linux-kernel - Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of the parent

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Date:   Thu, 31 May 2018 16:42:52 +0800
From:   Zefan Li <lizefan@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, <cgroups@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <kernel-team@...com>, <pjt@...gle.com>, <luto@...capital.net>,
        Mike Galbraith <efault@....de>,
        <torvalds@...ux-foundation.org>, Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of
 the parent

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.