linux-kernel - Re: [RFC PATCH v2] Introduce Hierarchical Per-CPU Counters

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Message-ID: <9f42144b-dca9-41b0-a80b-2e402cb5ab98@paulmck-laptop>
Date: Tue, 8 Apr 2025 14:00:39 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Sweet Tea Dorminy <sweettea@...gle.com>,
	Mateusz Guzik <mjguzik@...il.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Martin Liu <liumartin@...gle.com>,
	David Rientjes <rientjes@...gle.com>, christian.koenig@....com,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Johannes Weiner <hannes@...xchg.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christian Brauner <brauner@...nel.org>,
	Wei Yang <richard.weiyang@...il.com>,
	David Hildenbrand <david@...hat.com>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Al Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org, Yu Zhao <yuzhao@...gle.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH v2] Introduce Hierarchical Per-CPU Counters

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