On 1/14/26 15:36, 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 <[email protected]>
> Fixes: f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter")
> Link:
> https://lore.kernel.org/lkml/[email protected]/
> # [1]
> Signed-off-by: Mathieu Desnoyers <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: "Paul E. McKenney" <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Masami Hiramatsu <[email protected]>
> Cc: Mathieu Desnoyers <[email protected]>
> Cc: Dennis Zhou <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: Martin Liu <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: [email protected]
> Cc: Shakeel Butt <[email protected]>
> Cc: SeongJae Park <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Sweet Tea Dorminy <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: "Liam R . Howlett" <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Wei Yang <[email protected]>
> Cc: David Hildenbrand <[email protected]>
> Cc: Miaohe Lin <[email protected]>
> Cc: Al Viro <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: Yu Zhao <[email protected]>
> Cc: Roman Gushchin <[email protected]>
> Cc: Mateusz Guzik <[email protected]>
> Cc: Matthew Wilcox <[email protected]>
> Cc: Baolin Wang <[email protected]>
> Cc: Aboorva Devarajan <[email protected]>
> ---
> 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.
> ---
> include/linux/mm.h | 7 +++++++
> mm/oom_kill.c | 22 +++++++++++-----------
> 2 files changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6f959d8ca4b4..bfa1307264df 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2901,6 +2901,13 @@ static inline unsigned long get_mm_rss(struct
> mm_struct *mm)
> get_mm_counter(mm, MM_SHMEMPAGES);
> }
>
> +static inline unsigned long get_mm_rss_sum(struct mm_struct *mm)
> +{
> + return get_mm_counter_sum(mm, MM_FILEPAGES) +
> + get_mm_counter_sum(mm, MM_ANONPAGES) +
> + get_mm_counter_sum(mm, MM_SHMEMPAGES);
> +}
> +
> static inline unsigned long get_mm_hiwater_rss(struct mm_struct *mm)
> {
> return max(mm->hiwater_rss, get_mm_rss(mm));
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 5eb11fbba704..214cb8cb939b 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -228,7 +228,7 @@ long oom_badness(struct task_struct *p, unsigned long
> totalpages)
> * The baseline for the badness score is the proportion of RAM that each
> * task's rss, pagetable and swap space use.
> */
> - points = get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) +
> + points = get_mm_rss_sum(p->mm) + get_mm_counter_sum(p->mm, MM_SWAPENTS)
> +
> mm_pgtables_bytes(p->mm) / PAGE_SIZE;
> task_unlock(p);
>
> @@ -402,10 +402,10 @@ static int dump_task(struct task_struct *p, void *arg)
>
> pr_info("[%7d] %5d %5d %8lu %8lu %8lu %8lu %9lu %8ld %8lu %5hd
> %s\n",
> task->pid, from_kuid(&init_user_ns, task_uid(task)),
> - task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
> - get_mm_counter(task->mm, MM_ANONPAGES),
> get_mm_counter(task->mm, MM_FILEPAGES),
> - get_mm_counter(task->mm, MM_SHMEMPAGES),
> mm_pgtables_bytes(task->mm),
> - get_mm_counter(task->mm, MM_SWAPENTS),
> + task->tgid, task->mm->total_vm, get_mm_rss_sum(task->mm),
> + get_mm_counter_sum(task->mm, MM_ANONPAGES),
> get_mm_counter_sum(task->mm, MM_FILEPAGES),
> + get_mm_counter_sum(task->mm, MM_SHMEMPAGES),
> mm_pgtables_bytes(task->mm),
> + get_mm_counter_sum(task->mm, MM_SWAPENTS),
> task->signal->oom_score_adj, task->comm);
> task_unlock(task);
>
> @@ -604,9 +604,9 @@ static bool oom_reap_task_mm(struct task_struct *tsk,
> struct mm_struct *mm)
>
> pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB,
> file-rss:%lukB, shmem-rss:%lukB\n",
> task_pid_nr(tsk), tsk->comm,
> - K(get_mm_counter(mm, MM_ANONPAGES)),
> - K(get_mm_counter(mm, MM_FILEPAGES)),
> - K(get_mm_counter(mm, MM_SHMEMPAGES)));
> + K(get_mm_counter_sum(mm, MM_ANONPAGES)),
> + K(get_mm_counter_sum(mm, MM_FILEPAGES)),
> + K(get_mm_counter_sum(mm, MM_SHMEMPAGES)));
> out_finish:
> trace_finish_task_reaping(tsk->pid);
> out_unlock:
> @@ -960,9 +960,9 @@ static void __oom_kill_process(struct task_struct
> *victim, const char *message)
> mark_oom_victim(victim);
> pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB,
> file-rss:%lukB, shmem-rss:%lukB, UID:%u pgtables:%lukB oom_score_adj:%hd\n",
> message, task_pid_nr(victim), victim->comm, K(mm->total_vm),
> - K(get_mm_counter(mm, MM_ANONPAGES)),
> - K(get_mm_counter(mm, MM_FILEPAGES)),
> - K(get_mm_counter(mm, MM_SHMEMPAGES)),
> + K(get_mm_counter_sum(mm, MM_ANONPAGES)),
> + K(get_mm_counter_sum(mm, MM_FILEPAGES)),
> + K(get_mm_counter_sum(mm, MM_SHMEMPAGES)),
> from_kuid(&init_user_ns, task_uid(victim)),
> mm_pgtables_bytes(mm) >> 10, victim->signal->oom_score_adj);
> task_unlock(victim);
