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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 , Vlastimil Babka , Christian Brauner , Wei Yang , David Hildenbrand , Miaohe Lin , Al Viro , linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, Yu Zhao , Roman Gushchin , Mateusz Guzik , Matthew Wilcox , Baolin Wang , Aboorva Devarajan References: <20260111194958.1231477-1-mathieu.desnoyers@efficios.com> <20260111194958.1231477-3-mathieu.desnoyers@efficios.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQBPR01CA0111.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:1::11) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YT4PR01MB10616:EE_ X-MS-Office365-Filtering-Correlation-Id: ae72f84d-b569-47a8-e6f3-08de521212ed X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|1800799024; 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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 approach proposed here is to replace this by the hierarchical >> per-cpu counters, which bounds the inaccuracy based on the system >> topology with O(N*logN). > > The concept of hierarchical pcp counter is interesting and I am > definitely not opposed if there are more users that would benefit. > > From the OOM POV, IIUC the primary problem is that get_mm_counter > (percpu_counter_read_positive) is too imprecise on systems when the task > is moving around a large number of cpus. In the list of alternative > solutions I do not see percpu_counter_sum_positive to be mentioned. > oom_badness() is a really slow path and taking the slow path to > calculate a much more precise value seems acceptable. Have you > considered that option? I must admit I assumed that since there was already a mechanism in place to ensure it's not necessary to sum per-cpu counters when the oom killer is trying to select tasks, it must be because this O(nr_possible_cpus * nr_processes) operation must be too slow for the oom killer requirements. AFAIU, the oom killer is executed when the memory allocator fails to allocate memory, which can be within code paths which need to progress eventually. So even though it's a slow path compared to the allocator fast path, there must be at least _some_ expectations about it completing within a decent amount of time. What would that ballpark be ? To give an order of magnitude, I've tried modifying the upstream oom killer to use percpu_counter_sum_positive and compared it to the hierarchical approach: AMD EPYC 9654 96-Core (2 sockets) Within a KVM, configured with 256 logical cpus. nr_processes=40 nr_processes=10000 Counter sum: 0.4 ms 81.0 ms HPCC with 2-pass: 0.3 ms 9.3 ms So as we scale up the number of processes on large SMP systems, the latency caused by the oom killer task selection greatly increases with the counter sums compared with the hierarchical approach. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com