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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v7 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst
To:     Tejun Heo <tj@kernel.org>, Waiman Long <llong@redhat.com>
Cc:     Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Phil Auld <pauld@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>
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Date:   Fri, 27 Aug 2021 21:14:01 -0400
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On 8/27/21 7:35 PM, Tejun Heo wrote:
> Hello,
>
> On Fri, Aug 27, 2021 at 06:50:10PM -0400, Waiman Long wrote:
>> The cpu exclusivity rule is due to the setting of CPU_EXCLUSIVE bit. This is
>> a pre-existing condition unless you want to change how the
>> cpuset.cpu_exclusive works.
>>
>> So the new rules will be:
>>
>> 1) The "cpuset.cpus" is not empty and the list of CPUs are exclusive.
> Empty cpu list can be considered an exclusive one.
It doesn't make sense to me to have a partition with no cpu configured 
at all. I very much prefer the users to set cpuset.cpus first before 
turning it into a partition.
>
>> 2) The parent cgroup is a partition root (can be an invalid one).
> Does this mean a partition parent can't stop being a partition if one or
> more of its children become partitions? If so, it violates the rule that a
> descendant shouldn't be able to restrict what its ancestors can do.

No. As I said in the documentation, transitioning from partition root to 
member is allowed. Against, it is illogical to allow a cpuset to become 
a potential partition if it parent is not even a partition root at all. 
In the case that the parent is reverted back to a member, the child 
partitions will stay invalid forever unless the parent become a valid 
partition again.

>
>> 3) The "cpuset.cpus" is a subset of the parent's cpuset.cpus.allowed.
> Why not just go by effective? This would mean that a parent can't withdraw
> CPUs from its allowed set once descendants are configured. Restrictions like
> this are fine when the entire hierarchy is configured by a single entity but
> become awkward when configurations are multi-tiered, automated and dynamic.

The original rule is to be based on effective cpus. However, to properly 
handle the case of allowing offlined cpus to be included in the 
partition, I have to change it to cpu_allowed instead. I can certainly 
change it back to effective if you prefer.

>
>> 4) No child cgroup with cpuset enabled.
> idk, maybe? I'm having a hard time seeing the point in adding these
> restrictions when the state transitions are asynchronous anyway. Would it
> help if we try to separate what's absoluately and technically necessary and
> what seems reasonable or high bar and try to justify why each of the latter
> should be added?

This rule is there mainly for ease of implementation. Otherwise, I need 
to add additional code to handle the conversion of child cpusets which 
can be rather complex and require a lot more debugging. This rule will 
no longer apply once the cpuset becomes a partition root.

Cheers,
Longman