Re: [PATCH 1/2] mm: page allocator: Initialise ZLC for first zone eligible for zone_reclaim

[Christoph Lameter <cl@linux.com>]

Hmmm... Maybe we can bypass the checks?

Subject: [page allocator] Do not check watermarks if there is a page available on the per cpu freelists

One should be able to grab a page from the per cpu freelists if available.
The pages on the per cpu freelists are not accounted for in VM statistics
so getting a page from there has no impact on reclaim.

Check for this condition in get_page_from_freelist and short circuit
to the call to buffered_rmqueue if so.

Note that there is a race here. We may deplete the reserve pools by
one page if either the process is rescheduled on a different processor
or if another process grabs the last page from the per cpu freelist.

Signed-off-by: Christoph Lameter <cl@linux.com>

---
 mm/page_alloc.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c	2011-07-20 15:27:20.544825852 -0500
+++ linux-2.6/mm/page_alloc.c	2011-07-20 15:30:05.314824797 -0500
@@ -1666,6 +1666,16 @@ zonelist_scan:
 			!cpuset_zone_allowed_softwall(zone, gfp_mask))
 				goto try_next_zone;

+		/*
+		 * Short circuit allocation if we have a usable object on
+		 * the percpu freelist. Note that this can only be an
+		 * optimization since there is no guarantee that we will
+		 * be executing on the same cpu. Another process could also
+		 * be scheduled and take the available page from us.
+		 */
+		if (order == 0 && this_cpu_read(zone->pageset->pcp.count))
+			goto try_this_zone;
+
 		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
 		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
 			unsigned long mark;

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