linux-kernel - Re: [PATCH v1] cpuset: Avoid unnecessary partition invalidation

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Message-ID: <c8e234f4-2c27-4753-8f39-8ae83197efd3@redhat.com>
Date: Wed, 12 Nov 2025 11:40:18 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>,
 Sun Shaojie <sunshaojie@...inos.cn>
Cc: tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com, shuah@...nel.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1] cpuset: Avoid unnecessary partition invalidation

On 11/11/25 10:33 PM, Chen Ridong wrote:
>
> On 2025/11/12 10:11, Sun Shaojie wrote:
> Hello Shaojie,
>
>> Currently, when a non-exclusive cpuset's "cpuset.cpus" overlaps with a
>> partitioned sibling, the sibling's partition state becomes invalid.
>> However, this invalidation is often unnecessary.
>>
>> This can be observed in specific configuration sequences:
>>
>> Case 1: Partition created first, then non-exclusive cpuset overlaps
>>   #1> mkdir -p /sys/fs/cgroup/A1
>>   #2> echo "0-1" > /sys/fs/cgroup/A1/cpuset.cpus
>>   #3> echo "root" > /sys/fs/cgroup/A1/cpuset.cpus.partition
>>   #4> mkdir -p /sys/fs/cgroup/B1
>>   #5> echo "0-3" > /sys/fs/cgroup/B1/cpuset.cpus
>>   // A1's partition becomes "root invalid" - this is unnecessary
>>
>> Case 2: Non-exclusive cpuset exists first, then partition created
>>   #1> mkdir -p /sys/fs/cgroup/B1
>>   #2> echo "0-1" > /sys/fs/cgroup/B1/cpuset.cpus
>>   #3> mkdir -p /sys/fs/cgroup/A1
>>   #4> echo "0-1" > /sys/fs/cgroup/A1/cpuset.cpus
>>   #5> echo "root" > /sys/fs/cgroup/A1/cpuset.cpus.partition
>>   // A1's partition becomes "root invalid" - this is unnecessary
>>
>> In Case 1, the effective CPU mask of B1 can differ from its requested
>> mask. B1 can use CPUs 2-3 which don't overlap with A1's exclusive
>> CPUs (0-1), thus not violating A1's exclusivity requirement.
>>
>> In Case 2, B1 can inherit the effective CPUs from its parent, so there
>> is no need to invalidate A1's partition state.
>>
>> This patch relaxes the overlap check to only consider conflicts between
>> partitioned siblings, not between a partitioned cpuset and a regular
>> non-exclusive one.
>>
The current cgroup v2 exclusive cpuset behavior follows the v1 behavior 
of cpuset.cpus.exclusive flag. Even if we want to relax the cgroup v2 
behavior, we will still need to maintain the v1 behavior as we want to 
minimize any changes to cgroup v1.  IOW, we have to gate this change 
specific to v2.

Cheers,
Longman