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Mathieu Desnoyers 5898aa8f9a mm: fix OOM killer inaccuracy on large many-core systems
Use the precise, albeit slower, precise RSS counter sums for the OOM
killer task selection and console dumps.  The approximated value is too
imprecise 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 commit f1a7941243 ("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).

commit 82241a83cd ("mm: fix the inaccurate memory statistics issue for
users") introduced get_mm_counter_sum() for precise proc memory status
queries for some proc files.

The simple fix proposed here is to do the precise per-cpu counters sum
every time a counter value needs to be read.  This applies to the OOM
killer task selection, oom task console dumps (printk).

This change increases the latency introduced when the OOM killer executes
in favor of doing a more precise OOM target task selection.  Effectively,
the OOM killer iterates on all tasks, for all relevant page types, for
which the precise sum iterates on all possible CPUs.

As a reference, here is the execution time of the OOM killer before/after
the change:

AMD EPYC 9654 96-Core (2 sockets)
Within a KVM, configured with 256 logical cpus.

                                  |  before  |  after   |
----------------------------------|----------|----------|
nr_processes=40                   |  0.3 ms  |   0.5 ms |
nr_processes=10000                |  3.0 ms  |  80.0 ms |

Link: https://lkml.kernel.org/r/20260114143642.47333-1-mathieu.desnoyers@efficios.com
Fixes: f1a7941243 ("mm: convert mm's rss stats into percpu_counter")
Link: https://lore.kernel.org/lkml/20250331223516.7810-2-sweettea-kernel@dorminy.me/ # [1]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
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: Shakeel Butt <shakeel.butt@linux.dev>
Cc: SeongJae Park <sj@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R . Howlett" <liam.howlett@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
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: Yu Zhao <yuzhao@google.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2026-01-31 14:22:37 -08:00
arch mm: rename CONFIG_MEMORY_BALLOON -> CONFIG_BALLOON 2026-01-31 14:22:36 -08:00
block mm/block/fs: remove laptop_mode 2026-01-20 19:24:47 -08:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: seqiv - Do not use req->iv after crypto_aead_encrypt 2025-12-19 14:47:06 +08:00
Documentation mm: rename CONFIG_BALLOON_COMPACTION to CONFIG_BALLOON_MIGRATION 2026-01-31 14:22:36 -08:00
drivers zram: rename init_lock to dev_lock 2026-01-31 14:22:36 -08:00
fs mm/block/fs: remove laptop_mode 2026-01-20 19:24:47 -08:00
include mm: fix OOM killer inaccuracy on large many-core systems 2026-01-31 14:22:37 -08:00
init arch, mm: consolidate initialization of nodes, zones and memory map 2026-01-26 20:02:18 -08:00
io_uring io_uring-6.19-20260116 2026-01-16 20:56:56 -08:00
ipc Significant patch series in this pull request: 2025-12-06 14:01:20 -08:00
kernel Merge branch 'mm-hotfixes-stable' into mm-stable to pick up "mm/shmem, 2026-01-31 14:20:03 -08:00
lib alloc_tag: fix rw permission issue when handling boot parameter 2026-01-31 14:22:37 -08:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm mm: fix OOM killer inaccuracy on large many-core systems 2026-01-31 14:22:37 -08:00
net Including fixes from bluetooth, can and IPsec. 2026-01-15 10:11:11 -08:00
rust rust: bitops: fix missing _find_* functions on 32-bit ARM 2026-01-13 22:16:39 -05:00
samples samples/ftrace: Adjust LoongArch register restore order in direct calls 2025-12-31 15:19:25 +08:00
scripts Crypto library fixes for v6.19-rc5 2026-01-11 15:07:56 -10:00
security Landlock fix for v6.19-rc6 2026-01-18 15:15:47 -08:00
sound ALSA: hda/tas2781: Add newly-released HP laptop 2026-01-15 15:04:27 +01:00
tools mm/rmap: separate out fork-only logic on anon_vma_clone() 2026-01-26 20:02:22 -08:00
usr initramfs: add gen_init_cpio to hostprogs unconditionally 2025-11-26 21:55:40 +01:00
virt KVM fixes for 6.19-rc1 2025-12-18 18:38:45 +01:00
.clang-format s390/pci: Fix cyclic dead-lock in zpci_zdev_put() and zpci_scan_devices() 2025-12-14 11:03:58 +01:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: kbuild: add proc macro library support 2025-11-24 17:15:36 +01:00
.mailmap mailmap: add entry for Viacheslav Bocharov 2026-01-26 19:03:46 -08:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Update email address 2026-01-11 06:09:11 -10:00
Kbuild sched: Make migrate_{en,dis}able() inline 2025-09-25 09:57:16 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS MAINTAINERS: move memory balloon infrastructure to "MEMORY MANAGEMENT - BALLOON" 2026-01-31 14:22:36 -08:00
Makefile Linux 6.19-rc6 2026-01-18 15:42:45 -08:00
README README: restructure with role-based documentation and guidelines 2025-11-29 08:40:33 -07:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst



Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst