Message-ID: <aWe8IdTYEMJt3GTH@tiehlicka>
Date: Wed, 14 Jan 2026 16:54:09 +0100
From: Michal Hocko <mhocko@...e.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
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	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>, Martin Liu <liumartin@...gle.com>,
	David Rientjes <rientjes@...gle.com>, christian.koenig@....com,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	SeongJae Park <sj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Sweet Tea Dorminy <sweettea-kernel@...miny.me>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R . Howlett" <liam.howlett@...cle.com>,
	Mike Rapoport <rppt@...nel.org>,
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	Al Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
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	Aboorva Devarajan <aboorvad@...ux.ibm.com>
Subject: Re: [PATCH v2 1/1] mm: Fix OOM killer inaccuracy on large many-core
 systems

On Wed 14-01-26 09:36:42, 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@...e.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@...icios.com>

OOM is a rare situation - therefore a slow path - and handling taking
care of a huge imprecesion is much more important than adding ~100ms
overhead to calculate more precise memory consuption.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

-- 
Michal Hocko
SUSE Labs

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