Re: [RFD] Merge task counter into memcg

[Alexander Nikiforov <a.nikiforov-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>]

On 04/17/2012 05:09 AM, Frederic Weisbecker wrote:
> On Thu, Apr 12, 2012 at 07:56:44AM +0400, Alexander Nikiforov wrote:
>> On 04/11/2012 10:57 PM, Frederic Weisbecker wrote:
>>> Hi,
>>>
>>> While talking with Tejun about targetting the cgroup task counter subsystem
>>> for the next merge window, he suggested to check if this could be merged into
>>> the memcg subsystem rather than creating a new one cgroup subsystem just
>>> for task count limit purpose.
>>>
>>> So I'm pinging you guys to seek your insight.
>>>
>>> I assume not everybody in the Cc list knows what the task counter subsystem
>>> is all about. So here is a summary: this is a cgroup subsystem (latest version
>>> in https://lwn.net/Articles/478631/) that keeps track of the number of tasks
>>> present in a cgroup. Hooks are set in task fork/exit and cgroup migration to
>>> maintain this accounting visible to a special tasks.usage file. The user can
>>> set a limit on the number of tasks by writing on the tasks.limit file.
>>> Further forks or cgroup migration are then rejected if the limit is exceeded.
>>>
>>> This feature is especially useful to protect against forkbombs in containers.
>>> Or more generally to limit the resources on the number of tasks on a cgroup
>>> as it involves some kernel memory allocation.
>>>
>>> Now the dilemna is how to implement it?
>>>
>>> 1) As a standalone subsystem, as it stands currently (https://lwn.net/Articles/478631/)
>>>
>>> 2) As a feature in memcg, part of the memory.kmem.* files. This makes sense
>>> because this is about kernel memory allocation limitation. We could have a
>>> memory.kmem.tasks.count
>>>
>>> My personal opinion is that the task counter brings some overhead: a charge
>>> across the whole hierarchy at every fork, and the mirrored uncharge on task exit.
>>> And this overhead happens even in the off-case (when the task counter susbsystem
>>> is mounted but the limit is the default: ULLONG_MAX).
>>>
>>> So if we choose the second solution, this overhead will be added unconditionally
>>> to memcg.
>>> But I don't expect every users of memcg will need the task counter. So perhaps
>>> the overhead should be kept in its own separate subsystem.
>>>
>>> OTOH memory.kmem.* interface would have be a good fit.
>>>
>>> What do you think?
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>> Hi,
>>
>> I'm agree that this is memory related thing, but I prefer this as a
>> separate subsystem.
>> Yes it has some impact on a system, but on the other hand we will
>> have some very useful tool to track tasks state.
>> As I wrote before
>>
>> http://comments.gmane.org/gmane.linux.kernel.cgroups/1448
>>
>> it'll very useful to have event in the userspace about fork/exit
>> about group of the processes.
> I need more clarifications about your needs. The task counter susbsytem
> doesn't inform you about forks or exits unless you reach the limit on
> the number of tasks.
>
Hi Frederic,

yup now it doesn't have this functionality, but we can add it. Please 
look on my prevous post about this feature

http://comments.gmane.org/gmane.linux.kernel.cgroups/1448

Now userspace tools, for example libcgroup, can't catch event when 
process died/forked (or maybe moved to another group). Shortly when 
tasks file change.
According to this userspace tool go to inconsistency state when user 
manually kill process. Another example we want to balance number of 
process with several groups and make round-robin
between them. Now we have only 1 way to get notification about tasks 
file - inotify(), but this approach works only if you work from 
userspace with file (e.g. create struct file, for example with echo $$ 
/sys/abc/tasks), but when something happens from kernel side 
(do_fork()/do_exit) we cannot get any event about group of the process 
(we can scan tasks file and count number of PID, or work with
waitpid(), but IMHO this is ugly solutions)

Thx for your reply