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 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 Cc: Andrew Morton Cc: "Paul E. McKenney" Cc: Steven Rostedt Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: Dennis Zhou Cc: Tejun Heo Cc: Christoph Lameter Cc: Martin Liu Cc: David Rientjes Cc: christian.koenig@amd.com Cc: Shakeel Butt Cc: SeongJae Park Cc: Michal Hocko Cc: Johannes Weiner Cc: Sweet Tea Dorminy Cc: Lorenzo Stoakes Cc: "Liam R . Howlett" Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Christian Brauner Cc: Wei Yang Cc: David Hildenbrand Cc: Miaohe Lin Cc: Al Viro Cc: linux-mm@kvack.org Cc: stable@vger.kernel.org Cc: linux-trace-kernel@vger.kernel.org Cc: Yu Zhao Cc: Roman Gushchin Cc: Mateusz Guzik Cc: Matthew Wilcox Cc: Baolin Wang Cc: Aboorva Devarajan --- 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); -- 2.39.5