page allocator: use a pre-calculated value instead of num_online_nodes() in fast paths

num_online_nodes() is called in a number of places but most often by the
page allocator when deciding whether the zonelist needs to be filtered
based on cpusets or the zonelist cache.  This is actually a heavy function
and touches a number of cache lines.

This patch stores the number of online nodes at boot time and updates the
value when nodes get onlined and offlined.  The value is then used in a
number of important paths in place of num_online_nodes().

[rientjes@google.com: do not override definition of node_set_online() with macro]
Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2f8241f..7b9b601 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -875,7 +875,7 @@ static void return_unused_surplus_pages(struct hstate *h,
 	 * can no longer free unreserved surplus pages. This occurs when
 	 * the nodes with surplus pages have no free pages.
 	 */
-	unsigned long remaining_iterations = num_online_nodes();
+	unsigned long remaining_iterations = nr_online_nodes;
 
 	/* Uncommit the reservation */
 	h->resv_huge_pages -= unused_resv_pages;
@@ -904,7 +904,7 @@ static void return_unused_surplus_pages(struct hstate *h,
 			h->surplus_huge_pages--;
 			h->surplus_huge_pages_node[nid]--;
 			nr_pages--;
-			remaining_iterations = num_online_nodes();
+			remaining_iterations = nr_online_nodes;
 		}
 	}
 }