linux-kernel - Re: [PATCH v4] cpuset: Enable cpuset controller in default hierarchy

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Date:   Mon, 19 Mar 2018 17:41:13 -0400
From:   Waiman Long <longman@...hat.com>
To:     Mike Galbraith <efault@....de>, Tejun Heo <tj@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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,
        torvalds@...ux-foundation.org, Roman Gushchin <guro@...com>
Subject: Re: [PATCH v4] cpuset: Enable cpuset controller in default hierarchy

On 03/19/2018 04:49 PM, Mike Galbraith wrote:
> On Mon, 2018-03-19 at 08:34 -0700, Tejun Heo wrote:
>> Hello, Mike.
>>
>> On Thu, Mar 15, 2018 at 03:49:01AM +0100, Mike Galbraith wrote:
>>> Under the hood v2 details are entirely up to you.  My input ends at
>>> please don't leave dynamic partitioning standing at the dock when v2
>>> sails.
>> So, this isn't about implementation details but about what the
>> interface achieves - ie, what's the actual function?  The only thing I
>> can see is blocking the entity which is configuring the hierarchy from
>> making certain configs.  While that might be useful in some specific
>> use cases, it seems to miss the bar for becoming its own kernel
>> feature.  After all, nothing prevents the same entity from clearing
>> the exlusive bit and making the said changes.
> Yes, privileged contexts can maliciously or stupidly step all over one
> other no matter what you do (finite resource), but oxymoron creation
> (CPUs simultaneously balanced and isolated) should be handled.  If one
> context can allocate a set overlapping a set another context intends to
> or already has detached from scheduler domains, both are screwed.
>
> 	-Mike

The allocations of CPUs to child cgroups should be controlled by the
parent cgroup. It is the parent's fault if some CPUs are in both
balanced and isolated cgroups.

How about we don't allow turning off scheduling if the CPUs aren't
exclusive from the parent's perspective? So you can't create an isolated
cgroup if the CPUs aren't exclusive. Will this be a good enough compromise?

Cheers,
Longman