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

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Message-ID: <aP7Ft5Y0WEMGv7jX@fedora>
Date: Mon, 27 Oct 2025 09:07:03 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] lib/group_cpus: fix cross-NUMA CPU assignment in
 group_cpus_evenly

On Mon, Oct 20, 2025 at 08:46:46PM +0800, Ming Lei 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.
> 
> 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>

Hello,

ping...


Thanks,
Ming