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 Faw2j9CP 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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 2025-11-27 18:36, Gabriel Krisman Bertazi wrote: > The cost of the pcpu memory allocation is non-negligible for systems > with many cpus, and it is quite visible when forking a new task, as > reported in a few occasions. I've come to the same conclusion within the development of the hierarchical per-cpu counters. But while the mm_struct has a SLAB cache (initialized in kernel/fork.c:mm_cache_init()), there is no such thing for the per-mm per-cpu data. In the mm_struct, we have the following per-cpu data (please let me know if I missed any in the maze): - struct mm_cid __percpu *pcpu_cid (or equivalent through struct mm_mm_cid after Thomas Gleixner gets his rewrite upstream), - unsigned int __percpu *futex_ref, - NR_MM_COUNTERS rss_stats per-cpu counters. What would really reduce memory allocation overhead on fork is to move all those fields into a top level "struct mm_percpu_struct" as a first step. This would merge 3 per-cpu allocations into one when forking a new task. Then the second step is to create a mm_percpu_struct cache to bypass the per-cpu allocator. I suspect that by doing just that we'd get most of the performance benefits provided by the single-threaded special-case proposed here. I'm not against special casing single-threaded if it's still worth it after doing the underlying data structure layout/caching changes I'm proposing here, but I think we need to fix the memory allocation overhead issue first before working around it with special cases and added complexity. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com