+ lib-test-hierarchical-per-cpu-counters.patch added to mm-new branch

+ lib-test-hierarchical-per-cpu-counters.patch added to mm-new branch

[Andrew Morton]

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The patch titled
     Subject: lib: test hierarchical per-cpu counters
has been added to the -mm mm-new branch.  Its filename is
     lib-test-hierarchical-per-cpu-counters.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-test-hierarchical-per-cpu-counters.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
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fixup patches in mm-new.

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and is updated there most days

------------------------------------------------------
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: lib: test hierarchical per-cpu counters
Date: Tue, 17 Feb 2026 11:10:05 -0500

Introduce Kunit tests for hierarchical per-cpu counters.

Keep track of two sets of hierarchical counters, each meant to have the
same precise sum at any time, but distributed differently across the
topology.

Keep track of an atomic counter along with each hierarchical counter, for
sum validation.

Those tests cover:

- Single-threaded (no concurrency) updates.
- Concurrent updates of counters from various CPUs.

Perform the following validations:

- Compare the precise sum of counters with the sum tracked by
  an atomic counter.
- Compare the precise sum of two sets of hierarchical counters.
- Approximated comparison of hierarchical counter with atomic counter.
- Approximated comparison of two sets of hierarchical counters.
- Validate the bounds of approximation ranges.

Run with the following .kunit/.kunitconfig:

CONFIG_KUNIT=y
CONFIG_SMP=y
CONFIG_PREEMPT=y
CONFIG_NR_CPUS=32
CONFIG_HOTPLUG_CPU=y
CONFIG_PERCPU_COUNTER_TREE_TEST=y

With the following execution (to use SMP):

./tools/testing/kunit/kunit.py run --arch=x86_64 --qemu_args="-smp 12"

Link: https://lkml.kernel.org/r/20260217161006.1105611-3-mathieu.desnoyers@efficios.com
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Christan König <christian.koenig@amd.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Liam R . Howlett" <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Martin Liu <liumartin@google.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/Kconfig                           |   12 
 lib/tests/Makefile                    |    2 
 lib/tests/percpu_counter_tree_kunit.c |  351 ++++++++++++++++++++++++
 3 files changed, 365 insertions(+)

--- a/lib/Kconfig~lib-test-hierarchical-per-cpu-counters
+++ a/lib/Kconfig
@@ -52,6 +52,18 @@ config PACKING_KUNIT_TEST
 
 	  When in doubt, say N.
 
+config PERCPU_COUNTER_TREE_TEST
+	tristate "Hierarchical Per-CPU counter test" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  This builds Kunit tests for the hierarchical per-cpu counters.
+
+	  For more information on KUnit and unit tests in general,
+	  please refer to the KUnit documentation in Documentation/dev-tools/kunit/.
+
+	  When in doubt, say N.
+
 config BITREVERSE
 	tristate
 
--- a/lib/tests/Makefile~lib-test-hierarchical-per-cpu-counters
+++ a/lib/tests/Makefile
@@ -56,4 +56,6 @@ obj-$(CONFIG_UTIL_MACROS_KUNIT) += util_
 obj-$(CONFIG_RATELIMIT_KUNIT_TEST) += test_ratelimit.o
 obj-$(CONFIG_UUID_KUNIT_TEST) += uuid_kunit.o
 
+obj-$(CONFIG_PERCPU_COUNTER_TREE_TEST) += percpu_counter_tree_kunit.o
+
 obj-$(CONFIG_TEST_RUNTIME_MODULE)		+= module/
diff --git a/lib/tests/percpu_counter_tree_kunit.c a/lib/tests/percpu_counter_tree_kunit.c
new file mode 100644
--- /dev/null
+++ a/lib/tests/percpu_counter_tree_kunit.c
@@ -0,0 +1,351 @@
+// SPDX-License-Identifier: GPL-2.0+ OR MIT
+// SPDX-FileCopyrightText: 2026 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+
+#include <kunit/test.h>
+#include <linux/percpu_counter_tree.h>
+#include <linux/kthread.h>
+#include <linux/wait.h>
+#include <linux/random.h>
+
+struct multi_thread_test_data {
+	long increment;
+	int nr_inc;
+	int counter_index;
+};
+
+/* Hierarchical per-CPU counter instances. */
+static struct percpu_counter_tree counter[2];
+static struct percpu_counter_tree_level_item *items[2];
+
+/* Global atomic counters for validation. */
+static atomic_long_t global_counter[2];
+
+static struct wait_queue_head kernel_threads_wq;
+static atomic_t kernel_threads_to_run;
+
+static void complete_work(void)
+{
+	if (atomic_dec_and_test(&kernel_threads_to_run))
+		wake_up(&kernel_threads_wq);
+}
+
+static void hpcc_print_info(struct kunit *test)
+{
+	kunit_info(test, "Running test with %d CPUs\n", num_online_cpus());
+}
+
+static void add_to_counter(int counter_index, unsigned int nr_inc, long increment)
+{
+	unsigned int i;
+
+	for (i = 0; i < nr_inc; i++) {
+		percpu_counter_tree_add(&counter[counter_index], increment);
+		atomic_long_add(increment, &global_counter[counter_index]);
+	}
+}
+
+static void check_counters(struct kunit *test)
+{
+	int counter_index;
+
+	/* Compare each counter with its global counter. */
+	for (counter_index = 0; counter_index < 2; counter_index++) {
+		long v = atomic_long_read(&global_counter[counter_index]);
+		long approx_sum = percpu_counter_tree_approximate_sum(&counter[counter_index]);
+		unsigned long under_accuracy = 0, over_accuracy = 0;
+		long precise_min, precise_max;
+
+		/* Precise comparison. */
+		KUNIT_EXPECT_EQ(test, percpu_counter_tree_precise_sum(&counter[counter_index]), v);
+		KUNIT_EXPECT_EQ(test, 0, percpu_counter_tree_precise_compare_value(&counter[counter_index], v));
+
+		/* Approximate comparison. */
+		KUNIT_EXPECT_EQ(test, 0, percpu_counter_tree_approximate_compare_value(&counter[counter_index], v));
+
+		/* Accuracy limits checks. */
+		percpu_counter_tree_approximate_accuracy_range(&counter[counter_index], &under_accuracy, &over_accuracy);
+
+		KUNIT_EXPECT_GE(test, (long)(approx_sum - (v - under_accuracy)), 0);
+		KUNIT_EXPECT_LE(test, (long)(approx_sum - (v + over_accuracy)), 0);
+		KUNIT_EXPECT_GT(test, (long)(approx_sum - (v - under_accuracy - 1)), 0);
+		KUNIT_EXPECT_LT(test, (long)(approx_sum - (v + over_accuracy + 1)), 0);
+
+		/* Precise min/max range check. */
+		percpu_counter_tree_approximate_min_max_range(approx_sum, under_accuracy, over_accuracy, &precise_min, &precise_max);
+
+		KUNIT_EXPECT_GE(test, v - precise_min, 0);
+		KUNIT_EXPECT_LE(test, v - precise_max, 0);
+		KUNIT_EXPECT_GT(test, v - (precise_min - 1), 0);
+		KUNIT_EXPECT_LT(test, v - (precise_max + 1), 0);
+	}
+	/* Compare each counter with the second counter. */
+	KUNIT_EXPECT_EQ(test, percpu_counter_tree_precise_sum(&counter[0]), percpu_counter_tree_precise_sum(&counter[1]));
+	KUNIT_EXPECT_EQ(test, 0, percpu_counter_tree_precise_compare(&counter[0], &counter[1]));
+	KUNIT_EXPECT_EQ(test, 0, percpu_counter_tree_approximate_compare(&counter[0], &counter[1]));
+}
+
+static int multi_thread_worker_fn(void *data)
+{
+	struct multi_thread_test_data *td = data;
+
+	add_to_counter(td->counter_index, td->nr_inc, td->increment);
+	complete_work();
+	kfree(td);
+	return 0;
+}
+
+static void test_run_on_specific_cpu(struct kunit *test, int target_cpu, int counter_index, unsigned int nr_inc, long increment)
+{
+	struct task_struct *task;
+	struct multi_thread_test_data *td = kzalloc(sizeof(struct multi_thread_test_data), GFP_KERNEL);
+
+	KUNIT_EXPECT_PTR_NE(test, td, NULL);
+	td->increment = increment;
+	td->nr_inc = nr_inc;
+	td->counter_index = counter_index;
+	atomic_inc(&kernel_threads_to_run);
+	task = kthread_run_on_cpu(multi_thread_worker_fn, td, target_cpu, "kunit_multi_thread_worker");
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, task);
+}
+
+static void init_kthreads(void)
+{
+	atomic_set(&kernel_threads_to_run, 1);
+	init_waitqueue_head(&kernel_threads_wq);
+}
+
+static void fini_kthreads(void)
+{
+	/* Release our own reference. */
+	complete_work();
+	/* Wait for all others threads to run. */
+	__wait_event(kernel_threads_wq, (atomic_read(&kernel_threads_to_run) == 0));
+}
+
+static void test_sync_kthreads(void)
+{
+	fini_kthreads();
+	init_kthreads();
+}
+
+static void init_counters(struct kunit *test, unsigned long batch_size)
+{
+	int i, ret;
+
+	for (i = 0; i < 2; i++) {
+		items[i] = kzalloc(percpu_counter_tree_items_size(), GFP_KERNEL);
+		KUNIT_EXPECT_PTR_NE(test, items[i], NULL);
+		ret = percpu_counter_tree_init(&counter[i], items[i], batch_size, GFP_KERNEL);
+		KUNIT_EXPECT_EQ(test, ret, 0);
+
+		atomic_long_set(&global_counter[i], 0);
+	}
+}
+
+static void fini_counters(void)
+{
+	int i;
+
+	for (i = 0; i < 2; i++) {
+		percpu_counter_tree_destroy(&counter[i]);
+		kfree(items[i]);
+	}
+}
+
+enum up_test_inc_type {
+	INC_ONE,
+	INC_MINUS_ONE,
+	INC_RANDOM,
+};
+
+/*
+ * Single-threaded tests. Those use many threads to run on various CPUs,
+ * but synchronize for completion of each thread before running the
+ * next, effectively making sure there are no concurrent updates.
+ */
+static void do_hpcc_test_single_thread(struct kunit *test, int _cpu0, int _cpu1, enum up_test_inc_type type)
+{
+	unsigned long batch_size_order = 5;
+	int cpu0 = _cpu0;
+	int cpu1 = _cpu1;
+	int i;
+
+	init_counters(test, 1UL << batch_size_order);
+	init_kthreads();
+	for (i = 0; i < 10000; i++) {
+		long increment;
+
+		switch (type) {
+		case INC_ONE:
+			increment = 1;
+			break;
+		case INC_MINUS_ONE:
+			increment = -1;
+			break;
+		case INC_RANDOM:
+			increment = (long) get_random_long() % 50000;
+			break;
+		}
+		if (_cpu0 < 0)
+			cpu0 = cpumask_any_distribute(cpu_online_mask);
+		if (_cpu1 < 0)
+			cpu1 = cpumask_any_distribute(cpu_online_mask);
+		test_run_on_specific_cpu(test, cpu0, 0, 1, increment);
+		test_sync_kthreads();
+		test_run_on_specific_cpu(test, cpu1, 1, 1, increment);
+		test_sync_kthreads();
+		check_counters(test);
+	}
+	fini_kthreads();
+	fini_counters();
+}
+
+static void hpcc_test_single_thread_first(struct kunit *test)
+{
+	int cpu = cpumask_first(cpu_online_mask);
+
+	do_hpcc_test_single_thread(test, cpu, cpu, INC_ONE);
+	do_hpcc_test_single_thread(test, cpu, cpu, INC_MINUS_ONE);
+	do_hpcc_test_single_thread(test, cpu, cpu, INC_RANDOM);
+}
+
+static void hpcc_test_single_thread_first_random(struct kunit *test)
+{
+	int cpu = cpumask_first(cpu_online_mask);
+
+	do_hpcc_test_single_thread(test, cpu, -1, INC_ONE);
+	do_hpcc_test_single_thread(test, cpu, -1, INC_MINUS_ONE);
+	do_hpcc_test_single_thread(test, cpu, -1, INC_RANDOM);
+}
+
+static void hpcc_test_single_thread_random(struct kunit *test)
+{
+	do_hpcc_test_single_thread(test, -1, -1, INC_ONE);
+	do_hpcc_test_single_thread(test, -1, -1, INC_MINUS_ONE);
+	do_hpcc_test_single_thread(test, -1, -1, INC_RANDOM);
+}
+
+/* Multi-threaded SMP tests. */
+
+static void do_hpcc_multi_thread_increment_each_cpu(struct kunit *test, unsigned long batch_size, unsigned int nr_inc, long increment)
+{
+	int cpu;
+
+	init_counters(test, batch_size);
+	init_kthreads();
+	for_each_online_cpu(cpu) {
+		test_run_on_specific_cpu(test, cpu, 0, nr_inc, increment);
+		test_run_on_specific_cpu(test, cpu, 1, nr_inc, increment);
+	}
+	fini_kthreads();
+	check_counters(test);
+	fini_counters();
+}
+
+static void do_hpcc_multi_thread_increment_even_cpus(struct kunit *test, unsigned long batch_size, unsigned int nr_inc, long increment)
+{
+	int cpu;
+
+	init_counters(test, batch_size);
+	init_kthreads();
+	for_each_online_cpu(cpu) {
+		test_run_on_specific_cpu(test, cpu, 0, nr_inc, increment);
+		test_run_on_specific_cpu(test, cpu & ~1, 1, nr_inc, increment); /* even cpus. */
+	}
+	fini_kthreads();
+	check_counters(test);
+	fini_counters();
+}
+
+static void do_hpcc_multi_thread_increment_single_cpu(struct kunit *test, unsigned long batch_size, unsigned int nr_inc, long increment)
+{
+	int cpu;
+
+	init_counters(test, batch_size);
+	init_kthreads();
+	for_each_online_cpu(cpu) {
+		test_run_on_specific_cpu(test, cpu, 0, nr_inc, increment);
+		test_run_on_specific_cpu(test, cpumask_first(cpu_online_mask), 1, nr_inc, increment);
+	}
+	fini_kthreads();
+	check_counters(test);
+	fini_counters();
+}
+
+static void do_hpcc_multi_thread_increment_random_cpu(struct kunit *test, unsigned long batch_size, unsigned int nr_inc, long increment)
+{
+	int cpu;
+
+	init_counters(test, batch_size);
+	init_kthreads();
+	for_each_online_cpu(cpu) {
+		test_run_on_specific_cpu(test, cpu, 0, nr_inc, increment);
+		test_run_on_specific_cpu(test, cpumask_any_distribute(cpu_online_mask), 1, nr_inc, increment);
+	}
+	fini_kthreads();
+	check_counters(test);
+	fini_counters();
+}
+
+static void hpcc_test_multi_thread_batch_increment(struct kunit *test)
+{
+	unsigned long batch_size_order;
+
+	for (batch_size_order = 2; batch_size_order < 10; batch_size_order++) {
+		unsigned int nr_inc;
+
+		for (nr_inc = 1; nr_inc < 1024; nr_inc *= 2) {
+			long increment;
+
+			for (increment = 1; increment < 100000; increment *= 10) {
+				do_hpcc_multi_thread_increment_each_cpu(test, 1UL << batch_size_order, nr_inc, increment);
+				do_hpcc_multi_thread_increment_even_cpus(test, 1UL << batch_size_order, nr_inc, increment);
+				do_hpcc_multi_thread_increment_single_cpu(test, 1UL << batch_size_order, nr_inc, increment);
+				do_hpcc_multi_thread_increment_random_cpu(test, 1UL << batch_size_order, nr_inc, increment);
+			}
+		}
+	}
+}
+
+static void hpcc_test_multi_thread_random_walk(struct kunit *test)
+{
+	unsigned long batch_size_order = 5;
+	int loop;
+
+	for (loop = 0; loop < 100; loop++) {
+		int i;
+
+		init_counters(test, 1UL << batch_size_order);
+		init_kthreads();
+		for (i = 0; i < 1000; i++) {
+			long increment = (long) get_random_long() % 512;
+			unsigned int nr_inc = ((unsigned long) get_random_long()) % 1024;
+
+			test_run_on_specific_cpu(test, cpumask_any_distribute(cpu_online_mask), 0, nr_inc, increment);
+			test_run_on_specific_cpu(test, cpumask_any_distribute(cpu_online_mask), 1, nr_inc, increment);
+		}
+		fini_kthreads();
+		check_counters(test);
+		fini_counters();
+	}
+}
+
+static struct kunit_case hpcc_test_cases[] = {
+	KUNIT_CASE(hpcc_print_info),
+	KUNIT_CASE(hpcc_test_single_thread_first),
+	KUNIT_CASE(hpcc_test_single_thread_first_random),
+	KUNIT_CASE(hpcc_test_single_thread_random),
+	KUNIT_CASE(hpcc_test_multi_thread_batch_increment),
+	KUNIT_CASE(hpcc_test_multi_thread_random_walk),
+	{}
+};
+
+static struct kunit_suite hpcc_test_suite = {
+	.name = "percpu_counter_tree",
+	.test_cases = hpcc_test_cases,
+};
+
+kunit_test_suite(hpcc_test_suite);
+
+MODULE_DESCRIPTION("Test cases for hierarchical per-CPU counters");
+MODULE_LICENSE("Dual MIT/GPL");
_

Patches currently in -mm which might be from mathieu.desnoyers@efficios.com are

lib-introduce-hierarchical-per-cpu-counters.patch
lib-test-hierarchical-per-cpu-counters.patch
mm-improve-rss-counter-approximation-accuracy-for-proc-interfaces.patch