[PATCH v2 2/4] mm/page_alloc: Perform appropriate batching in drain_pages_zone

[Joshua Hahn <joshua.hahnjy@gmail.com>]

drain_pages_zone completely drains a zone of its pcp free pages by
repeatedly calling free_pcppages_bulk until pcp->count reaches 0.
In this loop, it already performs batched calls to ensure that
free_pcppages_bulk isn't called to free too many pages at once, and
relinquishes & reacquires the lock between each call to prevent
lock starvation from other processes.

However, the current batching does not prevent lock starvation. The
current implementation creates batches of
pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX, which has been seen in
Meta workloads to be up to 64 << 5 == 2048 pages.

While it is true that CONFIG_PCP_BATCH_SCALE_MAX is a config and
indeed can be adjusted by the system admin to be any number from
0 to 6, it's default value of 5 is still too high to be reasonable for
any system.

Instead, let's create batches of pcp->batch pages, which gives a more
reasonable 64 pages per call to free_pcppages_bulk. This gives other
processes a chance to grab the lock and prevents starvation. Each
individual call to drain_pages_zone may take longer, but we avoid the
worst case scenario of completely starving out other system-critical
threads from acquiring the pcp lock while 2048 pages are freed
one-by-one.

Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
 mm/page_alloc.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 77e7d9a5f149..b861b647f184 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2623,8 +2623,7 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
 		spin_lock(&pcp->lock);
 		count = pcp->count;
 		if (count) {
-			int to_drain = min(count,
-				pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
+			int to_drain = min(count, pcp->batch);
 
 			free_pcppages_bulk(zone, to_drain, pcp, 0);
 			count -= to_drain;
-- 
2.47.3