From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] cpuset: Enforce that a child's cpus must be a subset of
 the parent
Date: Thu, 31 May 2018 10:26:13 +0200
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To: Zefan Li <lizefan@huawei.com>
Cc: Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>, Ingo Molnar <mingo@redhat.com>, 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 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?