Re: [PATCH] control groups: documentation improvements

[Li Zefan <lizefan@huawei.com>]

On 2014/3/14 0:04, Glyn Normington wrote:
> Hi Tejun
> 
> Stepping back from the patch for a while, we'd like to explore the issues you raise. Please bear with us as we try to capture the ideas precisely.
> 
> Continued inline...
> 
> Regards,
> Glyn (& Steve Powell, copied)
> 
> On 10/03/2014 14:20, Tejun Heo wrote:
>> Hey,
>>
>> On Mon, Mar 10, 2014 at 02:17:21PM +0000, Glyn Normington wrote:
>>> Then we missed how to create a hierarchy with no associated
>>> subsystems. The only way I can think of is to use mount, specify no
>>> subsystems on -o (which defaults to all the subsystems defined in
>>> the kernel), and run it in a kernel with no subsystems defined
>>> (which seems unlikely these days).
>>>
>>> Is that what you had in mind or is there some other way of creating
>>> a hierarchy with no subsystems attached?
>> Hierarchy name should be specified "-o name=" for hierarchies w/o any
>> controllers.
> 
> According to cgroups.txt:
> 
>  When passing a name=<x> option for a new hierarchy, you need to
>  specify subsystems manually; the legacy behaviour of mounting all
>  subsystems when none are explicitly specified is not supported when
>  you give a subsystem a name.
> 
> So the documentation certainly does not make it clear that it is valid to specify no subsystems.
> 
> We tried this (on a 3.11 kernel) and can't get it to work:
> 
> # mount -t cgroup -o name=th none /home/glyn/h1

# mount -t cgroup -o name=th,none none /home/glyn/h1

and then

# mount -t cgroup -o name=th none /home/glyn/h2

> 
> mount: wrong fs type, bad option, bad superblock on none,
>     missing codepage or helper program, or other error
>     (for several filesystems (e.g. nfs, cifs) you might
>     need a /sbin/mount.<type> helper program) In some
>     cases useful info is found in syslog - try dmesg |
>     tail or so
> 
> Please could you supply an example which works?
> 
>>>>> Clarify that subsystems may be attached to multiple hierarchies,
>>>>> although this isn't very useful, and explain what happens.
>>>> And a subsystem may only be attached to a single hierarchy.
>>> Perhaps that's what should happen, but the following experiment
>>> demonstrates a subsystem being attached to two hierarchies:
>>>
>>> $ pwd
>>> /home/vagrant
>>> $ mkdir mem1
>>> $ mkdir mem2
>>> $ sudo su
>>> # mount -t cgroup -o memory none /home/vagrant/mem1
>>> # mount -t cgroup -o memory none /home/vagrant/mem2
>>> # cd mem1
>>> # mkdir inst1
>>> # ls inst1
>>> cgroup.clone_children  memory.failcnt ...
>>> # ls ../mem2
>>> cgroup.clone_children  inst1 memory.limit_in_bytes ...
>>> # cd inst1
>>> # echo 1000000 > memory.limit_in_bytes
>>> # cat memory.limit_in_bytes
>>> 1003520
>>> # cat ../../mem2/inst1/memory.limit_in_bytes
>>> 1003520
>>> # echo $$ > tasks
>>> # cat tasks
>>> 1365
>>> 1409
>>> # cat ../../mem2/inst1/tasks
>>> 1365
>>> 1411
>> You're mounting the same hierarchy twice.  Those are two views into
>> the same hierarchy.
>>
>> Thanks.
>>
> Yes, it does appear that this is what is going on, but to explain it this way turns out to be more complicated than one might expect. Here's an attempt...
> 

Yeah, this can only confuse people.

I don't think we need extra explanation, because we all know the same
filesystem can have more than one mount point, and cgroupfs is no
different.

> ---
>  A*hierarchy*  is a non-empty set of cgroups arranged in a tree and a
>  non-empty set of subsystems such that the cgroups in the hierarchy
>  partition all the tasks in the system (in other words, every task in the
>  system is in exactly one of the cgroups in the hierarchy) and each
>  subsystem attaches its own state to each cgroup in the hierarchy.
> 
>  There may be zero or more active hierarchies. Each hierarchy has one
>  or more views associated with it. A *view* is an instance of the cgroup
>  virtual filesystem. The tree of cgroups of a hierarchy is represented
>  by the directory tree in the cgroup virtual filesystem of each view of
>  the hierarchy. All the directory trees in the cgroup virtual filesystem
>  of the views of a given hierarchy have identical content.
> 
>  No subsystem may participate in more than one hierarchy.
> ---
> 
> We'd also need to explain the behaviour of mount and umount with respect to views. The first time a cgroup mount is performed with a given set of subsystems, a hierarchy is created and a view of the hierarchy is created and associated with a cgroup filesystem at the mount point. Subsequently, if another cgroup mount is performed with the same set of subsystems, no new hierarchy is created but a new view of the existing hierarchy is created and associated with the cgroup filesystem at the new mount point.
> 
> Unmounting a cgroup mount point destroys a particular view and destroys the hierarchy associated with the view if and only if the view is the only (remaining) view of the hierarchy.
> 
> So does introducing the concept of a view really help? The wording in the patch does without the concept by allowing two "hierarchies" to have identical content (so they are indistinguishable).
> 
> "views" don't seem to have any practical benefit. If we can avoid explaining concepts which have no use, we probably should. ;-)
> 
> 
> .
>