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Wed, 14 Jan 2026 19:21:35 +0000 Message-ID: <56bafa34-073e-4791-9f21-c625723ffc30@efficios.com> Date: Wed, 14 Jan 2026 14:21:34 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v16 2/3] mm: Improve RSS counter approximation accuracy for proc interfaces To: Michal Hocko Cc: Andrew Morton , linux-kernel@vger.kernel.org, "Paul E. 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 HMoh2llS 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026-01-14 11:48, Michal Hocko wrote: > On Wed 14-01-26 09:59:14, Mathieu Desnoyers wrote: >> Use hierarchical per-cpu counters for RSS tracking to improve the >> accuracy of per-mm RSS sum approximation on large many-core systems [1]. >> This improves the accuracy of the RSS values returned by proc >> interfaces. >> >> This is also a preparation step to introduce a 2-pass OOM killer task >> selection which leverages the approximation and accuracy ranges to >> quickly eliminate tasks which are outside of the range of the current >> selection, and thus reduce the latency introduced by execution of the >> OOM killer. >> >> 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). >> >> 4) Use a percise per-cpu counter sum for each counter value query: >> Requires iteration on each possible CPUs for each sum, which >> adds overhead (and thus increases OOM killer latency) on large >> many-core systems running many processes. >> >> The approach proposed here is to replace the per-cpu counters by the >> hierarchical per-cpu counters, which bounds the inaccuracy based on the >> system topology with O(N*logN). >> >> * Testing results: >> >> Test hardware: 2 sockets AMD EPYC 9654 96-Core Processor (384 logical CPUs total) >> >> Methodology: >> >> Comparing the current upstream implementation with the hierarchical >> counters is done by keeping both implementations wired up in parallel, >> and running a single-process, single-threaded program which hops >> randomly across CPUs in the system, calling mmap(2) and munmap(2) on >> random CPUs, keeping track of an array of allocated mappings, randomly >> choosing entries to either map or unmap. >> >> get_mm_counter() is instrumented to compare the upstream counter >> approximation to the precise value, and print the delta when going over >> a given threshold. The delta of the hierarchical counter approximation >> to the precise value is also printed for comparison. >> >> After a few minutes running this test, the upstream implementation >> counter approximation reaches a 1GB delta from the >> precise value, compared to 80MB delta with the hierarchical counter. >> The hierarchical counter provides a guaranteed maximum approximation >> inaccuracy of 192MB on that hardware topology. >> >> * Fast path implementation comparison >> >> The new inline percpu_counter_tree_add() uses a this_cpu_add_return() >> for the fast path (under a certain allocation size threshold). Above >> that, it calls a slow path which "trickles up" the carry to upper level >> counters with atomic_add_return. >> >> In comparison, the upstream counters implementation calls >> percpu_counter_add_batch which uses this_cpu_try_cmpxchg() on the fast >> path, and does a raw_spin_lock_irqsave above a certain threshold. >> >> The hierarchical implementation is therefore expected to have less >> contention on mid-sized allocations than the upstream counters because >> the atomic counters tracking those bits are only shared across nearby >> CPUs. In comparison, the upstream counters immediately use a global >> spinlock when reaching the threshold. >> >> * Benchmarks >> >> Using will-it-scale page_fault1 benchmarks to compare the upstream >> counters to the hierarchical counters. This is done with hyperthreading >> disabled. The speedup is within the standard deviation of the upstream >> runs, so the overhead is not significant. >> >> upstream hierarchical speedup >> page_fault1_processes -s 100 -t 1 614783 615558 +0.1% >> page_fault1_threads -s 100 -t 1 612788 612447 -0.1% >> page_fault1_processes -s 100 -t 96 37994977 37932035 -0.2% >> page_fault1_threads -s 100 -t 96 2484130 2504860 +0.8% >> page_fault1_processes -s 100 -t 192 71262917 71118830 -0.2% >> page_fault1_threads -s 100 -t 192 2446437 2469296 +0.1% >> >> This change depends on the following patch: >> "mm: Fix OOM killer inaccuracy on large many-core systems" [2] > > As mentioned in the previous patch, it would be great to explicitly > mention what is the memory price for the new tracking data structure. Yes, I can add the explanation here as well. > > Other than that this seems like a generally useful improvement for > larger systems and it is my understanding that it doesn't add almost any > overhead on small end systems, correct? Indeed, the impact is mostly on large many-core systems, not so much on smaller systems. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com