[PATCH v2] mm/page_alloc: speeding up the iteration of max_order

[Muchun Song]

When we free a page whose order is very close to MAX_ORDER and greater
than pageblock_order, it wastes some CPU cycles to increase max_order
to MAX_ORDER one by one and check the pageblock migratetype of that page
repeatedly especially when MAX_ORDER is much larger than pageblock_order.

Signed-off-by: Muchun Song <songmuc...@bytedance.com>
---
Changes in v2:
 - Rework the code suggested by Vlastimil. Thanks.

 mm/page_alloc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f91df593bf71..56e603eea1dd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1002,7 +1002,7 @@ static inline void __free_one_page(struct page *page,
        struct page *buddy;
        bool to_tail;
 
-       max_order = min_t(unsigned int, MAX_ORDER, pageblock_order + 1);
+       max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order);
 
        VM_BUG_ON(!zone_is_initialized(zone));
        VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
@@ -1015,7 +1015,7 @@ static inline void __free_one_page(struct page *page,
        VM_BUG_ON_PAGE(bad_range(zone, page), page);
 
 continue_merging:
-       while (order < max_order - 1) {
+       while (order < max_order) {
                if (compaction_capture(capc, page, order, migratetype)) {
                        __mod_zone_freepage_state(zone, -(1 << order),
                                                                migratetype);
@@ -1041,7 +1041,7 @@ static inline void __free_one_page(struct page *page,
                pfn = combined_pfn;
                order++;
        }
-       if (max_order < MAX_ORDER) {
+       if (order < MAX_ORDER - 1) {
                /* If we are here, it means order is >= pageblock_order.
                 * We want to prevent merge between freepages on isolate
                 * pageblock and normal pageblock. Without this, pageblock
@@ -1062,7 +1062,7 @@ static inline void __free_one_page(struct page *page,
                                                is_migrate_isolate(buddy_mt)))
                                goto done_merging;
                }
-               max_order++;
+               max_order = order + 1;
                goto continue_merging;
        }
 
-- 
2.11.0