[PATCH 3/3] mm/page_alloc: Introduce a new sysctl knob vm.pcp_batch_scale_max

[Yafang Shao <laoar.shao@gmail.com>]

The configuration parameter PCP_BATCH_SCALE_MAX poses challenges for
quickly experimenting with specific workloads in a production environment,
particularly when monitoring latency spikes caused by contention on the
zone->lock. To address this, a new sysctl parameter vm.pcp_batch_scale_max
is introduced as a more practical alternative.

To ultimately mitigate the zone->lock contention issue, several suggestions
have been proposed. One approach involves dividing large zones into multi
smaller zones, as suggested by Matthew[0], while another entails splitting
the zone->lock using a mechanism similar to memory arenas and shifting away
from relying solely on zone_id to identify the range of free lists a
particular page belongs to[1]. However, implementing these solutions is
likely to necessitate a more extended development effort.

Link: https://lore.kernel.org/linux-mm/ZnTrZ9mcAIRodnjx@casper.infradead.org/ [0]
Link: https://lore.kernel.org/linux-mm/20240705130943.htsyhhhzbcptnkcu@techsingularity.net/ [1]
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Rientjes <rientjes@google.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 15 +++++++++++++++
 include/linux/sysctl.h                  |  1 +
 kernel/sysctl.c                         |  2 +-
 mm/Kconfig                              | 11 -----------
 mm/page_alloc.c                         | 22 ++++++++++++++++------
 5 files changed, 33 insertions(+), 18 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index e86c968a7a0e..eb9e5216eefe 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -66,6 +66,7 @@ Currently, these files are in /proc/sys/vm:
 - page_lock_unfairness
 - panic_on_oom
 - percpu_pagelist_high_fraction
+- pcp_batch_scale_max
 - stat_interval
 - stat_refresh
 - numa_stat
@@ -864,6 +865,20 @@ 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.
 
+pcp_batch_scale_max
+===================
+
+In page allocator, PCP (Per-CPU pageset) is refilled and drained in
+batches.  The batch number is scaled automatically to improve page
+allocation/free throughput.  But too large scale factor may hurt
+latency.  This option sets the upper limit of scale factor to limit
+the maximum latency.
+
+The range for this parameter spans from 0 to 6, with a default value of 5.
+The value assigned to 'N' signifies that during each refilling or draining
+process, a maximum of (batch << N) pages will be involved, where "batch"
+represents the default batch size automatically computed by the kernel for
+each zone.
 
 stat_interval
 =============
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 09db2f2e6488..fb797f1c0ef7 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -52,6 +52,7 @@ struct ctl_dir;
 /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
 #define SYSCTL_MAXOLDUID		((void *)&sysctl_vals[10])
 #define SYSCTL_NEG_ONE			((void *)&sysctl_vals[11])
+#define SYSCTL_SIX			((void *)&sysctl_vals[12])
 
 extern const int sysctl_vals[];
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e0b917328cf9..430ac4f58eb7 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -82,7 +82,7 @@
 #endif
 
 /* shared constants to be used in various sysctls */
-const int sysctl_vals[] = { 0, 1, 2, 3, 4, 100, 200, 1000, 3000, INT_MAX, 65535, -1 };
+const int sysctl_vals[] = { 0, 1, 2, 3, 4, 100, 200, 1000, 3000, INT_MAX, 65535, -1, 6 };
 EXPORT_SYMBOL(sysctl_vals);
 
 const unsigned long sysctl_long_vals[] = { 0, 1, LONG_MAX };
diff --git a/mm/Kconfig b/mm/Kconfig
index b4cb45255a54..41fe4c13b7ac 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -663,17 +663,6 @@ config HUGETLB_PAGE_SIZE_VARIABLE
 config CONTIG_ALLOC
 	def_bool (MEMORY_ISOLATION && COMPACTION) || CMA
 
-config PCP_BATCH_SCALE_MAX
-	int "Maximum scale factor of PCP (Per-CPU pageset) batch allocate/free"
-	default 5
-	range 0 6
-	help
-	  In page allocator, PCP (Per-CPU pageset) is refilled and drained in
-	  batches.  The batch number is scaled automatically to improve page
-	  allocation/free throughput.  But too large scale factor may hurt
-	  latency.  This option sets the upper limit of scale factor to limit
-	  the maximum latency.
-
 config PHYS_ADDR_T_64BIT
 	def_bool 64BIT
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2b76754a48e0..703eec22a997 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -273,6 +273,7 @@ int min_free_kbytes = 1024;
 int user_min_free_kbytes = -1;
 static int watermark_boost_factor __read_mostly = 15000;
 static int watermark_scale_factor = 10;
+static int pcp_batch_scale_max = 5;
 
 /* movable_zone is the "real" zone pages in ZONE_MOVABLE are taken from */
 int movable_zone;
@@ -2310,7 +2311,7 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
 	int count = READ_ONCE(pcp->count);
 
 	while (count) {
-		int to_drain = min(count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
+		int to_drain = min(count, pcp->batch << pcp_batch_scale_max);
 		count -= to_drain;
 
 		spin_lock(&pcp->lock);
@@ -2438,7 +2439,7 @@ static int nr_pcp_free(struct per_cpu_pages *pcp, int batch, int high, bool free
 
 	/* Free as much as possible if batch freeing high-order pages. */
 	if (unlikely(free_high))
-		return min(pcp->count, batch << CONFIG_PCP_BATCH_SCALE_MAX);
+		return min(pcp->count, batch << pcp_batch_scale_max);
 
 	/* Check for PCP disabled or boot pageset */
 	if (unlikely(high < batch))
@@ -2470,7 +2471,7 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
 		return 0;
 
 	if (unlikely(free_high)) {
-		pcp->high = max(high - (batch << CONFIG_PCP_BATCH_SCALE_MAX),
+		pcp->high = max(high - (batch << pcp_batch_scale_max),
 				high_min);
 		return 0;
 	}
@@ -2540,9 +2541,9 @@ static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp,
 	} else if (pcp->flags & PCPF_PREV_FREE_HIGH_ORDER) {
 		pcp->flags &= ~PCPF_PREV_FREE_HIGH_ORDER;
 	}
-	if (pcp->free_count < (batch << CONFIG_PCP_BATCH_SCALE_MAX))
+	if (pcp->free_count < (batch << pcp_batch_scale_max))
 		pcp->free_count = min(pcp->free_count + (1 << order),
-				      batch << CONFIG_PCP_BATCH_SCALE_MAX);
+				      batch << pcp_batch_scale_max);
 	high = nr_pcp_high(pcp, zone, batch, free_high);
 	if (pcp->count >= high) {
 		free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
@@ -2884,7 +2885,7 @@ static int nr_pcp_alloc(struct per_cpu_pages *pcp, struct zone *zone, int order)
 		 * subsequent allocation of order-0 pages without any freeing.
 		 */
 		if (batch <= max_nr_alloc &&
-		    pcp->alloc_factor < CONFIG_PCP_BATCH_SCALE_MAX)
+		    pcp->alloc_factor < pcp_batch_scale_max)
 			pcp->alloc_factor++;
 		batch = min(batch, max_nr_alloc);
 	}
@@ -6251,6 +6252,15 @@ static struct ctl_table page_alloc_sysctl_table[] = {
 		.proc_handler	= percpu_pagelist_high_fraction_sysctl_handler,
 		.extra1		= SYSCTL_ZERO,
 	},
+	{
+		.procname	= "pcp_batch_scale_max",
+		.data		= &pcp_batch_scale_max,
+		.maxlen		= sizeof(pcp_batch_scale_max),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_SIX,
+	},
 	{
 		.procname	= "lowmem_reserve_ratio",
 		.data		= &sysctl_lowmem_reserve_ratio,
-- 
2.43.5