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

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Message-ID: <06d74676-258e-43b7-ae61-d2102bab3926@huaweicloud.com>
Date: Thu, 20 Nov 2025 08:57:51 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Michal Koutný <mkoutny@...e.com>,
 Sun Shaojie <sunshaojie@...inos.cn>
Cc: llong@...hat.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 2025/11/19 21:20, Michal Koutný wrote:
> On Wed, Nov 19, 2025 at 06:57:49PM +0800, Sun Shaojie <sunshaojie@...inos.cn> wrote:
>> Currently, when setting a cpuset's cpuset.cpus to a value that conflicts
>> with its sibling partition, the sibling's partition state becomes invalid.
>> However, this invalidation is often unnecessary. If the cpuset being
>> modified is exclusive, it should invalidate itself upon conflict.
>>
>> This patch applies only to the following two cases：
>>
>> Assume the machine has 4 CPUs (0-3).
>>
>>    root cgroup
>>       /    \
>>     A1      B1
>>
>> Case 1: A1 is exclusive, B1 is non-exclusive, set B1's cpuset.cpus
>>
>>  Table 1.1: Before applying this patch
>>  Step                                       | A1's prstate | B1's prstate |
>>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>>  #3> echo "0" > B1/cpuset.cpus              | root invalid | member       |
>>
>> After step #3, A1 changes from "root" to "root invalid" because its CPUs
>> (0-1) overlap with those requested by B1 (0). However, B1 can actually
>> use CPUs 2-3(from B1's parent), so it would be more reasonable for A1 to
>> remain as "root."
>>
>>  Table 1.2: After applying this patch
>>  Step                                       | A1's prstate | B1's prstate |
>>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>>  #3> echo "0" > B1/cpuset.cpus              | root         | member       |
>>
>> Case 2: Both A1 and B1 are exclusive, set B1's cpuset.cpus
> 
> (Thanks for working this out, Shaojie.)
> 
>>
>>  Table 2.1: Before applying this patch
>>  Step                                       | A1's prstate | B1's prstate |
>>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>>  #3> echo "2" > B1/cpuset.cpus              | root         | member       |
>>  #4> echo "root" > B1/cpuset.cpus.partition | root         | root         |
>>  #5> echo "1-2" > B1/cpuset.cpus            | root invalid | root invalid |
>>
>> After step #4, B1 can exclusively use CPU 2. Therefore, at step #5,
>> regardless of what conflicting value B1 writes to cpuset.cpus, it will
>> always have at least CPU 2 available. This makes it unnecessary to mark
>> A1 as "root invalid".
>>
>>  Table 2.2: After applying this patch
>>  Step                                       | A1's prstate | B1's prstate |
>>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>>  #3> echo "2" > B1/cpuset.cpus              | root         | member       |
>>  #4> echo "root" > B1/cpuset.cpus.partition | root         | root         |
>>  #5> echo "1-2" > B1/cpuset.cpus            | root         | root invalid |
>>
>> In summary, regardless of how B1 configures its cpuset.cpus, there will
>> always be available CPUs in B1's cpuset.cpus.effective. Therefore, there
>> is no need to change A1 from "root" to "root invalid".
> 
> Admittedly, I don't like this change because it relies on implicit
> preference ordering between siblings (here first comes, first served)

Agree. If we only invalidate the latter one, I think regardless of the implementation approach, we
may end up with different results depending on the order of operations.

> and so the effective config cannot be derived just from the applied
> values :-/
> 
> Do you actually want to achieve this or is it an implementation
> side-effect of the Case 1 scenario that you want to achieve?
> 
> 
> Thanks,
> Michal

-- 
Best regards,
Ridong