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

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Message-ID: <f90d7646-99a3-48f5-ba2e-850c73592080@gentwo.org>
Date: Tue, 8 Apr 2025 09:37:18 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc: Sweet Tea Dorminy <sweettea@...gle.com>, Mateusz Guzik <mjguzik@...il.com>, 
    linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, 
    "Paul E. McKenney" <paulmck@...nel.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, Mathieu Desnoyers wrote:

> - Minimize contention when incrementing and decrementing counters,
> - Provide fast access to a sum approximation,

In general I like this as a abstraction of the Zoned VM counters in
vmstat.c that will make the scalable counters there useful elsewhere.

> It aims at fixing the per-mm RSS tracking which has become too
> inaccurate for OOM killer purposes on large many-core systems [1].

There are numerous cases where these issues occur. I know of a few I could
use something like this.

> The hierarchical per-CPU counters propagate a sum approximation through
> a binary tree. When reaching the batch size, the carry is propagated
> through a binary tree which consists of log2(nr_cpu_ids) levels. The
> batch size for each level is twice the batch size of the prior level.

A binary tree? Could we do this N-way? Otherwise the tree will be 8 levels
on a 512 cpu machine. Given the inflation of the number of cpus this
scheme better work up to 8K cpus.

> +int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter);
> +int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b);
> +int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v);

Precise? Concurrent counter updates can occur while determining the global
value. People may get confused.

Also maybe there would be a need for a function to collape the values into
the global if f.e. a cpu goes off line or in order to switch off OS
activities on a cpu.