[PATCH] mm: reduce lock contention of pcp buffer refill

[Alexander Halbuer]

The `rmqueue_bulk` function batches the allocation of multiple elements to
refill the per-CPU buffers into a single hold of the zone lock. Each
element is allocated and checked using the `check_pcp_refill` function.
The check touches every related struct page which is especially expensive
for higher order allocations (huge pages). This patch reduces the time
holding the lock by moving the check out of the critical section similar
to the `rmqueue_buddy` function which allocates a single element.
Measurements of parallel allocation-heavy workloads show a reduction of
the average huge page allocation latency of 50 percent for two cores and
nearly 90 percent for 24 cores.

Signed-off-by: Alexander Halbuer <halbuer@xxxxxxxxxxxxxxxxxxx>
---
 mm/page_alloc.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0745aedebb37..4b80438b1f59 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3119,6 +3119,8 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 {
 	unsigned long flags;
 	int i, allocated = 0;
+	struct list_head *prev_tail = list->prev;
+	struct page *pos, *n;
 
 	spin_lock_irqsave(&zone->lock, flags);
 	for (i = 0; i < count; ++i) {
@@ -3127,9 +3129,6 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 		if (unlikely(page == NULL))
 			break;
 
-		if (unlikely(check_pcp_refill(page, order)))
-			continue;
-
 		/*
 		 * Split buddy pages returned by expand() are received here in
 		 * physical page order. The page is added to the tail of
@@ -3141,7 +3140,6 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 		 * pages are ordered properly.
 		 */
 		list_add_tail(&page->pcp_list, list);
-		allocated++;
 		if (is_migrate_cma(get_pcppage_migratetype(page)))
 			__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
 					      -(1 << order));
@@ -3155,6 +3153,22 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 	 */
 	__mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
 	spin_unlock_irqrestore(&zone->lock, flags);
+
+	/*
+	 * Pages are appended to the pcp list without checking to reduce the
+	 * time holding the zone lock. Checking the appended pages happens right
+	 * after the critical section while still holding the pcp lock.
+	 */
+	pos = list_first_entry(prev_tail, struct page, pcp_list);
+	list_for_each_entry_safe_from(pos, n, list, pcp_list) {
+		if (unlikely(check_pcp_refill(pos, order))) {
+			list_del(&pos->pcp_list);
+			continue;
+		}
+
+		allocated++;
+	}
+
 	return allocated;
 }
 
-- 
2.39.1