[RFC PATCH v5 0/2] mm: Fix OOM killer inaccuracy on large many-core systems

[RFC PATCH v5 0/2] mm: Fix OOM killer inaccuracy on large many-core systems

[Mathieu Desnoyers]

Introduce hierarchical per-cpu counters and use them for rss tracking to
fix the per-mm RSS tracking which has become too inaccurate for OOM
killer purposes on large many-core systems.

The approach proposed here is to replace this by the hierarchical
per-cpu counters, which bounds the inaccuracy based on the system
topology with O(N*logN).

Testing and feedback are welcome!

Thanks,

Mathieu

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Martin Liu <liumartin@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: christian.koenig@amd.com
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Sweet Tea Dorminy <sweettea@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R . Howlett" <Liam.Howlett@Oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-mm@kvack.org
Cc: linux-trace-kernel@vger.kernel.org
Cc: Yu Zhao <yuzhao@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>

Mathieu Desnoyers (2):
  lib: Introduce hierarchical per-cpu counters
  mm: Fix OOM killer inaccuracy on large many-core systems

 include/linux/mm.h                  |  10 +-
 include/linux/mm_types.h            |   4 +-
 include/linux/percpu_counter_tree.h | 108 ++++++++
 include/trace/events/kmem.h         |   2 +-
 kernel/fork.c                       |  31 ++-
 lib/Makefile                        |   1 +
 lib/percpu_counter_tree.c           | 393 ++++++++++++++++++++++++++++
 7 files changed, 532 insertions(+), 17 deletions(-)
 create mode 100644 include/linux/percpu_counter_tree.h
 create mode 100644 lib/percpu_counter_tree.c

-- 
2.39.5

[RFC PATCH v5 1/2] lib: Introduce hierarchical per-cpu counters

[Mathieu Desnoyers]

* Motivation

The purpose of this hierarchical split-counter scheme is to:

- Minimize contention when incrementing and decrementing counters,
- Provide fast access to a sum approximation,
- Provide a sum approximation with an acceptable accuracy level when
  scaling to many-core systems.
- Provide approximate and precise comparison of two counters, and
  between a counter and a value.

It aims at fixing the per-mm RSS tracking which has become too
inaccurate for OOM killer purposes on large many-core systems [1].

* Design

The hierarchical per-CPU counters propagate a sum approximation through
a N-way tree. When reaching the batch size, the carry is propagated
through a binary tree which consists of logN(nr_cpu_ids) levels. The
batch size for each level is twice the batch size of the prior level.

Example propagation diagram with 8 cpus through a binary tree:

Level 0:  0    1    2    3    4    5    6    7
          |   /     |   /     |   /     |   /
          |  /      |  /      |  /      |  /
          | /       | /       | /       | /
Level 1:  0         1         2         3
          |       /           |       /
          |    /              |    /
          | /                 | /
Level 2:  0                   1
          |               /
          |         /
          |   /
Level 3:  0

For a binary tree, the maximum inaccuracy is bound by:
   batch_size * log2(nr_cpus) * nr_cpus
which evolves with O(n*log(n)) as the number of CPUs increases.

For a N-way tree, the maximum inaccuracy can be pre-calculated
based on the the N-arity of each level and the batch size.

Link: https://lore.kernel.org/lkml/20250331223516.7810-2-sweettea-kernel@dorminy.me/ # [1]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Martin Liu <liumartin@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: christian.koenig@amd.com
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Sweet Tea Dorminy <sweettea@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R . Howlett" <Liam.Howlett@Oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-mm@kvack.org
Cc: linux-trace-kernel@vger.kernel.org
Cc: Yu Zhao <yuzhao@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
---
Changes since v3:
- Add gfp flags to init function.

Changes since v2:
- Introduce N-way tree to reduce tree depth on larger systems.

Changes since v1:
- Remove percpu_counter_tree_precise_sum_unbiased from public header,
  make this function static,
- Introduce precise and approximate comparisons between two counters,
- Reorder the struct percpu_counter_tree fields,
- Introduce approx_sum field, which points to the approximate sum
  for the percpu_counter_tree_approximate_sum() fast path.
---
 include/linux/percpu_counter_tree.h | 108 ++++++++
 lib/Makefile                        |   1 +
 lib/percpu_counter_tree.c           | 393 ++++++++++++++++++++++++++++
 3 files changed, 502 insertions(+)
 create mode 100644 include/linux/percpu_counter_tree.h
 create mode 100644 lib/percpu_counter_tree.c

diff --git a/include/linux/percpu_counter_tree.h b/include/linux/percpu_counter_tree.h
new file mode 100644
index 000000000000..cb7a97c8a823
--- /dev/null
+++ b/include/linux/percpu_counter_tree.h
@@ -0,0 +1,108 @@
+/* SPDX-License-Identifier: GPL-2.0+ OR MIT */
+/* SPDX-FileCopyrightText: 2025 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> */
+
+#ifndef _PERCPU_COUNTER_TREE_H
+#define _PERCPU_COUNTER_TREE_H
+
+#include <linux/cleanup.h>
+#include <linux/preempt.h>
+#include <linux/atomic.h>
+#include <linux/percpu.h>
+
+struct percpu_counter_tree_level_item {
+	atomic_t count;
+} ____cacheline_aligned_in_smp;
+
+struct percpu_counter_tree {
+	/* Fast-path fields. */
+	unsigned int __percpu *level0;
+	unsigned int level0_bit_mask;
+	union {
+		unsigned int *i;
+		atomic_t *a;
+	} approx_sum;
+	int bias;			/* bias for counter_set */
+
+	/* Slow-path fields. */
+	struct percpu_counter_tree_level_item *items;
+	unsigned int batch_size;
+	unsigned int inaccuracy;	/* approximation imprecise within ± inaccuracy */
+};
+
+int percpu_counter_tree_init(struct percpu_counter_tree *counter, unsigned int batch_size, gfp_t gfp_flags);
+void percpu_counter_tree_destroy(struct percpu_counter_tree *counter);
+void percpu_counter_tree_add_slowpath(struct percpu_counter_tree *counter, int inc);
+int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter);
+int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b);
+int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v);
+int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b);
+int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v);
+void percpu_counter_tree_set_bias(struct percpu_counter_tree *counter, int bias);
+void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v);
+unsigned int percpu_counter_tree_inaccuracy(struct percpu_counter_tree *counter);
+
+/* Fast paths */
+
+static inline
+int percpu_counter_tree_carry(int orig, int res, int inc, unsigned int bit_mask)
+{
+	if (inc < 0) {
+		inc = -(-inc & ~(bit_mask - 1));
+		/*
+		 * xor bit_mask: underflow.
+		 *
+		 * If inc has bit set, decrement an additional bit if
+		 * there is _no_ bit transition between orig and res.
+		 * Else, inc has bit cleared, decrement an additional
+		 * bit if there is a bit transition between orig and
+		 * res.
+		 */
+		if ((inc ^ orig ^ res) & bit_mask)
+			inc -= bit_mask;
+	} else {
+		inc &= ~(bit_mask - 1);
+		/*
+		 * xor bit_mask: overflow.
+		 *
+		 * If inc has bit set, increment an additional bit if
+		 * there is _no_ bit transition between orig and res.
+		 * Else, inc has bit cleared, increment an additional
+		 * bit if there is a bit transition between orig and
+		 * res.
+		 */
+		if ((inc ^ orig ^ res) & bit_mask)
+			inc += bit_mask;
+	}
+	return inc;
+}
+
+static inline
+void percpu_counter_tree_add(struct percpu_counter_tree *counter, int inc)
+{
+	unsigned int bit_mask = counter->level0_bit_mask, orig, res;
+
+	if (!inc)
+		return;
+	/* Make sure the fast and slow paths use the same cpu number. */
+	guard(migrate)();
+	res = this_cpu_add_return(*counter->level0, inc);
+	orig = res - inc;
+	inc = percpu_counter_tree_carry(orig, res, inc, bit_mask);
+	if (!inc)
+		return;
+	percpu_counter_tree_add_slowpath(counter, inc);
+}
+
+static inline
+int percpu_counter_tree_approximate_sum(struct percpu_counter_tree *counter)
+{
+	unsigned int v;
+
+	if (!counter->level0_bit_mask)
+		v = READ_ONCE(*counter->approx_sum.i);
+	else
+		v = atomic_read(counter->approx_sum.a);
+	return (int) (v + (unsigned int)READ_ONCE(counter->bias));
+}
+
+#endif  /* _PERCPU_COUNTER_TREE_H */
diff --git a/lib/Makefile b/lib/Makefile
index f07b24ce1b3f..1d86c97ee39b 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -191,6 +191,7 @@ obj-$(CONFIG_TEXTSEARCH_KMP) += ts_kmp.o
 obj-$(CONFIG_TEXTSEARCH_BM) += ts_bm.o
 obj-$(CONFIG_TEXTSEARCH_FSM) += ts_fsm.o
 obj-$(CONFIG_SMP) += percpu_counter.o
+obj-$(CONFIG_SMP) += percpu_counter_tree.o
 obj-$(CONFIG_AUDIT_GENERIC) += audit.o
 obj-$(CONFIG_AUDIT_COMPAT_GENERIC) += compat_audit.o
 
diff --git a/lib/percpu_counter_tree.c b/lib/percpu_counter_tree.c
new file mode 100644
index 000000000000..da6fccf9f883
--- /dev/null
+++ b/lib/percpu_counter_tree.c
@@ -0,0 +1,393 @@
+// SPDX-License-Identifier: GPL-2.0+ OR MIT
+// SPDX-FileCopyrightText: 2025 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+
+/*
+ * Split Counters With Tree Approximation Propagation
+ *
+ * * Propagation diagram when reaching batch size thresholds (± batch size):
+ *
+ * Example diagram for 8 CPUs:
+ *
+ * log2(8) = 3 levels
+ *
+ * At each level, each pair propagates its values to the next level when
+ * reaching the batch size thresholds.
+ *
+ * Counters at levels 0, 1, 2 can be kept on a single byte (±128 range),
+ * although it may be relevant to keep them on 32-bit counters for
+ * simplicity. (complexity vs memory footprint tradeoff)
+ *
+ * Counter at level 3 can be kept on a 32-bit counter.
+ *
+ * Level 0:  0    1    2    3    4    5    6    7
+ *           |   /     |   /     |   /     |   /
+ *           |  /      |  /      |  /      |  /
+ *           | /       | /       | /       | /
+ * Level 1:  0         1         2         3
+ *           |       /           |       /
+ *           |    /              |    /
+ *           | /                 | /
+ * Level 2:  0                   1
+ *           |               /
+ *           |         /
+ *           |   /
+ * Level 3:  0
+ *
+ * * Approximation inaccuracy:
+ *
+ * BATCH(level N): Level N batch size.
+ *
+ * Example for BATCH(level 0) = 32.
+ *
+ * BATCH(level 0) =  32
+ * BATCH(level 1) =  64
+ * BATCH(level 2) = 128
+ * BATCH(level N) = BATCH(level 0) * 2^N
+ *
+ *            per-counter     global
+ *            inaccuracy      inaccuracy
+ * Level 0:   [ -32 ..  +31]  ±256  (8 * 32)
+ * Level 1:   [ -64 ..  +63]  ±256  (4 * 64)
+ * Level 2:   [-128 .. +127]  ±256  (2 * 128)
+ * Total:      ------         ±768  (log2(nr_cpu_ids) * BATCH(level 0) * nr_cpu_ids)
+ *
+ * -----
+ *
+ * Approximate Sum Carry Propagation
+ *
+ * Let's define a number of counter bits for each level, e.g.:
+ *
+ * log2(BATCH(level 0)) = log2(32) = 5
+ *
+ *               nr_bit        value_mask                      range
+ * Level 0:      5 bits        v                             0 ..  +31
+ * Level 1:      1 bit        (v & ~((1UL << 5) - 1))        0 ..  +63
+ * Level 2:      1 bit        (v & ~((1UL << 6) - 1))        0 .. +127
+ * Level 3:     25 bits       (v & ~((1UL << 7) - 1))        0 .. 2^32-1
+ *
+ * Note: Use a full 32-bit per-cpu counter at level 0 to allow precise sum.
+ *
+ * Note: Use cacheline aligned counters at levels above 0 to prevent false sharing.
+ *       If memory footprint is an issue, a specialized allocator could be used
+ *       to eliminate padding.
+ *
+ * Example with expanded values:
+ *
+ * counter_add(counter, inc):
+ *
+ *         if (!inc)
+ *                 return;
+ *
+ *         res = percpu_add_return(counter @ Level 0, inc);
+ *         orig = res - inc;
+ *         if (inc < 0) {
+ *                 inc = -(-inc & ~0b00011111);  // Clear used bits
+ *                 // xor bit 5: underflow
+ *                 if ((inc ^ orig ^ res) & 0b00100000)
+ *                         inc -= 0b00100000;
+ *         } else {
+ *                 inc &= ~0b00011111;           // Clear used bits
+ *                 // xor bit 5: overflow
+ *                 if ((inc ^ orig ^ res) & 0b00100000)
+ *                         inc += 0b00100000;
+ *         }
+ *         if (!inc)
+ *                 return;
+ *
+ *         res = atomic_add_return(counter @ Level 1, inc);
+ *         orig = res - inc;
+ *         if (inc < 0) {
+ *                 inc = -(-inc & ~0b00111111);  // Clear used bits
+ *                 // xor bit 6: underflow
+ *                 if ((inc ^ orig ^ res) & 0b01000000)
+ *                         inc -= 0b01000000;
+ *         } else {
+ *                 inc &= ~0b00111111;           // Clear used bits
+ *                 // xor bit 6: overflow
+ *                 if ((inc ^ orig ^ res) & 0b01000000)
+ *                         inc += 0b01000000;
+ *         }
+ *         if (!inc)
+ *                 return;
+ *
+ *         res = atomic_add_return(counter @ Level 2, inc);
+ *         orig = res - inc;
+ *         if (inc < 0) {
+ *                 inc = -(-inc & ~0b01111111);  // Clear used bits
+ *                 // xor bit 7: underflow
+ *                 if ((inc ^ orig ^ res) & 0b10000000)
+ *                         inc -= 0b10000000;
+ *         } else {
+ *                 inc &= ~0b01111111;           // Clear used bits
+ *                 // xor bit 7: overflow
+ *                 if ((inc ^ orig ^ res) & 0b10000000)
+ *                         inc += 0b10000000;
+ *         }
+ *         if (!inc)
+ *                 return;
+ *
+ *         atomic_add(counter @ Level 3, inc);
+ */
+
+#include <linux/percpu_counter_tree.h>
+#include <linux/cpumask.h>
+#include <linux/percpu.h>
+#include <linux/atomic.h>
+#include <linux/module.h>
+#include <linux/errno.h>
+#include <linux/slab.h>
+#include <linux/math.h>
+
+#define MAX_NR_LEVELS 5
+
+struct counter_config {
+	unsigned int nr_items;
+	unsigned char nr_levels;
+	unsigned char n_arity_order[MAX_NR_LEVELS];
+};
+
+/*
+ * nr_items is the number of items in the tree for levels 1 to and
+ * including the final level (approximate sum). It excludes the level 0
+ * per-cpu counters.
+ */
+static const struct counter_config per_nr_cpu_order_config[] = {
+	[0] =	{ .nr_items = 1,	.nr_levels = 0,		.n_arity_order = { 0 } },
+	[1] =	{ .nr_items = 3,	.nr_levels = 1,		.n_arity_order = { 1 } },
+	[2] =	{ .nr_items = 3,	.nr_levels = 2,		.n_arity_order = { 1, 1 } },
+	[3] =	{ .nr_items = 7,	.nr_levels = 3,		.n_arity_order = { 1, 1, 1 } },
+	[4] =	{ .nr_items = 7,	.nr_levels = 3,		.n_arity_order = { 2, 1, 1 } },
+	[5] =	{ .nr_items = 11,	.nr_levels = 3,		.n_arity_order = { 2, 2, 1 } },
+	[6] =	{ .nr_items = 21,	.nr_levels = 3,		.n_arity_order = { 2, 2, 2 } },
+	[7] =	{ .nr_items = 21,	.nr_levels = 3,		.n_arity_order = { 3, 2, 2 } },
+	[8] =	{ .nr_items = 37,	.nr_levels = 3,		.n_arity_order = { 3, 3, 2 } },
+	[9] =	{ .nr_items = 73,	.nr_levels = 3,		.n_arity_order = { 3, 3, 3 } },
+	[10] =	{ .nr_items = 149,	.nr_levels = 4,		.n_arity_order = { 3, 3, 2, 2 } },
+	[11] =	{ .nr_items = 293,	.nr_levels = 4,		.n_arity_order = { 3, 3, 3, 2 } },
+	[12] =	{ .nr_items = 585,	.nr_levels = 4,		.n_arity_order = { 3, 3, 3, 3 } },
+	[13] =	{ .nr_items = 1173,	.nr_levels = 5,		.n_arity_order = { 3, 3, 3, 2, 2 } },
+	[14] =	{ .nr_items = 2341,	.nr_levels = 5,		.n_arity_order = { 3, 3, 3, 3, 2 } },
+	[15] =	{ .nr_items = 4681,	.nr_levels = 5,		.n_arity_order = { 3, 3, 3, 3, 3 } },
+	[16] =	{ .nr_items = 4681,	.nr_levels = 5,		.n_arity_order = { 4, 3, 3, 3, 3 } },
+	[17] =	{ .nr_items = 8777,	.nr_levels = 5,		.n_arity_order = { 4, 4, 3, 3, 3 } },
+	[18] =	{ .nr_items = 17481,	.nr_levels = 5,		.n_arity_order = { 4, 4, 4, 3, 3 } },
+	[19] =	{ .nr_items = 34953,	.nr_levels = 5,		.n_arity_order = { 4, 4, 4, 4, 3 } },
+	[20] =	{ .nr_items = 69905,	.nr_levels = 5,		.n_arity_order = { 4, 4, 4, 4, 4 } },
+};
+
+static const struct counter_config *counter_config;
+static unsigned int nr_cpus_order, inaccuracy_multiplier;
+
+int percpu_counter_tree_init(struct percpu_counter_tree *counter, unsigned int batch_size, gfp_t gfp_flags)
+{
+	/* Batch size must be power of 2 */
+	if (!batch_size || (batch_size & (batch_size - 1)))
+		return -EINVAL;
+	counter->batch_size = batch_size;
+	counter->bias = 0;
+	counter->level0 = alloc_percpu_gfp(unsigned int, gfp_flags);
+	if (!counter->level0)
+		return -ENOMEM;
+	if (!nr_cpus_order) {
+		counter->items = NULL;
+		counter->approx_sum.i = per_cpu_ptr(counter->level0, 0);
+		counter->level0_bit_mask = 0;
+	} else {
+		counter->items = kzalloc(counter_config->nr_items *
+					 sizeof(struct percpu_counter_tree_level_item),
+					 gfp_flags);
+		if (!counter->items) {
+			free_percpu(counter->level0);
+			return -ENOMEM;
+		}
+		counter->approx_sum.a = &counter->items[counter_config->nr_items - 1].count;
+		counter->level0_bit_mask = 1UL << get_count_order(batch_size);
+	}
+	counter->inaccuracy = batch_size * inaccuracy_multiplier;
+	return 0;
+}
+
+void percpu_counter_tree_destroy(struct percpu_counter_tree *counter)
+{
+	free_percpu(counter->level0);
+	kfree(counter->items);
+}
+
+/* Called with migration disabled. */
+void percpu_counter_tree_add_slowpath(struct percpu_counter_tree *counter, int inc)
+{
+	unsigned int level_items, nr_levels = counter_config->nr_levels,
+		     level, n_arity_order, bit_mask;
+	struct percpu_counter_tree_level_item *item = counter->items;
+	unsigned int cpu = smp_processor_id();
+
+	WARN_ON_ONCE(!nr_cpus_order);	/* Should never be called for 1 cpu. */
+
+	n_arity_order = counter_config->n_arity_order[0];
+	bit_mask = counter->level0_bit_mask << n_arity_order;
+	level_items = 1U << (nr_cpus_order - n_arity_order);
+
+	for (level = 1; level < nr_levels; level++) {
+		atomic_t *count = &item[cpu & (level_items - 1)].count;
+		unsigned int orig, res;
+
+		res = atomic_add_return_relaxed(inc, count);
+		orig = res - inc;
+		inc = percpu_counter_tree_carry(orig, res, inc, bit_mask);
+		if (!inc)
+			return;
+		item += level_items;
+		n_arity_order = counter_config->n_arity_order[level];
+		level_items >>= n_arity_order;
+		bit_mask <<= n_arity_order;
+	}
+	atomic_add(inc, counter->approx_sum.a);
+}
+
+/*
+ * Precise sum. Perform the sum of all per-cpu counters.
+ */
+static int percpu_counter_tree_precise_sum_unbiased(struct percpu_counter_tree *counter)
+{
+	unsigned int sum = 0;
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		sum += *per_cpu_ptr(counter->level0, cpu);
+	return (int) sum;
+}
+
+int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter)
+{
+	return percpu_counter_tree_precise_sum_unbiased(counter) + READ_ONCE(counter->bias);
+}
+
+/*
+ * Do an approximate comparison of two counters.
+ * Return 0 if counters do not differ by more than the sum of their
+ * respective inaccuracy ranges,
+ * Return -1 if counter @a less than counter @b,
+ * Return 1 if counter @a is greater than counter @b.
+ */
+int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b)
+{
+	int count_a = percpu_counter_tree_approximate_sum(a),
+	    count_b = percpu_counter_tree_approximate_sum(b);
+
+	if (abs(count_a - count_b) <= (a->inaccuracy + b->inaccuracy))
+		return 0;
+	if (count_a < count_b)
+		return -1;
+	return 1;
+}
+
+/*
+ * Do an approximate comparison of a counter against a given value.
+ * Return 0 if the value is within the inaccuracy range of the counter,
+ * Return -1 if the value less than counter,
+ * Return 1 if the value is greater than counter.
+ */
+int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v)
+{
+	int count = percpu_counter_tree_approximate_sum(counter);
+
+	if (abs(v - count) <= counter->inaccuracy)
+		return 0;
+	if (count < v)
+		return -1;
+	return 1;
+}
+
+/*
+ * Do a precise comparison of two counters.
+ * Return 0 if the counters are equal,
+ * Return -1 if counter @a less than counter @b,
+ * Return 1 if counter @a is greater than counter @b.
+ */
+int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b)
+{
+	int count_a = percpu_counter_tree_approximate_sum(a),
+	    count_b = percpu_counter_tree_approximate_sum(b);
+
+	if (abs(count_a - count_b) <= (a->inaccuracy + b->inaccuracy)) {
+		if (b->inaccuracy < a->inaccuracy) {
+			count_a = percpu_counter_tree_precise_sum(a);
+			if (abs(count_a - count_b) <= b->inaccuracy)
+				count_b = percpu_counter_tree_precise_sum(b);
+		} else {
+			count_b = percpu_counter_tree_precise_sum(b);
+			if (abs(count_a - count_b) <= a->inaccuracy)
+				count_a = percpu_counter_tree_precise_sum(a);
+		}
+	}
+	if (count_a > count_b)
+		return -1;
+	if (count_a > count_b)
+		return 1;
+	return 0;
+}
+
+/*
+ * Do a precise comparision of a counter against a given value.
+ * Return 0 if the value is equal to the counter,
+ * Return -1 if the value less than counter,
+ * Return 1 if the value is greater than counter.
+ */
+int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v)
+{
+	int count = percpu_counter_tree_approximate_sum(counter);
+
+	if (abs(v - count) <= counter->inaccuracy)
+		count = percpu_counter_tree_precise_sum(counter);
+	if (count < v)
+		return -1;
+	if (count > v)
+		return 1;
+	return 0;
+}
+
+void percpu_counter_tree_set_bias(struct percpu_counter_tree *counter, int bias)
+{
+	WRITE_ONCE(counter->bias, bias);
+}
+
+void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v)
+{
+	percpu_counter_tree_set_bias(counter,
+				     v - percpu_counter_tree_precise_sum_unbiased(counter));
+}
+
+unsigned int percpu_counter_tree_inaccuracy(struct percpu_counter_tree *counter)
+{
+	return counter->inaccuracy;
+}
+
+static unsigned int __init calculate_inaccuracy_multiplier(void)
+{
+	unsigned int nr_levels = counter_config->nr_levels, level;
+	unsigned int level_items = 1U << nr_cpus_order;
+	unsigned int inaccuracy = 0, batch_size = 1;
+
+	for (level = 0; level < nr_levels; level++) {
+		unsigned int n_arity_order = counter_config->n_arity_order[level];
+
+		inaccuracy += batch_size * level_items;
+		batch_size <<= n_arity_order;
+		level_items >>= n_arity_order;
+	}
+	return inaccuracy;
+}
+
+static int __init percpu_counter_startup(void)
+{
+
+	nr_cpus_order = get_count_order(nr_cpu_ids);
+	if (WARN_ON_ONCE(nr_cpus_order >= ARRAY_SIZE(per_nr_cpu_order_config))) {
+		printk(KERN_ERR "Unsupported number of CPUs (%u)\n", nr_cpu_ids);
+		return -1;
+	}
+	counter_config = &per_nr_cpu_order_config[nr_cpus_order];
+	inaccuracy_multiplier = calculate_inaccuracy_multiplier();
+	return 0;
+}
+module_init(percpu_counter_startup);
-- 
2.39.5

[RFC PATCH v5 2/2] mm: Fix OOM killer inaccuracy on large many-core systems

[Mathieu Desnoyers]

Use hierarchical per-cpu counters for rss tracking to fix the per-mm RSS
tracking which has become too inaccurate for OOM killer purposes on
large many-core systems.

The following rss tracking issues were noted by Sweet Tea Dorminy [1],
which lead to picking wrong tasks as OOM kill target:

  Recently, several internal services had an RSS usage regression as part of a
  kernel upgrade. Previously, they were on a pre-6.2 kernel and were able to
  read RSS statistics in a backup watchdog process to monitor and decide if
  they'd overrun their memory budget. Now, however, a representative service
  with five threads, expected to use about a hundred MB of memory, on a 250-cpu
  machine had memory usage tens of megabytes different from the expected amount
  -- this constituted a significant percentage of inaccuracy, causing the
  watchdog to act.

  This was a result of f1a7941243c1 ("mm: convert mm's rss stats into
  percpu_counter") [1].  Previously, the memory error was bounded by
  64*nr_threads pages, a very livable megabyte. Now, however, as a result of
  scheduler decisions moving the threads around the CPUs, the memory error could
  be as large as a gigabyte.

  This is a really tremendous inaccuracy for any few-threaded program on a
  large machine and impedes monitoring significantly. These stat counters are
  also used to make OOM killing decisions, so this additional inaccuracy could
  make a big difference in OOM situations -- either resulting in the wrong
  process being killed, or in less memory being returned from an OOM-kill than
  expected.

Here is a (possibly incomplete) list of the prior approaches that were
used or proposed, along with their downside:

1) Per-thread rss tracking: large error on many-thread processes.

2) Per-CPU counters: up to 12% slower for short-lived processes and 9%
   increased system time in make test workloads [1]. Moreover, the
   inaccuracy increases with O(n^2) with the number of CPUs.

3) Per-NUMA-node counters: requires atomics on fast-path (overhead),
   error is high with systems that have lots of NUMA nodes (32 times
   the number of NUMA nodes).

The approach proposed here is to replace this by the hierarchical
per-cpu counters, which bounds the inaccuracy based on the system
topology with O(N*logN).

Link: https://lore.kernel.org/lkml/20250331223516.7810-2-sweettea-kernel@dorminy.me/ # [1]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Martin Liu <liumartin@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: christian.koenig@amd.com
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Sweet Tea Dorminy <sweettea@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R . Howlett" <Liam.Howlett@Oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-mm@kvack.org
Cc: linux-trace-kernel@vger.kernel.org
Cc: Yu Zhao <yuzhao@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
---
Change since v4:
- get_mm_counter needs to return 0 or a positive value.
---
 include/linux/mm.h          | 10 ++++++----
 include/linux/mm_types.h    |  4 ++--
 include/trace/events/kmem.h |  2 +-
 kernel/fork.c               | 31 +++++++++++++++++++++----------
 4 files changed, 30 insertions(+), 17 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e51dba8398f7..18ccb51dad88 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2705,28 +2705,30 @@ static inline bool get_user_page_fast_only(unsigned long addr,
  */
 static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
 {
-	return percpu_counter_read_positive(&mm->rss_stat[member]);
+	int v = percpu_counter_tree_approximate_sum(&mm->rss_stat[member]);
+
+	return v > 0 ? v : 0;
 }
 
 void mm_trace_rss_stat(struct mm_struct *mm, int member);
 
 static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
 {
-	percpu_counter_add(&mm->rss_stat[member], value);
+	percpu_counter_tree_add(&mm->rss_stat[member], value);
 
 	mm_trace_rss_stat(mm, member);
 }
 
 static inline void inc_mm_counter(struct mm_struct *mm, int member)
 {
-	percpu_counter_inc(&mm->rss_stat[member]);
+	percpu_counter_tree_add(&mm->rss_stat[member], 1);
 
 	mm_trace_rss_stat(mm, member);
 }
 
 static inline void dec_mm_counter(struct mm_struct *mm, int member)
 {
-	percpu_counter_dec(&mm->rss_stat[member]);
+	percpu_counter_tree_add(&mm->rss_stat[member], -1);
 
 	mm_trace_rss_stat(mm, member);
 }
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 56d07edd01f9..85b15109106a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -18,7 +18,7 @@
 #include <linux/page-flags-layout.h>
 #include <linux/workqueue.h>
 #include <linux/seqlock.h>
-#include <linux/percpu_counter.h>
+#include <linux/percpu_counter_tree.h>
 #include <linux/types.h>
 
 #include <asm/mmu.h>
@@ -1059,7 +1059,7 @@ struct mm_struct {
 
 		unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
 
-		struct percpu_counter rss_stat[NR_MM_COUNTERS];
+		struct percpu_counter_tree rss_stat[NR_MM_COUNTERS];
 
 		struct linux_binfmt *binfmt;
 
diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index f74925a6cf69..d6199b99c771 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -477,7 +477,7 @@ TRACE_EVENT(rss_stat,
 		__entry->mm_id = mm_ptr_to_hash(mm);
 		__entry->curr = !!(current->mm == mm);
 		__entry->member = member;
-		__entry->size = (percpu_counter_sum_positive(&mm->rss_stat[member])
+		__entry->size = (percpu_counter_tree_approximate_sum(&mm->rss_stat[member])
 							    << PAGE_SHIFT);
 	),
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 168681fc4b25..dd458adc5543 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -129,6 +129,11 @@
  */
 #define MAX_THREADS FUTEX_TID_MASK
 
+/*
+ * Batch size of rss stat approximation
+ */
+#define RSS_STAT_BATCH_SIZE	32
+
 /*
  * Protected counters by write_lock_irq(&tasklist_lock)
  */
@@ -843,11 +848,10 @@ static void check_mm(struct mm_struct *mm)
 			 "Please make sure 'struct resident_page_types[]' is updated as well");
 
 	for (i = 0; i < NR_MM_COUNTERS; i++) {
-		long x = percpu_counter_sum(&mm->rss_stat[i]);
-
-		if (unlikely(x))
-			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
-				 mm, resident_page_types[i], x);
+		if (unlikely(percpu_counter_tree_precise_compare_value(&mm->rss_stat[i], 0) != 0))
+			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%d\n",
+				 mm, resident_page_types[i],
+				 percpu_counter_tree_precise_sum(&mm->rss_stat[i]));
 	}
 
 	if (mm_pgtables_bytes(mm))
@@ -930,6 +934,8 @@ static void cleanup_lazy_tlbs(struct mm_struct *mm)
  */
 void __mmdrop(struct mm_struct *mm)
 {
+	int i;
+
 	BUG_ON(mm == &init_mm);
 	WARN_ON_ONCE(mm == current->mm);
 
@@ -945,8 +951,8 @@ void __mmdrop(struct mm_struct *mm)
 	put_user_ns(mm->user_ns);
 	mm_pasid_drop(mm);
 	mm_destroy_cid(mm);
-	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
-
+	for (i = 0; i < NR_MM_COUNTERS; i++)
+		percpu_counter_tree_destroy(&mm->rss_stat[i]);
 	free_mm(mm);
 }
 EXPORT_SYMBOL_GPL(__mmdrop);
@@ -1285,6 +1291,8 @@ static void mmap_init_lock(struct mm_struct *mm)
 static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	struct user_namespace *user_ns)
 {
+	int i;
+
 	mt_init_flags(&mm->mm_mt, MM_MT_FLAGS);
 	mt_set_external_lock(&mm->mm_mt, &mm->mmap_lock);
 	atomic_set(&mm->mm_users, 1);
@@ -1332,15 +1340,18 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	if (mm_alloc_cid(mm, p))
 		goto fail_cid;
 
-	if (percpu_counter_init_many(mm->rss_stat, 0, GFP_KERNEL_ACCOUNT,
-				     NR_MM_COUNTERS))
-		goto fail_pcpu;
+	for (i = 0; i < NR_MM_COUNTERS; i++) {
+		if (percpu_counter_tree_init(&mm->rss_stat[i], RSS_STAT_BATCH_SIZE, GFP_KERNEL_ACCOUNT))
+			goto fail_pcpu;
+	}
 
 	mm->user_ns = get_user_ns(user_ns);
 	lru_gen_init_mm(mm);
 	return mm;
 
 fail_pcpu:
+	for (i--; i >= 0; i--)
+		percpu_counter_tree_destroy(&mm->rss_stat[i]);
 	mm_destroy_cid(mm);
 fail_cid:
 	destroy_context(mm);
-- 
2.39.5

Re: [RFC PATCH v5 2/2] mm: Fix OOM killer inaccuracy on large many-core systems

[Mathieu Desnoyers]

On 2025-07-03 13:38, Mathieu Desnoyers wrote:
[...]
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e51dba8398f7..18ccb51dad88 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2705,28 +2705,30 @@ static inline bool get_user_page_fast_only(unsigned long addr,
>    */
>   static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
>   {
> -	return percpu_counter_read_positive(&mm->rss_stat[member]);
> +	int v = percpu_counter_tree_approximate_sum(&mm->rss_stat[member]);
> +
> +	return v > 0 ? v : 0;
>   }

In a follow up version, I plan to introduce a new
percpu_counter_tree_approximate_sum_positive() to wrap this
 >= 0 logic.

Thanks,

Mathieu



-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com