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

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Message-Id: <20251126122943.2099086-1-sunshaojie@kylinos.cn>
Date: Wed, 26 Nov 2025 20:29:43 +0800
From: Sun Shaojie <sunshaojie@...inos.cn>
To: chenridong@...weicloud.com
Cc: cgroups@...r.kernel.org,
	hannes@...xchg.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	llong@...hat.com,
	mkoutny@...e.com,
	shuah@...nel.org,
	sunshaojie@...inos.cn,
	tj@...nel.org
Subject: Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on cpuset.cpus conflict.

Hi, Ridong,

On Mon, 24 Nov 2025 19:33:54, Chen Ridong wrote:
>On 2025/11/24 18:20, Sun Shaojie wrote:
>> I'm sorry, I didn't quite understand the point you were trying to make with this example.
>> 
>> If that's the case
>> 
>>      root cgroup
>>           |
>>           A1
>>        / /  \ \
>>      A2 A3  A4 A5
>> 
>>  #1> echo "0-1" > A1/cpuset.cpus
>>  #2> echo "root" > A1/cpuset.cpus.partition
>>  #3> echo "0-1" > A2/cpuset.cpus
>>  #4> echo "root" > A2/cpuset.cpus.partition
>>  mkdir A4
>>  mkdir A5
>>  echo "0" > A4/cpuset.cpus
>
>If we don't apply your patch, A2 will be invalidated.
>
>>  echo $$ > A4/cgroup.procs  ->This will return an error because A4's effective CPUs are empty.
>>  echo "1" > A5/cpuset.cpus
>>  echo $$ > A5/cgroup.procs  ->This will return an error because A5's effective CPUs are empty.
>> 
>> Even with this patch applied, this result will not change.
>> 
>
>You can have a try, the result I got:
>
># mkdir A1
># echo "0-1" > A1/cpuset.cpus
># echo "root" > A1/cpuset.cpus.partition
># cd A1/
># mkdir A2
># mkdir A4
># mkdir A5
># echo "0-1" > A2/cpuset.cpus
># echo "root" > A2/cpuset.cpus.partition
>#
># echo "0" > A4/cpuset.cpus
># cat A2/cpuset.cpus
>0-1
># cat A2/cpuset.cpus.partition
>root invalid
># cat A4/cpuset.cpus.effective
>0

A4's cpuset.cpus.effective is 0 because A2 changed from root to root invalid. 
However, the purpose of this patch is precisely to keep A2 as "root".

Before 'echo "0" > A4/cpuset.cpus', A4 is aware that its cpuset.cpus.effective
is empty and that its parent's cpuset.cpus.effective is also empty. Therefore,
after executing 'echo "0" > A4/cpuset.cpus', A4 should anticipate the 
possibility that it may not be allocated any available CPUs.

Thanks,
Sun Shaojie