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 change for v10: The new patch 3/3 changes the implementation of
the oom killer task selection to a 2-pass algorithm, where the first
pass uses the fast approximation provided by the hierarchical percpu
counters, and the second pass does a precise sum for all tasks which
have badness values within the range of the approximation accuracy.

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.

The one request I did not have time to fulfill yet is to port the
tests from the librseq feature branch implementation (userspace) to the
kernel selftests.

This series is based on v6.18.

Andrew, are you interested to try this out 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>

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                  |  58 ++-
 include/linux/mm_types.h            |   4 +-
 include/linux/oom.h                 |  12 +-
 include/linux/percpu_counter_tree.h | 242 ++++++++++
 include/trace/events/kmem.h         |   2 +-
 init/main.c                         |   2 +
 kernel/fork.c                       |  24 +-
 lib/Makefile                        |   1 +
 lib/percpu_counter_tree.c           | 705 ++++++++++++++++++++++++++++
 mm/oom_kill.c                       |  72 ++-
 11 files changed, 1089 insertions(+), 35 deletions(-)
 create mode 100644 include/linux/percpu_counter_tree.h
 create mode 100644 lib/percpu_counter_tree.c

-- 
2.39.5