linux-kernel - Re: [PATCH v11 7/9] cpuset: Expose cpus.effective and mems.effective on cgroup v2 root

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Message-ID: <4857a9db-ebf5-24f8-c42d-d795f5c75854@redhat.com>
Date:   Fri, 20 Jul 2018 12:19:29 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        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 v11 7/9] cpuset: Expose cpus.effective and mems.effective
 on cgroup v2 root

On 07/20/2018 11:56 AM, Tejun Heo wrote:
> Hello, Peter.
>
> On Fri, Jul 20, 2018 at 05:44:54PM +0200, Peter Zijlstra wrote:
>>> The currently proposed implementation is somewhere in the middle.  It
>>> kinda gets there by restricting a partition to be a child of another
>>> partition, which may be okay but it does make the whole delegation
>>> mechanism less useful.
>> So the implementation does not set ownership of the 'partition' file to
>> that of the parent directory? Because _that_ is what I understood from
>> Waiman (many versions ago). And that _does_ allow delegation to work
>> nicely.
> So, that part isn't the problem.  The problem is that if we allow
> partitioning nested further away from ancestor, the descendant would
> be able to take away reources from the removed ancestor.  Waiman
> avoids this problem by only allowing partitions to nest but then it's
> kinda weird cuz it's just a separate tree.

The rationale behind the current design is that a parent is allowed to
create a child partition if it fully owns all the cpus it has. That is
true only if it is a partition itself. IOW, making a cpuset a partition
won't affect any ancestors other than the parent.


>>>>> 2. take away any given cpu from ist subtree.
>>>> I really hate this obsession of yours and doubly so for partitions. But
>>>> why would this currently not be allowed?
>>> Well, sorry that you hate it.  It's a fundamental architectural
>>> constraint.  If it can't satisfy that, it should't be in cgroup.
>> So is hierarchical behaviour; but you seem willing to forgo that.
> Well, we do have root or first-level only things.  Things just need to
> be properly delegatable when they're hierarchical (and things should
> be hierarchical only when that actually means something).  While not
> desirable, considering the history, accomodating this specific usage
> that way seems acceptable to me.

I am not against the idea of making it hierarchical eventually. I am
just hoping to get thing going by merging the patchset in its current
form and then we can make it hierarchical in a followup patch.

Cheers,
Longman