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Thu, 15 Jan 2026 17:21:28 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id /EqUOhgiaWlcDAAAD6G6ig (envelope-from ); Thu, 15 Jan 2026 17:21:28 +0000 Message-ID: Date: Thu, 15 Jan 2026 18:21:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/1] mm: Fix OOM killer inaccuracy on large many-core systems Content-Language: en-US To: Mathieu Desnoyers , Andrew Morton Cc: linux-kernel@vger.kernel.org, Michal Hocko , "Paul E. McKenney" , Steven Rostedt , Masami Hiramatsu , Dennis Zhou , Tejun Heo , Christoph Lameter , Martin Liu , David Rientjes , christian.koenig@amd.com, Shakeel Butt , SeongJae Park , Johannes Weiner , Sweet Tea Dorminy , Lorenzo Stoakes , "Liam R . Howlett" , Mike Rapoport , Suren Baghdasaryan , Christian Brauner , Wei Yang , David Hildenbrand , Miaohe Lin , Al Viro , linux-mm@kvack.org, stable@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Yu Zhao , Roman Gushchin , Mateusz Guzik , Matthew Wilcox , Baolin Wang , Aboorva Devarajan References: <20260114143642.47333-1-mathieu.desnoyers@efficios.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <20260114143642.47333-1-mathieu.desnoyers@efficios.com> Content-Type: text/plain; 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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 Acked-by: Vlastimil Babka > 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);