[patch v6] mm: vmstat_kunit: add synthetic benchmark for mm stats

[David Rientjes <rientjes@google.com>]

From: Christoph Lameter <cl@gentwo.org>

Add a synthetic benchmark that can be used to measure performance of MM
statistics.  This is used to analyze any improvements or regressions in
functions that are frequently used in hot code paths.

This is the first of three tests that measure the execution times of
critical MM functions anticipated to be used by developers to measure the
impact of any core changes that would end up causing workload performance
issues.  Vmstats are embedded into MM hotpaths, so any regression can
cause performance and scalability impact for users.  Google has carried
these tests internally and kept them updated since Christoph posted them
back in 2009 for this purpose.

This can also be used to detect regressions over time, from kernel
version to kernel version, independent of development use cases.

The test is run by KUnit or doing modprobe vmstat_kunit directly.

Sample output:
 KTAP version 1
 1..1
 KTAP version 1
 # Subtest: vmstat
 # module: vmstat_kunit
      1..3
      # vmstat_test_inc_dec_zone_page_state: 10000 ops: inc_zone_page_state -> 8 cycles (4 ns/op), dec_zone_page_state -> 9 cycles (4 ns/op)
      ok 1 vmstat_test_inc_dec_zone_page_state
      # vmstat_test_interleaved_zone_page_state: 10000 ops: inc/dec pair -> 17 cycles (8 ns/op)
      ok 2 vmstat_test_interleaved_zone_page_state
      # vmstat_test_count_vm_event: 10000 ops: count_vm_event -> 4 cycles (2 ns/op)
      ok 3 vmstat_test_count_vm_event
 # vmstat: pass:3 fail:0 skip:0 total:3
 # Totals: pass:3 fail:0 skip:0 total:3
 ok 1 vmstat

Assisted-by: Gemini:gemini-3.8-flash
Signed-off-by: Christoph Lameter <cl@gentwo.org>
Signed-off-by: David Rientjes <rientjes@google.com>
---
v6:
 - rebase on top of 7.4-rc1
 - resend per akpm

DavidH said "Works for me. The tests are rather small and I see value in
them." and I was this >< close to infering that as his Acked-by, but will
leave it to him to do that explicitly.

Once we're happy with this change, I'll apply the same treatment to the
proposed pgalloc and slab tests.

 MAINTAINERS             |   1 +
 mm/Kconfig              |  12 +++
 mm/Makefile             |   1 +
 mm/tests/vmstat_kunit.c | 157 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 171 insertions(+)
 create mode 100644 mm/tests/vmstat_kunit.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 93abbb315065..c6aa67db55cd 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17551,6 +17551,7 @@ F:	mm/mmzone.c
 F:	mm/pagewalk.c
 F:	mm/pgtable-generic.c
 F:	mm/ptdump.c
+F:	mm/tests/vmstat_kunit.c
 F:	mm/util.c
 F:	mm/vmpressure.c
 F:	mm/vmstat.c
diff --git a/mm/Kconfig b/mm/Kconfig
index afff1b25b543..33f269f6e8da 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1490,6 +1490,18 @@ config LAZY_MMU_MODE_KUNIT_TEST
 
 	  If unsure, say N.
 
+config VMSTAT_KUNIT_TEST
+	tristate "KUnit test for VM statistics" if !KUNIT_ALL_TESTS
+	depends on VM_EVENT_COUNTERS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Enable this option to test and benchmark the performance of VM
+	  statistics updates (zone page state and VM event counters), which
+	  are used frequently in hot memory management code paths.
+
+	  If unsure, say N.
+
 config ARCH_HAS_CHECK_WX
 	bool
 
diff --git a/mm/Makefile b/mm/Makefile
index 2a3ec53d62ee..b671c5060a3a 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -148,4 +148,5 @@ obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
 obj-$(CONFIG_EXECMEM) += execmem.o
 obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
 obj-$(CONFIG_LAZY_MMU_MODE_KUNIT_TEST) += tests/lazy_mmu_mode_kunit.o
+obj-$(CONFIG_VMSTAT_KUNIT_TEST) += tests/vmstat_kunit.o
 obj-$(CONFIG_MEM_ALLOC_PROFILING) += alloc_tag.o
diff --git a/mm/tests/vmstat_kunit.c b/mm/tests/vmstat_kunit.c
new file mode 100644
index 000000000000..899c957091d1
--- /dev/null
+++ b/mm/tests/vmstat_kunit.c
@@ -0,0 +1,157 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * KUnit synthetic performance benchmark for VM statistics.
+ *
+ * (C) 2009 Linux Foundation, Christoph Lameter <cl@gentwo.org>
+ * (C) 2026 Google LLC, David Rientjes <rientjes@google.com>
+ */
+#include <kunit/test.h>
+#include <linux/mm.h>
+#include <linux/vmstat.h>
+#include <linux/timex.h>
+#include <linux/ktime.h>
+#include <linux/math64.h>
+
+#define TEST_COUNT 10000
+
+static void vmstat_test_free_page(void *arg)
+{
+	__free_page((struct page *)arg);
+}
+
+/*
+ * Test 1: Sequential inc_zone_page_state() followed by dec_zone_page_state().
+ * Net change to zone counters is 0.
+ */
+static void vmstat_test_inc_dec_zone_page_state(struct kunit *test)
+{
+	struct page *page;
+	cycles_t time1, time2, time;
+	u64 t1_ns, t2_ns;
+	u64 inc_cycles, dec_cycles;
+	u64 inc_ns, dec_ns;
+	unsigned int i;
+
+	page = alloc_page(GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, page);
+	KUNIT_ASSERT_EQ(test, kunit_add_action_or_reset(test, vmstat_test_free_page, page), 0);
+
+	/* Benchmark inc_zone_page_state() */
+	time1 = get_cycles();
+	t1_ns = ktime_get_ns();
+	for (i = 0; i < TEST_COUNT; i++)
+		inc_zone_page_state(page, NR_MLOCK);
+	t2_ns = ktime_get_ns();
+	time2 = get_cycles();
+
+	time = time2 - time1;
+	inc_cycles = div_u64(time, TEST_COUNT);
+	inc_ns = div_u64(t2_ns - t1_ns, TEST_COUNT);
+
+	/* Benchmark dec_zone_page_state() */
+	time1 = get_cycles();
+	t1_ns = ktime_get_ns();
+	for (i = 0; i < TEST_COUNT; i++)
+		dec_zone_page_state(page, NR_MLOCK);
+	t2_ns = ktime_get_ns();
+	time2 = get_cycles();
+
+	time = time2 - time1;
+	dec_cycles = div_u64(time, TEST_COUNT);
+	dec_ns = div_u64(t2_ns - t1_ns, TEST_COUNT);
+
+	if (inc_cycles || dec_cycles)
+		kunit_info(test, "%u ops: inc_zone_page_state -> %llu cycles (%llu ns/op), dec_zone_page_state -> %llu cycles (%llu ns/op)\n",
+			   TEST_COUNT, inc_cycles, inc_ns, dec_cycles, dec_ns);
+	else
+		kunit_info(test, "%u ops: inc_zone_page_state -> %llu ns/op, dec_zone_page_state -> %llu ns/op\n",
+			   TEST_COUNT, inc_ns, dec_ns);
+}
+
+/*
+ * Test 2: Paired inc_zone_page_state() and dec_zone_page_state().
+ * Net change to zone counters is 0.
+ */
+static void vmstat_test_interleaved_zone_page_state(struct kunit *test)
+{
+	struct page *page;
+	cycles_t time1, time2, time;
+	u64 t1_ns, t2_ns;
+	u64 avg_cycles, avg_ns;
+	unsigned int i;
+
+	page = alloc_page(GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, page);
+	KUNIT_ASSERT_EQ(test, kunit_add_action_or_reset(test, vmstat_test_free_page, page), 0);
+
+	time1 = get_cycles();
+	t1_ns = ktime_get_ns();
+	for (i = 0; i < TEST_COUNT; i++) {
+		inc_zone_page_state(page, NR_MLOCK);
+		dec_zone_page_state(page, NR_MLOCK);
+	}
+	t2_ns = ktime_get_ns();
+	time2 = get_cycles();
+
+	time = time2 - time1;
+	avg_cycles = div_u64(time, TEST_COUNT);
+	avg_ns = div_u64(t2_ns - t1_ns, TEST_COUNT);
+
+	if (avg_cycles)
+		kunit_info(test, "%u ops: inc/dec pair -> %llu cycles (%llu ns/op)\n",
+			   TEST_COUNT, avg_cycles, avg_ns);
+	else
+		kunit_info(test, "%u ops: inc/dec pair -> %llu ns/op\n",
+			   TEST_COUNT, avg_ns);
+}
+
+/*
+ * Test 3: count_vm_event() benchmark.
+ * Restores counter balance with count_vm_events(item, -TEST_COUNT).
+ */
+static void vmstat_test_count_vm_event(struct kunit *test)
+{
+	cycles_t time1, time2, time;
+	u64 t1_ns, t2_ns;
+	u64 avg_cycles, avg_ns;
+	unsigned int i;
+
+	time1 = get_cycles();
+	t1_ns = ktime_get_ns();
+	for (i = 0; i < TEST_COUNT; i++)
+		count_vm_event(SLABS_SCANNED);
+	t2_ns = ktime_get_ns();
+	time2 = get_cycles();
+
+	/* Restore balance */
+	count_vm_events(SLABS_SCANNED, -TEST_COUNT);
+
+	time = time2 - time1;
+	avg_cycles = div_u64(time, TEST_COUNT);
+	avg_ns = div_u64(t2_ns - t1_ns, TEST_COUNT);
+
+	if (avg_cycles)
+		kunit_info(test, "%u ops: count_vm_event -> %llu cycles (%llu ns/op)\n",
+			   TEST_COUNT, avg_cycles, avg_ns);
+	else
+		kunit_info(test, "%u ops: count_vm_event -> %llu ns/op\n",
+			   TEST_COUNT, avg_ns);
+}
+
+static struct kunit_case vmstat_test_cases[] = {
+	KUNIT_CASE(vmstat_test_inc_dec_zone_page_state),
+	KUNIT_CASE(vmstat_test_interleaved_zone_page_state),
+	KUNIT_CASE(vmstat_test_count_vm_event),
+	{}
+};
+
+static struct kunit_suite vmstat_test_suite = {
+	.name = "vmstat",
+	.test_cases = vmstat_test_cases,
+};
+kunit_test_suite(vmstat_test_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Christoph Lameter <cl@gentwo.org>");
+MODULE_AUTHOR("David Rientjes <rientjes@google.com>");
+MODULE_DESCRIPTION("KUnit benchmark test for VM statistics");