[PATCH v16 0/3] Improve proc RSS accuracy and OOM killer latency

[Mathieu Desnoyers]

This series use the hierarchical tree counter approximation (hpcc) to:

* Increase accuracy of approximated RSS counters exposed through proc
  interfaces:

With a test program hopping across CPUs doing frequent mmap/munmap
operations, the upstream implementation approximation reaches a 1GB
delta from the precise value after a few minutes, compared to a 80MB
delta with the hierarchical counter. The hierarchical counter provides a
guaranteed maximum approximation inaccuracy of 192MB on that hardware
topology.
 
* 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 |

I'm sending this series to gather feedback. I plan to re-submit it for
inclusion into mm-new _after_ the next merge window closes, so the bug
fix "mm: Fix OOM killer inaccuracy on large many-core systems" can be
tested in the current release cycle.

Andrew, if you have a prior version of this specific series in mm-new,
please drop it for now.

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-mathieu.desnoyers@xxxxxxxxxxxx/T/#t
https://lore.kernel.org/linux-mm/20251224173810.648699-1-mathieu.desnoyers@xxxxxxxxxxxx/T/#t
https://lore.kernel.org/linux-mm/20260114143642.47333-1-mathieu.desnoyers@xxxxxxxxxxxx/

Thanks,

Mathieu

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Martin Liu <liumartin@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: christian.koenig@xxxxxxx
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: SeongJae Park <sj@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Sweet Tea Dorminy <sweettea-kernel@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: "Liam R . Howlett" <liam.howlett@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Aboorva Devarajan <aboorvad@xxxxxxxxxxxxx>

Mathieu Desnoyers (3):
  lib: Introduce hierarchical per-cpu counters
  mm: Improve RSS counter approximation accuracy for proc interfaces
  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                            |  49 +-
 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, 1295 insertions(+), 53 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