From mboxrd@z Thu Jan  1 00:00:00 1970
From: Zefan Li <lizefan@huawei.com>
Subject: Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of
 the parent
Date: Thu, 31 May 2018 16:12:34 +0800
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 <5B0F4F09.9050100@huawei.com>
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To: Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>, Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@fb.com, pjt@google.com, luto@amacapital.net, Mike Galbraith <efault@gmx.de>, torvalds@linux-foundation.org, Roman Gushchin <guro@fb.com>, Juri Lelli <juri.lelli@redhat.com>, Patrick Bellasi <patrick.bellasi@arm.com>

On 2018/5/31 9:25, Zefan Li wrote:
> Hi Waiman,
> 
> On 2018/5/30 21:46, Waiman Long wrote:
>> It was found that the cpuset.cpus could contain CPUs that are not listed
>> in their parent's cpu list as shown by the command sequence below:
>>
>>   # echo "+cpuset" >cgroup.subtree_control
>>   # mkdir g1
>>   # echo 0-5 >g1/cpuset.cpus
>>   # mkdir g1/g11
>>   # echo "+cpuset" > g1/cgroup.subtree_control
>>   # echo 6-11 >g1/g11/cpuset.cpus
>>   # grep -R . g1 | grep "\.cpus"
>>   g1/cpuset.cpus:0-5
>>   g1/cpuset.cpus.effective:0-5
>>   g1/g11/cpuset.cpus:6-11
>>   g1/g11/cpuset.cpus.effective:0-5
>>
>> As the intersection of g11's cpus and that of g1 is empty, the effective
>> cpus of g11 is just that of g1. The check in update_cpumask() is now
>> corrected to make sure that cpus in a child cpus must be a subset of
>> its parent's cpus. The error "write error: Invalid argument" will now
>> be reported in the above case.
>>
> 
> We made the distinction between user-configured CPUs and effective CPUs
> in commit 7e88291beefbb758, so actually it's not a bug.
> 

I remember the original reason is to support restoration of the original
cpu after cpu offline->online. We use user-configured CPUs to remember
if the cpu should be restored in the cpuset after it's onlined.