[PATCH] Fix up per-cpu page batch sizes

The code to clamp batch sizes to 2^n - 1 went missing and an extra
check got added, which must have been a hunk of the "higer order pcp
batch refills" work sneaking in.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b257720..3b21a13 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1772,16 +1772,16 @@ static int __devinit zone_batchsize(struct zone *zone)
                batch = 1;
 
        /*
-        * We will be trying to allcoate bigger chunks of contiguous
-        * memory of the order of fls(batch).  This should result in
-        * better cache coloring.
+        * Clamp the batch to a 2^n - 1 value. Having a power
+        * of 2 value was found to be more likely to have
+        * suboptimal cache aliasing properties in some cases.
         *
-        * A sanity check also to ensure that batch is still in limits.
+        * For example if 2 tasks are alternately allocating
+        * batches of pages, one task can end up with a lot
+        * of pages of one half of the possible page colors
+        * and the other with pages of the other colors.
         */
-       batch = (1 << fls(batch + batch/2));
-
-       if (fls(batch) >= (PAGE_SHIFT + MAX_ORDER - 2))
-               batch = PAGE_SHIFT + ((MAX_ORDER - 1 - PAGE_SHIFT)/2);
+       batch = (1 << (fls(batch + batch/2)-1)) - 1;
 
        return batch;
 }