 zonelist_scan:
        zonelist_rescan = false;
 
+       /* Bypass cpuset limitation if allocate from kernel thread context */
+       if (current->flags & PF_KTHREAD)
+               alloc_flags &= ~ALLOC_CPUSET;
        /*
         * Scan zonelist, looking for a zone with enough free.
         * See also __cpuset_node_allowed_softwall() comment in kernel/cpuset.c.
-- 
 Kirill A. Shutemov

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