mm, pcp: decrease PCP high if free pages < high watermark

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.

Link: https://lkml.kernel.org/r/20231016053002.756205-9-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 775abc899e80..b92ab001e146 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1016,6 +1016,7 @@ 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)