Re: [RFD] Merge task counter into memcg

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/04/12 3:57), 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?


Sounds interesting to me. Hm, does your 'overhead' of task accounting is
enough large to be visible to users ? How performance regression is big ?

BTW, now, all memcg's limit interfaces use 'bytes' as an unit of accounting.
It's a small concern to me to have mixture of bytes and numbers of objects
for accounting. But I think increasing number of subsystem is not very good....
 
Regards,
-Kame