[Devel] Re: How to draw values for /proc/stat

[Glauber Costa]

On 12/12/2011 11:06 AM, Glauber Costa wrote:
> On 12/12/2011 04:31 AM, KAMEZAWA Hiroyuki wrote:
>> On Sun, 11 Dec 2011 15:50:56 +0100
>> Glauber Costa<glommer at parallels.com> wrote:
>>
>>> On 12/09/2011 03:55 PM, Glauber Costa wrote:
>>>> On 12/09/2011 12:03 PM, Peter Zijlstra wrote:
>>>>> On Mon, 2011-12-05 at 07:32 -0200, Glauber Costa wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Specially Peter and Paul, but all the others:
>>>>>>
>>>>>> As you can see in https://lkml.org/lkml/2011/12/4/178, and in my
>>>>>> answer
>>>>>> to that, there is a question - one I've asked before but without that
>>>>>> much of an audience - of whether /proc files read from process
>>>>>> living on
>>>>>> cgroups should display global or per-cgroup resources.
>>>>>>
>>>>>> In the past, I was arguing for a knob to control that, but I recently
>>>>>> started to believe that a knob here will only overcomplicate matters:
>>>>>> if you live in a cgroup, you should display only the resources you
>>>>>> can
>>>>>> possibly use. Global is for whoever is in the main cgroup.
>>>>>>
>>>>>> Now, it comes two questions:
>>>>>> 1) Do you agree with that, for files like /proc/stat ? I think the
>>>>>> most
>>>>>> important part is to be consistent inside the system, regardless
>>>>>> of what
>>>>>> is done
>>>>>
>>>>> Personally I don't give a rats arse about (/proc vs) cgroups :-)
>>>>> Currently /proc is unaffected by whatever cgroup you happen to be
>>>>> in and
>>>>> that seems to make some sort of sense.
>>>>>
>>>>> Namespaces seem to be about limiting visibility, cgroups about
>>>>> controlling resources.
>>>>>
>>>>> The two things are hopelessly disjoint atm, but I believe someone was
>>>>> looking at this mess.
>>>>
>>>> I did take a look at this (if anyone else was, I'd like to know so we
>>>> can share some ideas), but I am not convinced we should do anything to
>>>> join them anymore. We virtualization people are to the best of my
>>>> knowledge the only ones doing namespaces. Cgroups, OTOH, got a lot
>>>> bigger.
>>>>
>>>> What I am mostly concerned about now, is how consistent they will be.
>>>> /proc always being always global indeed does make sense, but my
>>>> question
>>>> still stands: if you live in a resource-controlled world, why should
>>>> you
>>>> even see resources you will never own ?
>>>>
>>>>
>>>>> IOW a /proc namespace coupled to cgroup scope would do what you want.
>>>>> Now my head hurts..
>>>>
>>>> Mine too. The idea is good, but too broad. Boils down to: How do you
>>>> couple them? And none of the methods I thought about seemed to make any
>>>> sense.
>>>>
>>>> If we really want to have the values in /proc being opted-in, I think
>>>> Kamezawa's idea of a mount option is the winner so far.
>>>>
>>>
>>> Ok:
>>>
>>> How about the following patch to achieve this ?
>>
>> Hmm, What I thought was mount option for procfs. Containers will mount
>> its own
>> /proc file systems. Do you have any pros. / cons. ?
>> IIUC, cgroup can be mounted per subsystems. Then, options can be
>> passed per
>> subsystems. It's a mess but we don't need to bring this to procfs.
>>
>> How about
>>
>> # mount -t procfs proc /container_root/proc -o cgroup_aware
>>
>> to show cgroup aware procfs ? I think this will be easy to be used with
>> namespace/chroot, etc.
>>
>
> Don't think it works.
>
> Because whoever mounts the proc filesystem, may not want to be isolated.
> But we want him to be.
>
> As an example from our usecase, procfs is mounted inside a container. We
> can't assume the container is willing to cooperate. So we need to
> establish this from the outside. We can of course force options to be
> always added to a procfs mount if it comes from the container, but it is
> way more messier than this.
>
> per-cgroup knobs works fine for this because the container cannot
> possibly see it or change it in any circumstance.
> per-namespace would work as well, but then I don't see how to specify a
> want/don't want flag in a sane way.
>

There is another aspect of this as well - that I myself was overlooking.
/proc is not the only place in which this knob to work.

Think of syscalls like sysinfo, for instance. We'd also like this 
information to come from a cgroup-specific place. Possibly other places 
as well.

This is one more reason for me to believe that if we are going for a 
switch, it needs to live in the cgroup - and also that "proc_overlay" is 
quite a bad name - but that's okay since this small patch was just a 
proof of concept to get the discussion going.