Re: [PATCH v2 1/1] mm: Fix OOM killer inaccuracy on large many-core systems

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 1/14/26 10:36 PM, Mathieu Desnoyers wrote:
> 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 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.
> 
> 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 82241a83cd15 ("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 |
> 
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Fixes: f1a7941243c1 ("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>
> 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: stable@vger.kernel.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>
> ---
> This patch replaces v1. It's aimed at mm-new.
> 
> Changes since v1:
> - Only change the oom killer RSS values from approximated to precise
>    sums. Do not change other RSS values users.
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>