Re: [RFC PATCH 0/4] cgroups: Start a basic rlimit subsystem

[Frederic Weisbecker]

On Tue, Jun 21, 2011 at 10:08:26AM -0700, Paul Menage wrote:
> Hi Frederick,
> 
> On Sun, Jun 19, 2011 at 4:51 PM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> > This starts a basic rlimit cgroup subsystem with only the
> > equivalent of RLIMIT_NPROC yet. This can be useful to limit
> > the global effects of a local fork bomb for example (local
> > in term of a cgroup).
> 
> My general thoughts on this are:
> 
> - do we really want an "rlimit" subsystem rather than grouping things
> functionally? We definitely shouldn't just stuff things in here
> because they happen to be controlled via setrlimit currently. Also,
> some limits might fit more appropriately in other subsystems. (E.g.
> max locked memory should be a memcg field, and real-time priority
> should be in the cpu subsystem if it's not already subsumed by
> existing functionality). Grouping "rlimit" things together in a single
> subsystem reduces flexibility, since you can't then mount them on
> separate hierarchies. (This is actually related to one of my regrets
> about the original implementation of cgroups - the cpuset subsystem
> should have been split into a "cpunode" subsystem and a "memnode"
> subsystem, since the two parts of cpusets had no requirement to be
> located together - they were only linked since before cgroups there
> was no way to mount them separately).
> 
> A lot of the rlimit values are more for the benefit of the process (to
> prevent runaways) rather than for resource isolation - data segment
> size, file size, stack size, pending signals, virtual memory limits
> fall into that category, i think - they're all resource usage that
> falls under existing cgroup resource limits, such as
> memory.limit_in_bytes.

Yeah I all agree with you. To mimic rlmit inside a cgroup subsystem
would be a bad thing given we already have subsystems where some of
the rlimit options can fit and moreover your message made me read
again the part about hierarchies in cgroup documentation. I
eventually understood the idea/point of building parallel hierarchies with
different subsystems mounted in it, and thus eventually I understand
your point about the problem on flexibility implied by an everything-rlimit
subsystem.

> Task count is a little blurry in this regard - the main resources that
> you can consume with a fork bomb are CPU cycles and memory, both of
> which are already isolated by existing subsystems, so arguably there
> shouldn't be a need to control the number of tasks itself. But I'm
> prepared to believe that there are still bits of the kernel that have
> arbitrary machine-wide limits that can be hit simply by forking a
> massive number of processes, even if they're not using much memory or
> CPU cycles.

Yeah I've just asked Johannes Weiner about that and he told me
can't use memory limits for that as these don't handle kernel
memory.

> So for this case, I'd suggest that the best option is to have a
> numtasks subsystem with "count" and "limit" files. Future rlimit
> options can go in their own subsystems or be attached to existing
> subsystems if that makes sense.

Agreed about future rlimit options, but building a single purpose
numtask subsystem looks a bit strange. Just because it looks too much
single purpose. OTOH I can't figure out any other kind of future
limitation that should fit aside in a very similar topic, enough
that we wouldn't care about seperating both for flexibility.

So I guess I'm going to take that way.

Thanks!
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