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

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Message-ID: <d5d635df-94f3-4909-afe3-f2e6141afa32@redhat.com>
Date: Wed, 26 Nov 2025 14:43:50 -0500
From: Waiman Long <llong@...hat.com>
To: Michal Koutný <mkoutny@...e.com>,
 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.

On 11/26/25 9:13 AM, Michal Koutný wrote:
> On Mon, Nov 24, 2025 at 05:30:47PM -0500, Waiman Long <llong@...hat.com> wrote:
>> In the example above, the final configuration is A1:0-1 & B1:1-2. As the cpu
>> lists overlap, we can't have both of them as valid partition roots. So
>> either one of A1 or B1 is valid or they are both invalid. The current code
>> makes them both invalid no matter the operation ordering.  This patch will
>> make one of them valid given the operation ordering above. To minimize
>> partition invalidation, we will have to live with the fact that it will be
>> first-come first-serve as noted by Michal. I am not against this, we just
>> have to document it. However, the following operation order will still make
>> both of them invalid:
> I'm skeptical of the FCFS behavior since I'm afraid it may be subject to
> race conditions in practice.
> BTW should cpuset.cpus and cpuset.cpus.exclusive have different behavior
> in this regard?

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.

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?

Cheers,
Longman