linux-kernel - Re: [PATCH v2 1/4] cgroup: Child cgroup creation not allowed on invalid domain

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Date:   Mon, 24 Jul 2017 13:48:51 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de, torvalds@...ux-foundation.org,
        guro@...com
Subject: Re: [PATCH v2 1/4] cgroup: Child cgroup creation not allowed on
 invalid domain

On 07/22/2017 09:43 AM, Tejun Heo wrote:
> Hello, Waiman.
>
> On Fri, Jul 21, 2017 at 04:34:50PM -0400, Waiman Long wrote:
>> When thread mode is used, it is possible that some cgroups may be
>> in an invalid state. Currently users may not be aware that they are
>> invalid until they try to migrate tasks over. This patch disallows
>> child cgroup creation on invalid domain. This adds one more failure
>> point in reminding users that they are dealing with invalid domains.
>> It also minimizes the number of invalid domains outstanding as much
>> as possible.
> It's a bit inconsistent because we can reach the same forbidden state
> by turning a sibling cgroup threaded.  Please consider the following.
>
>      A
>     / \
>    B   C
>         \
> 	 D
>
> Let's say all are domains and we make B threaded.  A becomes the
> threaded domain, C and D become invalid, which is the configuration
> you're trying to prevent.  We can either enabling threaded on B too or
> relax type modifications further so that people can make C threaded
> which makes sense given that that would lead to a topology which has
> to supported anyway (if C were threaded before D was created, it'd
> look the same).
>
> So, I'm leaning more towards relaxing restrictions and tightening it,
> and given that we have to expose invalid state anyway, I think there's
> actual benefit in doing so as it gives more flexibility while building
> the hierarchy.

Yes, I totally understand that we could have this happened in a sibling
node. It is just my idea to make users become more aware that they are
dealing with an invalid domain cgroup. It was just a suggestion on my
part and I am totally fine if it is not merged.

Cheers,
Longman