[PATCH 3/3] hugetlb: Decrease hugetlb_lock cycling in gather_surplus_huge_pages

[Adam Litke <agl@us.ibm.com>]

To reduce hugetlb_lock acquisitions and releases when freeing excess
surplus pages, scan the page list in two parts.  First, transfer the needed
pages to the hugetlb pool.  Then drop the lock and free the remaining
pages back to the buddy allocator.

In the common case there are zero excess pages and no lock operations are
required.

Thanks Mel Gorman for this improvement.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Cc: Mel Gorman <mel@csn.ul.ie>
---

 mm/hugetlb.c |   17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f6ce740..b0d48bf 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -350,11 +350,19 @@ retry:
 	resv_huge_pages += delta;
 	ret = 0;
 free:
+	/* Free the needed pages to the hugetlb pool */
 	list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
+		if ((--needed) < 0)
+			break;
 		list_del(&page->lru);
-		if ((--needed) >= 0)
-			enqueue_huge_page(page);
-		else {
+		enqueue_huge_page(page);
+	}
+
+	/* Free unnecessary surplus pages to the buddy allocator */
+	if (!list_empty(&surplus_list)) {
+		spin_unlock(&hugetlb_lock);
+		list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
+			list_del(&page->lru);
 			/*
 			 * The page has a reference count of zero already, so
 			 * call free_huge_page directly instead of using
@@ -362,10 +370,9 @@ free:
 			 * unlocked which is safe because free_huge_page takes
 			 * hugetlb_lock before deciding how to free the page.
 			 */
-			spin_unlock(&hugetlb_lock);
 			free_huge_page(page);
-			spin_lock(&hugetlb_lock);
 		}
+		spin_lock(&hugetlb_lock);
 	}
 
 	return ret;

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