mempolicy: use MPOL_F_LOCAL to Indicate Preferred Local Policy

Now that we're using "preferred local" policy for system default, we need to
make this as fast as possible.  Because of the variable size of the mempolicy
structure [based on size of nodemasks], the preferred_node may be in a
different cacheline from the mode.  This can result in accessing an extra
cacheline in the normal case of system default policy.  Suspect this is the
cause of an observed 2-3% slowdown in page fault testing relative to kernel
without this patch series.

To alleviate this, use an internal mode flag, MPOL_F_LOCAL in the mempolicy
flags member which is guaranteed [?] to be in the same cacheline as the mode
itself.

Verified that reworked mempolicy now performs slightly better on 25-rc8-mm1
for both anon and shmem segments with system default and vma [preferred local]
policy.

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 6 deletions

diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt
index 13cca5a3cf17..bad16d3f6a47 100644
--- a/Documentation/vm/numa_memory_policy.txt
+++ b/Documentation/vm/numa_memory_policy.txt
@@ -176,12 +176,11 @@ Components of Memory Policies
         containing the cpu where the allocation takes place.
 
             Internally, the Preferred policy uses a single node--the
-            preferred_node member of struct mempolicy.  A "distinguished
-            value of this preferred_node, currently '-1', is interpreted
-            as "the node containing the cpu where the allocation takes
-            place"--local allocation.  "Local" allocation policy can be
-            viewed as a Preferred policy that starts at the node containing
-            the cpu where the allocation takes place.
+            preferred_node member of struct mempolicy.  When the internal
+            mode flag MPOL_F_LOCAL is set, the preferred_node is ignored and
+            the policy is interpreted as local allocation.  "Local" allocation
+            policy can be viewed as a Preferred policy that starts at the node
+            containing the cpu where the allocation takes place.
 
             It is possible for the user to specify that local allocation is
             always preferred by passing an empty nodemask with this mode.