Re: [PATCH REPOST RFC cgroup/for-3.7] cgroup: mark subsystems with broken hierarchy support and whine if cgroups are nested for them

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

On 09/11/2012 04:38 PM, Li Zefan wrote:
> On 2012/9/11 6:33, Tejun Heo wrote:
>> (forgot cc'ing containers / cgroups mailing lists and used the old
>>  address for Li.  Reposting.  Sorry for the noise.)
>>
>> Currently, cgroup hierarchy support is a mess.  cpu related subsystems
>> behave correctly - configuration, accounting and control on a parent
>> properly cover its children.  blkio and freezer completely ignore
>> hierarchy and treat all cgroups as if they're directly under the root
>> cgroup.  Others show yet different behaviors.
>>
>> These differing interpretations of cgroup hierarchy make using cgroup
>> confusing and it impossible to co-mount controllers into the same
>> hierarchy and obtain sane behavior.
>>
>> Eventually, we want full hierarchy support from all subsystems and
>> probably a unified hierarchy.  Users using separate hierarchies
>> expecting completely different behaviors depending on the mounted
>> subsystem is deterimental to making any progress on this front.
>>
>> This patch adds cgroup_subsys.broken_hierarchy and sets it to %true
>> for controllers which are lacking in hierarchy support.  The goal of
>> this patch is two-fold.
>>
>> * Move users away from using hierarchy on currently non-hierarchical
>>   subsystems, so that implementing proper hierarchy support on those
>>   doesn't surprise them.
>>
>> * Keep track of which controllers are broken how and nudge the
>>   subsystems to implement proper hierarchy support.
>>
>> For now, start with a single warning message.  We can whine louder
>> later on.
>>
>> (I tried to document what's broken and how it should be fixed.  If I
>>  got something wrong, please let me know.)
>>
> 
> So isn't cpuset broken too? child cpuset's cpu mask isn't necessary a subset
> of the parent's if the cpu_exclusive flag is not set.
> 

Li is right, cpu_exclusive is pretty much cpuset's version of memcg's
use_hierarchy.

the only problem is that it seem to be by design, unlike memcg's, which
was there to bypass a performance problem to the best of my knowledge.
Even then, we should maybe think about start enforcing that flag as well.