linux-kernel - Re: [PATCH 5/6] cgroup: implement cgroup v2 thread support

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Date:   Mon, 17 Jul 2017 16:56:32 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>
Cc:     lizefan@...wei.com, hannes@...xchg.org, 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 5/6] cgroup: implement cgroup v2 thread support

On 07/17/2017 10:14 AM, Peter Zijlstra wrote:
> On Sun, Jul 16, 2017 at 10:07:20PM -0400, Tejun Heo wrote:
>> v4: - Updated to marking each cgroup threaded as suggested by PeterZ.
>>
>> +On creation, a cgroup is always a domain cgroup and can be made
>> +threaded by writing "threaded" to the "cgroup.type" file.  The
>> +operation is single direction::
>> +
>> +  # echo threaded > cgroup.type
>> +
>> +Once threaded, the cgroup can't be made a domain again.  To enable the
>> +thread mode, the following conditions must be met.
>> +
>> +- As the cgroup will join the parent's resource domain.  The parent
>> +  must either be a valid (threaded) domain or a threaded cgroup.
>> +
>> +- The cgroup must be empty.  No enabled controllers, child cgroups or
>> +  processes.
>> +
>> +Topology-wise, a cgroup can be in an invalid state.  Please consider
>> +the following toplogy::
>> +
>> +  A (threaded domain) - B (threaded) - C (domain, just created)
>> +
>> +C is created as a domain but isn't connected to a parent which can
>> +host child domains.  C can't be used until it is turned into a
>> +threaded cgroup.  "cgroup.type" file will report "domain (invalid)" in
>> +these cases.  Operations which fail due to invalid topology use
>> +EOPNOTSUPP as the errno.
>> +
>> +A domain cgroup is turned into a threaded domain when one of its child
>> +cgroup becomes threaded or threaded controllers are enabled in the
>> +"cgroup.subtree_control" file while there are processes in the cgroup.
>> +A threaded domain reverts to a normal domain when the conditions
>> +clear.
> AFAICT this is not in fact what I suggested... :/
>
> My proposal did not have that invalid state. It would simply refuse to
> change the type from thread to domain in the case where the parent is
> not a domain.
>
> Also, my proposal maintained the normal property inheritance rules. A
> child cgroup's creation 'type' would be that of its parent and not
> always be 'domain'.

I agree with Peter on this. A new cgroup created under a threaded cgroup
should always be threaded. I don't see a need for an intermediate
invalid domain.

Cheers,
Longman