[PATCH -V3 8/9] mm, pcp: decrease PCP high if free pages < high watermark

[Huang Ying]

One target of PCP is to minimize pages in PCP if the system free pages
is too few.  To reach that target, when page reclaiming is active for
the zone (ZONE_RECLAIM_ACTIVE), we will stop increasing PCP high in
allocating path, decrease PCP high and free some pages in freeing
path.  But this may be too late because the background page reclaiming
may introduce latency for some workloads.  So, in this patch, during
page allocation we will detect whether the number of free pages of the
zone is below high watermark.  If so, we will stop increasing PCP high
in allocating path, decrease PCP high and free some pages in freeing
path.  With this, we can reduce the possibility of the premature
background page reclaiming caused by too large PCP.

The high watermark checking is done in allocating path to reduce the
overhead in hotter freeing path.

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        | 33 +++++++++++++++++++++++++++++++--
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ec3f7daedcc7..c88770381aaf 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1018,6 +1018,7 @@ enum zone_flags {
 					 * Cleared when kswapd is woken.
 					 */
 	ZONE_RECLAIM_ACTIVE,		/* kswapd may be scanning the zone. */
+	ZONE_BELOW_HIGH,		/* zone is below high watermark. */
 };
 
 static inline unsigned long zone_managed_pages(struct zone *zone)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8382ad2cdfd4..253fc7d0498e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2407,7 +2407,13 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
 		return min(batch << 2, pcp->high);
 	}
 
-	if (pcp->count >= high && high_min != high_max) {
+	if (high_min == high_max)
+		return high;
+
+	if (test_bit(ZONE_BELOW_HIGH, &zone->flags)) {
+		pcp->high = max(high - (batch << pcp->free_factor), high_min);
+		high = max(pcp->count, high_min);
+	} else if (pcp->count >= high) {
 		int need_high = (batch << pcp->free_factor) + batch;
 
 		/* pcp->high should be large enough to hold batch freed pages */
@@ -2457,6 +2463,10 @@ static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp,
 	if (pcp->count >= high) {
 		free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
 				   pcp, pindex);
+		if (test_bit(ZONE_BELOW_HIGH, &zone->flags) &&
+		    zone_watermark_ok(zone, 0, high_wmark_pages(zone),
+				      ZONE_MOVABLE, 0))
+			clear_bit(ZONE_BELOW_HIGH, &zone->flags);
 	}
 }
 
@@ -2763,7 +2773,7 @@ static int nr_pcp_alloc(struct per_cpu_pages *pcp, struct zone *zone, int order)
 	 * If we had larger pcp->high, we could avoid to allocate from
 	 * zone.
 	 */
-	if (high_min != high_max && !test_bit(ZONE_RECLAIM_ACTIVE, &zone->flags))
+	if (high_min != high_max && !test_bit(ZONE_BELOW_HIGH, &zone->flags))
 		high = pcp->high = min(high + batch, high_max);
 
 	if (!order) {
@@ -3225,6 +3235,25 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 			}
 		}
 
+		/*
+		 * Detect whether the number of free pages is below high
+		 * watermark.  If so, we will decrease pcp->high and free
+		 * PCP pages in free path to reduce the possibility of
+		 * premature page reclaiming.  Detection is done here to
+		 * avoid to do that in hotter free path.
+		 */
+		if (test_bit(ZONE_BELOW_HIGH, &zone->flags))
+			goto check_alloc_wmark;
+
+		mark = high_wmark_pages(zone);
+		if (zone_watermark_fast(zone, order, mark,
+					ac->highest_zoneidx, alloc_flags,
+					gfp_mask))
+			goto try_this_zone;
+		else
+			set_bit(ZONE_BELOW_HIGH, &zone->flags);
+
+check_alloc_wmark:
 		mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
 		if (!zone_watermark_fast(zone, order, mark,
 				       ac->highest_zoneidx, alloc_flags,
-- 
2.39.2