[PATCH -V2 09/10] mm, pcp: avoid to reduce PCP high unnecessarily

[Huang Ying <ying.huang@xxxxxxxxx>]

In PCP high auto-tuning algorithm, to minimize idle pages in PCP, in
periodic vmstat updating kworker (via refresh_cpu_vm_stats()), we will
decrease PCP high to try to free possible idle PCP pages.  One issue
is that even if the page allocating/freeing depth is larger than
maximal PCP high, we may reduce PCP high unnecessarily.

To avoid the above issue, in this patch, we will track the minimal PCP
page count.  And, the periodic PCP high decrement will not more than
the recent minimal PCP page count.  So, only detected idle pages will
be freed.

On a 2-socket Intel server with 224 logical CPU, we run 8 kbuild
instances in parallel (each with `make -j 28`) in 8 cgroup.  This
simulates the kbuild server that is used by 0-Day kbuild service.
With the patch, The number of pages allocated from zone (instead of
from PCP) decreases 21.4%.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Johannes Weiner <jweiner@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
---
 include/linux/mmzone.h |  1 +
 mm/page_alloc.c        | 15 ++++++++++-----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8a19e2af89df..35b78c7522a7 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -682,6 +682,7 @@ enum zone_watermarks {
 struct per_cpu_pages {
 	spinlock_t lock;	/* Protects lists field */
 	int count;		/* number of pages in the list */
+	int count_min;		/* minimal number of pages in the list recently */
 	int high;		/* high watermark, emptying needed */
 	int high_min;		/* min high watermark */
 	int high_max;		/* max high watermark */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 08b74c65b88a..d7b602822ab3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2166,19 +2166,20 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
  */
 int decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
 {
-	int high_min, to_drain, batch;
+	int high_min, decrease, to_drain, batch;
 	int todo = 0;
 
 	high_min = READ_ONCE(pcp->high_min);
 	batch = READ_ONCE(pcp->batch);
 	/*
-	 * Decrease pcp->high periodically to try to free possible
-	 * idle PCP pages.  And, avoid to free too many pages to
-	 * control latency.
+	 * Decrease pcp->high periodically to free idle PCP pages counted
+	 * via pcp->count_min.  And, avoid to free too many pages to
+	 * control latency.  This caps pcp->high decrement too.
 	 */
 	if (pcp->high > high_min) {
+		decrease = min(pcp->count_min, pcp->high / 5);
 		pcp->high = max3(pcp->count - (batch << PCP_BATCH_SCALE_MAX),
-				 pcp->high * 4 / 5, high_min);
+				 pcp->high - decrease, high_min);
 		if (pcp->high > high_min)
 			todo++;
 	}
@@ -2191,6 +2192,8 @@ int decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
 		todo++;
 	}
 
+	pcp->count_min = pcp->count;
+
 	return todo;
 }
 
@@ -2828,6 +2831,8 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order,
 		page = list_first_entry(list, struct page, pcp_list);
 		list_del(&page->pcp_list);
 		pcp->count -= 1 << order;
+		if (pcp->count < pcp->count_min)
+			pcp->count_min = pcp->count;
 	} while (check_new_pages(page, order));
 
 	return page;
-- 
2.39.2