[PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction

[Mel Gorman]

This introduces a new sysctl vm.percpu_pagelist_high_fraction. It is
similar to the old vm.percpu_pagelist_fraction except it only adjusts
pcp->high to potentially reduce zone->lock contention while preserving
allocation latency when PCP lists have to be refilled.

  # grep -E "high:|batch" /proc/zoneinfo | tail -2
              high:  649
              batch: 63

  # sysctl vm.percpu_pagelist_high_fraction=8
  # grep -E "high:|batch" /proc/zoneinfo | tail -2
              high:  35071
              batch: 63

  # sysctl vm.percpu_pagelist_high_fraction=64
              high:  4383
              batch: 63

  # sysctl vm.percpu_pagelist_high_fraction=0
              high:  649
              batch: 63

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/vm.rst | 20 +++++++++
 include/linux/mmzone.h                  |  3 ++
 kernel/sysctl.c                         |  8 ++++
 mm/page_alloc.c                         | 56 +++++++++++++++++++++++--
 4 files changed, 83 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 2fcafccb53a8..415f2aebf59b 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -64,6 +64,7 @@ files can be found in mm/swap.c.
 - overcommit_ratio
 - page-cluster
 - panic_on_oom
+- percpu_pagelist_high_fraction
 - stat_interval
 - stat_refresh
 - numa_stat
@@ -789,6 +790,25 @@ panic_on_oom=2+kdump gives you very strong tool to investigate
 why oom happens. You can get snapshot.
 
 
+percpu_pagelist_high_fraction
+=============================
+
+This is the fraction of pages at most (high mark pcp->high) in each zone that
+are allocated for each per cpu page list.  The min value for this is 8.  It
+means that we do not allow more than 1/8th of pages in each zone to be
+allocated in any single per_cpu_pagelist.  This entry only changes the value
+of hot per cpu pagelists.  User can specify a number like 100 to allocate
+1/100th of each zone to each per cpu page list.
+
+The batch value of each per cpu pagelist remains the same regardless of the
+value of the high fraction so allocation latencies are unaffected.
+
+The initial value is zero. Kernel uses this value to set the high pcp->high
+mark based on the low watermark for the zone and the number of local
+online CPUs.  If the user writes '0' to this sysctl, it will revert to
+this default behavior.
+
+
 stat_interval
 =============
 
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index a0606239a167..e20d98c62beb 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1040,12 +1040,15 @@ int watermark_scale_factor_sysctl_handler(struct ctl_table *, int, void *,
 extern int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES];
 int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *, int, void *,
 		size_t *, loff_t *);
+int percpu_pagelist_high_fraction_sysctl_handler(struct ctl_table *, int,
+		void *, size_t *, loff_t *);
 int sysctl_min_unmapped_ratio_sysctl_handler(struct ctl_table *, int,
 		void *, size_t *, loff_t *);
 int sysctl_min_slab_ratio_sysctl_handler(struct ctl_table *, int,
 		void *, size_t *, loff_t *);
 int numa_zonelist_order_handler(struct ctl_table *, int,
 		void *, size_t *, loff_t *);
+extern int percpu_pagelist_high_fraction;
 extern char numa_zonelist_order[];
 #define NUMA_ZONELIST_ORDER_LEN	16
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 4e5ac50a1af0..9eb9d1f987d9 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2889,6 +2889,14 @@ static struct ctl_table vm_table[] = {
 		.extra1		= SYSCTL_ONE,
 		.extra2		= &one_thousand,
 	},
+	{
+		.procname	= "percpu_pagelist_high_fraction",
+		.data		= &percpu_pagelist_high_fraction,
+		.maxlen		= sizeof(percpu_pagelist_high_fraction),
+		.mode		= 0644,
+		.proc_handler	= percpu_pagelist_high_fraction_sysctl_handler,
+		.extra1		= SYSCTL_ZERO,
+	},
 	{
 		.procname	= "page_lock_unfairness",
 		.data		= &sysctl_page_lock_unfairness,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d8f8044781c4..08f9e5027ed4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -120,6 +120,7 @@ typedef int __bitwise fpi_t;
 
 /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
 static DEFINE_MUTEX(pcp_batch_high_lock);
+#define MIN_PERCPU_PAGELIST_HIGH_FRACTION (8)
 
 struct pagesets {
 	local_lock_t lock;
@@ -181,6 +182,7 @@ EXPORT_SYMBOL(_totalram_pages);
 unsigned long totalreserve_pages __read_mostly;
 unsigned long totalcma_pages __read_mostly;
 
+int percpu_pagelist_high_fraction;
 gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
 DEFINE_STATIC_KEY_MAYBE(CONFIG_INIT_ON_ALLOC_DEFAULT_ON, init_on_alloc);
 EXPORT_SYMBOL(init_on_alloc);
@@ -6670,7 +6672,8 @@ static int zone_batchsize(struct zone *zone)
 #endif
 }
 
-static int zone_highsize(struct zone *zone, int cpu_online)
+static int
+zone_highsize(struct zone *zone, unsigned long total_pages, int cpu_online)
 {
 #ifdef CONFIG_MMU
 	int high;
@@ -6683,7 +6686,7 @@ static int zone_highsize(struct zone *zone, int cpu_online)
 	 * not be online yet.
 	 */
 	nr_local_cpus = max(1U, cpumask_weight(cpumask_of_node(zone_to_nid(zone)))) + cpu_online;
-	high = low_wmark_pages(zone) / nr_local_cpus;
+	high = total_pages / nr_local_cpus;
 
 	return high;
 #else
@@ -6749,14 +6752,21 @@ static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long h
 
 /*
  * Calculate and set new high and batch values for all per-cpu pagesets of a
- * zone based on the zone's size.
+ * zone based on the zone's size and the percpu_pagelist_high_fraction sysctl.
  */
 static void zone_set_pageset_high_and_batch(struct zone *zone, int cpu_online)
 {
 	int new_high, new_batch;
 
+	if (!percpu_pagelist_high_fraction) {
+		new_high = zone_highsize(zone, low_wmark_pages(zone), cpu_online);
+	} else {
+		new_high = zone_highsize(zone,
+			zone_managed_pages(zone) / percpu_pagelist_high_fraction,
+			cpu_online);
+	}
+
 	new_batch = max(1, zone_batchsize(zone));
-	new_high = zone_highsize(zone, cpu_online);
 
 	if (zone->pageset_high == new_high &&
 	    zone->pageset_batch == new_batch)
@@ -8443,6 +8453,44 @@ int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *table, int write,
 	return 0;
 }
 
+/*
+ * percpu_pagelist_high_fraction - changes the pcp->high for each zone on each
+ * cpu. It is the fraction of total pages in each zone that a hot per cpu
+ * pagelist can have before it gets flushed back to buddy allocator.
+ */
+int percpu_pagelist_high_fraction_sysctl_handler(struct ctl_table *table,
+		int write, void *buffer, size_t *length, loff_t *ppos)
+{
+	struct zone *zone;
+	int old_percpu_pagelist_high_fraction;
+	int ret;
+
+	mutex_lock(&pcp_batch_high_lock);
+	old_percpu_pagelist_high_fraction = percpu_pagelist_high_fraction;
+
+	ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
+	if (!write || ret < 0)
+		goto out;
+
+	/* Sanity checking to avoid pcp imbalance */
+	if (percpu_pagelist_high_fraction &&
+	    percpu_pagelist_high_fraction < MIN_PERCPU_PAGELIST_HIGH_FRACTION) {
+		percpu_pagelist_high_fraction = old_percpu_pagelist_high_fraction;
+		ret = -EINVAL;
+		goto out;
+	}
+
+	/* No change? */
+	if (percpu_pagelist_high_fraction == old_percpu_pagelist_high_fraction)
+		goto out;
+
+	for_each_populated_zone(zone)
+		zone_set_pageset_high_and_batch(zone, 0);
+out:
+	mutex_unlock(&pcp_batch_high_lock);
+	return ret;
+}
+
 #ifndef __HAVE_ARCH_RESERVED_KERNEL_PAGES
 /*
  * Returns the number of pages that arch has reserved but
-- 
2.26.2