On Tue, 13 Jan 2026 14:47:34 -0500 Mathieu Desnoyers
<[email protected]> wrote:
> Use the precise, albeit slower, precise RSS counter sums for the OOM
> killer task selection and proc statistics. The approximated value is
> too imprecise on large many-core systems.
Thanks.
Problem: if I also queue your "mm: Reduce latency of OOM killer task
selection" series then this single patch won't get tested, because the
larger series erases this patch, yes?
Obvious solution: aim this patch at next-merge-window and let's look at
the larger series for the next -rc cycle. Thoughts?
> 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).
>
> 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, to the /proc statistics, and to the oom mark_victim
> trace event.
>
> Note that 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. This change renames
> get_mm_counter_sum() to get_mm_counter(), thus moving the rest of the
> proc files to the precise sum.
Please confirm - switching /proc functions from get_mm_counter_sum() to
get_mm_counter_sum() doesn't actually change anything, right? It would
be concerning to add possible overhead to things like task_statm().
> This change effectively 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 |
That seems acceptable.
