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

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Date:	Fri, 19 Jun 2015 14:30:43 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH] mm, thp: respect MPOL_PREFERRED policy with non-local node

Vlastimil Babka <vbabka@...e.cz> 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@...e.cz>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Michal Hocko <mhocko@...e.cz>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
  

-aneesh

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