Re: [PATCH] cgroup_pids: add fork limit

[Tejun Heo <tj@kernel.org>]

Hello, Max.

On Tue, Nov 10, 2015 at 04:37:46PM +0100, Max Kellermann wrote:
> > But what's the resource here?
> 
> CPU consumption and memory bandwidth.  A fork/clone is an operation

Both are abstracted as CPU usage and controlled by the cpu controller.

> that puts considerable load on a machine, most of which happens in
> kernel space (copying page tables etc.).
> 
> > All first-order resources which can be consumed by forking
> > repeatedly already have proper controllers.
> 
> They do?

Yes.

> I can limit CPU time with RLIMIT_CPU, but that's per-process and thus
> completely useless.  There's no cgroup controller with such a feature.

There's the cpu controller

> There's "cpu" which changes priority, "cpuset" selects CPUs, "cpuacct"

The cpu controller can limit both in terms of relative weight and
absolute CPU cycle bandwidth.

> only does accounting and "freezer" (somewhat related).  But nothing
> that limits CPU usage according to configured rules.
> 
> I can limit absolute memory usage with memcg, which is a good thing,
> but is orthogonal to this feature.  Note that there are already
> various RLIMITs about memory usage, and despite that, memcg was
> merged due to RLIMIT shortcomings.
> 
> "pids" was merged even though there already was RLIMIT_NPROC.  Again,
> RLIMITs have their shortcomings.

Because pids turned out to be a first-order resource which is not
contrained by memory due to the limited pid space.

> But which controllers can I use to achieve the same effect as my fork
> limit feature?  Did I miss one?

Apparently.

> > What's the point of adding an extra second-order controller?
> 
> I explained that, and you just cited my explanation.
> 
> > Where do we go from there?  Limit on the number of syscalls?
> 
> No idea.  Are these questions really relevant for my patch?

Well, it's relevant to the fact that it's failing to distinguish what
are actual resources and what aren't.

Thanks.

-- 
tejun