Re: [PATCH] mm, thp: respect MPOL_PREFERRED policy with non-local node

From: Aneesh Kumar K.V
Date: Fri Jun 19 2015 - 05:01:14 EST

Vlastimil Babka <vbabka@xxxxxxx> writes:

> Since commit 077fcf116c8c ("mm/thp: allocate transparent hugepages on local
> node"), we handle THP allocations on page fault in a special way - for
> non-interleave memory policies, the allocation is only attempted on the node
> local to the current CPU, if the policy's nodemask allows the node.
> This is motivated by the assumption that THP benefits cannot offset the cost
> of remote accesses, so it's better to fallback to base pages on the local node
> (which might still be available, while huge pages are not due to
> fragmentation) than to allocate huge pages on a remote node.
> The nodemask check prevents us from violating e.g. MPOL_BIND policies where
> the local node is not among the allowed nodes. However, the current
> implementation can still give surprising results for the MPOL_PREFERRED policy
> when the preferred node is different than the current CPU's local node.
> In such case we should honor the preferred node and not use the local node,
> which is what this patch does. If hugepage allocation on the preferred node
> fails, we fall back to base pages and don't try other nodes, with the same
> motivation as is done for the local node hugepage allocations.
> The patch also moves the MPOL_INTERLEAVE check around to simplify the hugepage
> specific test.
> The difference can be demonstrated using in-tree transhuge-stress test on the
> following 2-node machine where half memory on one node was occupied to show
> the difference.

> Without -p parameter, hugepage restriction to CPU-local node works as before.
> Fixes: 077fcf116c8c ("mm/thp: allocate transparent hugepages on local node")
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>


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