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McKenney" To: "Christoph Lameter (Ampere)" Cc: Mathieu Desnoyers , Sweet Tea Dorminy , Mateusz Guzik , linux-kernel@vger.kernel.org, Andrew Morton , Steven Rostedt , Masami Hiramatsu , Dennis Zhou , Tejun Heo , Martin Liu , David Rientjes , christian.koenig@amd.com, Shakeel Butt , Johannes Weiner , Lorenzo Stoakes , "Liam R . Howlett" , 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 , Matthew Wilcox Subject: Re: [RFC PATCH v2] Introduce Hierarchical Per-CPU Counters Message-ID: <9f42144b-dca9-41b0-a80b-2e402cb5ab98@paulmck-laptop> Reply-To: paulmck@kernel.org References: <20250408160508.991738-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 2D89B1C0008 X-Stat-Signature: m4pmb3iye8uirtq71kqygfgyc8zsqjt1 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1744146041-188163 X-HE-Meta: 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 C+LO9Hbm 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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 Tue, Apr 08, 2025 at 01:44:25PM -0700, Christoph Lameter (Ampere) wrote: > On Tue, 8 Apr 2025, Mathieu Desnoyers wrote: > > > Currently percpu_counter_tree_precise_sum_unbiased() iterates on each > > possible cpu, which does not require cpu hotplug integration. > > Well that looks like a performance issue if you have a system that can > expand to 8K cpus but currently only has 16 or so online. RCU handles this by iterating from zero to nr_cpu_ids, which is set during early boot. It also builds its tree-shaped data structures during early boot based on nr_cpu_ids. Thanx, Paul