linux-kernel - Re: [PATCH] mm: use per-numa-node atomics instead of percpu

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Message-ID: <a89cb4d9-088e-4ed6-afde-f1b097de8db9@efficios.com>
Date: Thu, 27 Mar 2025 16:35:35 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Mateusz Guzik <mjguzik@...il.com>,
 Sweet Tea Dorminy <sweettea-kernel@...miny.me>
Cc: 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>, Christoph Lameter <cl@...ux.com>,
 Martin Liu <liumartin@...gle.com>, David Rientjes <rientjes@...gle.com>,
 Jani Nikula <jani.nikula@...el.com>, Sweet Tea Dorminy
 <sweettea@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Christian Brauner <brauner@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 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-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Yu Zhao <yuzhao@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH] mm: use per-numa-node atomics instead of percpu_counters

On 2025-03-26 19:36, Mateusz Guzik wrote:
[...]
> 
> Hell, it may be your patch as is can be easily repurposed to
> decentralize the main percpu counter? I mean perhaps there is no need
> for any fancy hierarchical structure.

Here is an initial attempt at a design document for the hierarchical
percpu counters. Feedback welcome!

Split Counters With Binary Tree Approximation Propagation
=========================================================

Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
March 27, 2025

* Propagation diagram when reaching batch size thresholds (± batch size):

Example diagram for 8 CPUs:

log2(8) = 3 levels

At each level, each pair propagates its values to the next level when
reaching the batch size thresholds.

Counters at levels 0, 1, 2 can be kept on a single byte (±128 range).
Counter at level 3 can be kept on a 32/64-bit counter.

Level 0:  0    1    2    3    4    5    6    7
           |   /     |   /     |   /     |   /
           |  /      |  /      |  /      |  /
           | /       | /       | /       | /
Level 1:  0         1         2         3
           |       /           |       /
           |    /              |    /
           | /                 | /
Level 2:  0                   1
           |               /
           |         /
           |   /
Level 3:  0

* Inaccuracy:

BATCH(level N): Level N batch size.

Example for BATCH(level 0) = 4

BATCH(level 0) =  4
BATCH(level 1) =  8
BATCH(level 2) = 16
BATCH(level N) = BATCH(level 0) * 2^N

           per-counter     global
           inaccuracy      inaccuracy
Level 0:    ±  3          ± 24  (8 * 3)
Level 1:    ±  7          ± 28  (4 * 7)
Level 2:    ± 15          ± 30  (2 * 15)
Total:      ------        ± 82  (log2(nr_cpus) * BATCH(level 0) * nr_cpus - Sum[0 .. log2(nr_cpus) - 1](nr_cpus / 2^n)

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com