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

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Message-ID: <8897b4e1-b5ba-807d-7bc4-5a6080a93462@gentwo.org>
Date: Tue, 8 Apr 2025 14:21:59 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: "Paul E. McKenney" <paulmck@...nel.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, 8 Apr 2025, Paul E. McKenney wrote:

> 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.

nr_cpu_ids is better but there are funky things like the default bios of a
major server vendor indicating 256 or so possible cpus although only 2
were installed. Thus nr_cpu_id is 256. Presumably some hardware
configurations can support onlining 256 cpus....