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Subject: Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of
 the parent
To: Peter Zijlstra <peterz@infradead.org>
References: <1527687991-1431-1-git-send-email-longman@redhat.com>
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CC: Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>,
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        Juri Lelli <juri.lelli@redhat.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>
From: Zefan Li <lizefan@huawei.com>
Message-ID: <5B0FB58C.9030705@huawei.com>
Date: Thu, 31 May 2018 16:42:52 +0800
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On 2018/5/31 16:26, Peter Zijlstra wrote:
> On Thu, May 31, 2018 at 04:12:34PM +0800, Zefan Li wrote:
>> 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.
> 
> AFAICT you can do that and still have the child a subset of the parent,
> no?
> .

Sure. IIRC this was suggested by Tejun as he had done something similar to devcgroup.