Re: [PATCH] cgroup: remove unnecessary empty check when enabling threaded mode

[Waiman Long]

On 07/23/2017 08:18 AM, Tejun Heo wrote:
> cgroup_enable_threaded() checks that the cgroup doesn't have any tasks
> or children and fails the operation if so.  This test is unnecessary
> because the first part is already checked by
> cgroup_can_be_thread_root() and the latter is unnecessary.  The latter
> actually cause a behavioral oddity.  Please consider the following
> hierarchy.  All cgroups are domains.
>
>     A
>    / \
>   B   C
>        \
>         D
>
> If B is made threaded, C and D becomes invalid domains.  Due to the no
> children restriction, threaded mode can't be enabled on C.  For C and
> D, the only thing the user can do is removal.
>
> There is no reason for this restriction.  Remove it.
>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> ---
> Hello,
>
> So, the only thing inconsistent there was the extra restriction when
> enabling threaded mode when all the necessary conditions are already
> checked by when verifying the parent's domain.
>
> Thanks.
>
>  Documentation/cgroup-v2.txt |    5 +++--
>  kernel/cgroup/cgroup.c      |    7 -------
>  2 files changed, 3 insertions(+), 9 deletions(-)
>
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -274,8 +274,9 @@ thread mode, the following conditions mu
>  - 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.
> +- When the parent is an unthreaded domain, it must not have any domain
> +  controllers enabled or populated domain children.  The root is
> +  exempt from this requirement.
>  
>  Topology-wise, a cgroup can be in an invalid state.  Please consider
>  the following toplogy::
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3147,13 +3147,6 @@ static int cgroup_enable_threaded(struct
>  		return -EOPNOTSUPP;
>  
>  	/*
> -	 * Allow enabling thread mode only on empty cgroups to avoid
> -	 * implicit migrations and recursive operations.
> -	 */
> -	if (cgroup_has_tasks(cgrp) || css_has_online_children(&cgrp->self))
> -		return -EBUSY;
> -
> -	/*
>  	 * The following shouldn't cause actual migrations and should
>  	 * always succeed.
>  	 */

Acked-by: Waiman Long <longman@xxxxxxxxxx>

While you are at it, the one comment that I have with
cgroup_enable_threaded() is that it does not check to see if the cgroup
parent exists. I thought for a while the first time I saw it, but then
realized that cgroup.type wasn't show in root. I think a comment about
this will make the code less puzzling to causal reader.

Cheers,
Longman