mm: Reduce latency of OOM killer task selection

Hi Andrew,

This series use the hierarchical tree counter approximation (hpcc) to
implement the OOM killer task selection with a 2-pass algorithm. This is
a latency reduction improvement of the OOM killer task selection.

Testing the execution time of select_bad_process() with a single
tail -f /dev/zero:

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

                                      | precise sum |   hpcc   |
    ----------------------------------|-------------|----------|
    nr_processes=40                   |    0.5 ms   |   0.3 ms |
    nr_processes=10000                |   80.0 ms   |   7.9 ms |

Notable changes for v15: This series becomes a OOM killer latency
improvement, rather than a bug fix.

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

https://lore.kernel.org/linux-mm/20251224173358.647691-1-...
https://lore.kernel.org/linux-mm/20251224173810.648699-1-...
https://lore.kernel.org/linux-mm/20260113194734.28983-1-m...

This series replaces v14, aimed at mm-new.

Thanks,

Matheu

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: 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: Use hierarchical per-cpu counters for RSS tracking
  mm: Reduce latency of OOM killer task selection with 2-pass algorithm

 .../core-api/percpu-counter-tree.rst          |  75 ++
 fs/proc/base.c                                |   2 +-
 include/linux/mm.h                            |  46 +-
 include/linux/mm_types.h                      |  54 +-
 include/linux/oom.h                           |  11 +-
 include/linux/percpu_counter_tree.h           | 367 ++++++++++
 include/trace/events/kmem.h                   |   2 +-
 init/main.c                                   |   2 +
 kernel/fork.c                                 |  22 +-
 lib/Makefile                                  |   1 +
 lib/percpu_counter_tree.c                     | 679 ++++++++++++++++++
 mm/oom_kill.c                                 |  84 ++-
 12 files changed, 1296 insertions(+), 49 deletions(-)
 create mode 100644 Documentation/core-api/percpu-counter-tree.rst
 create mode 100644 include/linux/percpu_counter_tree.h
 create mode 100644 lib/percpu_counter_tree.c

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2.39.5