linux-kernel - Re: [PATCH] lib/group_cpus: fix cross-NUMA CPU assignment in group_cpus

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Message-Id: <20251221112354.3a0ee9e1824f2cac9572d170@linux-foundation.org>
Date: Sun, 21 Dec 2025 11:23:54 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ming Lei <ming.lei@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
 linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>, Wangyang Guo
 <wangyang.guo@...el.com>
Subject: Re: [PATCH] lib/group_cpus: fix cross-NUMA CPU assignment in
 group_cpus_evenly

On Mon, 20 Oct 2025 20:46:46 +0800 Ming Lei <ming.lei@...hat.com> wrote:

> When numgrps > nodes, group_cpus_evenly() can incorrectly assign CPUs
> from different NUMA nodes to the same group due to the wrapping logic.
> Then poor block IO performance is caused because of remote IO completion.
> And it can be avoided completely in case of `numgrps > nodes` because
> each numa node may includes more CPUs than group's.

Please quantify "poor block IO performance", to help people understand
the userspace-visible effect of this change.

> The issue occurs when curgrp reaches last_grp and wraps to 0. This causes
> CPUs from later-processed nodes to be added to groups that already contain
> CPUs from earlier-processed nodes, violating NUMA locality.
> 
> Example with 8 NUMA nodes, 16 groups:
> - Each node gets 2 groups allocated
> - After processing nodes, curgrp reaches 16
> - Wrapping to 0 causes CPUs from node N to be added to group 0 which
>   already has CPUs from node 0
> 
> Fix this by adding find_next_node_group() helper that searches for the
> next group (starting from 0) that already contains CPUs from the same
> NUMA node. When wrapping is needed, use this helper instead of blindly
> wrapping to 0, ensuring CPUs are only added to groups within the same
> NUMA node.
> 
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
>  lib/group_cpus.c | 28 +++++++++++++++++++++++++---

The patch overlaps (a lot) with Wangyang Guo's "lib/group_cpus: make
group CPU cluster aware".  I did a lot of surgery but got stuck on the
absence of node_to_cpumask, so I guess the patch has bitrotted.

Please update the changelog as above and redo this patch against
Wangyang's patch (which will be in linux-next very soon).

Also, it would be great if you and Wangyang were to review and test
each other's changes, thanks.