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 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 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.

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).

Notable changes for v13:

- One uninitialized variable fix for oom_task_origin case.
- percpu_counter_tree_set needs to use atomic_long_set in UP build.
- percpu_counter_tree_precise_sum needs to return long type.

I've done moderate testing of this series on a 256-core VM with 128GB
RAM. Figuring out whether this indeed helps solve issues with real-life
workloads will require broader feedback from the community.

This series is based on v6.19-rc4, on top of the following two
preparation series:

https://lore.kernel.org/linux-mm/20251224173358.647691-1-mathieu.desnoyers@efficios.com/T/#t
https://lore.kernel.org/linux-mm/20251224173810.648699-1-mathieu.desnoyers@efficios.com/T/#t

Andrew, this series replaces v12, for testing in mm-new.

Thanks!

Mathieu

Link: https://lore.kernel.org/lkml/20250331223516.7810-2-sweettea-kernel@dorminy.me/ # [1]
To: 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: 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: 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>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>

Mathieu Desnoyers (3):
  lib: Introduce hierarchical per-cpu counters
  mm: Fix OOM killer inaccuracy on large many-core systems
  mm: Implement precise OOM killer task selection

 fs/proc/base.c                      |   2 +-
 include/linux/mm.h                  |  49 +-
 include/linux/mm_types.h            |  54 ++-
 include/linux/oom.h                 |  11 +-
 include/linux/percpu_counter_tree.h | 344 ++++++++++++++
 include/trace/events/kmem.h         |   2 +-
 init/main.c                         |   2 +
 kernel/fork.c                       |  22 +-
 lib/Makefile                        |   1 +
 lib/percpu_counter_tree.c           | 702 ++++++++++++++++++++++++++++
 mm/oom_kill.c                       |  84 +++-
 11 files changed, 1223 insertions(+), 50 deletions(-)
 create mode 100644 include/linux/percpu_counter_tree.h
 create mode 100644 lib/percpu_counter_tree.c

-- 
2.39.5