Re: [RFD] Merge task counter into memcg

[Frederic Weisbecker <fweisbec@gmail.com>]

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?
> >--
> >To unsubscribe from this list: send the line "unsubscribe cgroups" in
> >the body of a message to majordomo@vger.kernel.org
> >More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >
> 
> 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.