linux-kernel - Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on cpuset.cpus conflict.

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Message-ID: <3jkvuruuxdykpxjjdwhuqjfqi4nrtxojotswaoc7ehuwxp4s32@hfrvfato6q5b>
Date: Mon, 8 Dec 2025 15:32:01 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Waiman Long <llong@...hat.com>
Cc: Sun Shaojie <sunshaojie@...inos.cn>, chenridong@...weicloud.com, 
	cgroups@...r.kernel.org, hannes@...xchg.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, shuah@...nel.org, tj@...nel.org
Subject: Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on
 cpuset.cpus conflict.

Hi Waiman.

On Wed, Nov 26, 2025 at 02:43:50PM -0500, Waiman Long <llong@...hat.com> wrote:
> Modification to cpumasks are all serialized by the cpuset_mutex. If you are
> referring to 2 or more tasks doing parallel updates to various cpuset
> control files of sibling cpusets, the results can actually vary depending on
> the actual serialization results of those operations.

I meant the latter when the difference in results when concurrent tasks
do the update (e.g. two containers start in parallel), I don't see an
issue with the race wrt consistency of in-kernel data. We're on the same
page here.

> One difference between cpuset.cpus and cpuset.cpus.exclusive is the fact
> that operations on cpuset.cpus.exclusive can fail if the result is not
> exclusive WRT sibling cpusets, but becoming a valid partition is guaranteed
> unless none of the exclusive CPUs are passed down from the parent. The use
> of cpuset.cpus.exclusive is required for creating remote partition.
> 
> OTOH, changes to cpuset.cpus will never fail, but becoming a valid partition
> root is not guaranteed and is limited to the creation of local partition
> only.
> 
> Does that answer your question?

It does help my understanding. Do you envision that remote and local
partitions should be used together (in one subtree)?

Thanks,
Michal

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