Re: [PATCH 5/9] x86/intel_rdt: Add new cgroup and Class of service management

[Tejun Heo <tj@kernel.org>]

Hello, Vikas.

On Wed, Jul 01, 2015 at 03:21:06PM -0700, Vikas Shivappa wrote:
> This patch adds a cgroup subsystem for Intel Resource Director
> Technology(RDT) feature and Class of service(CLOSid) management which is
> part of common RDT framework.  This cgroup would eventually be used by
> all sub-features of RDT and hence be associated with the common RDT
> framework as well as sub-feature specific framework.  However current
> patch series only adds cache allocation sub-feature specific code.
> 
> When a cgroup directory is created it has a CLOSid associated with it
> which is inherited from its parent.  The Closid is mapped to a
> cache_mask which represents the L3 cache allocation to the cgroup.
> Tasks belonging to the cgroup get to fill the cache represented by the
> cache_mask.

First of all, I apologize for being so late.  I've been thinking about
it but the thoughts didn't quite crystalize (which isn't to say that
it's very crystal now) until recently.  If I understand correctly,
there are a couple suggested use cases for explicitly managing cache
usage.

1. Pinning known hot areas of memory in cache.

2. Explicitly regulating cache usage so that cacheline allocation can
   be better than CPU itself doing it.

#1 isn't part of this patchset, right?  Is there any plan for working
towards this too?

For #2, it is likely that the targeted use cases would involve threads
of a process or at least cooperating processes and having a simple API
which just goes "this (or the current) thread is only gonna use this
part of cache" would be a lot easier to use and actually beneficial.

I don't really think it makes sense to implement a fully hierarchical
cgroup solution when there isn't the basic affinity-adjusting
interface and it isn't clear whether fully hierarchical resource
distribution would be necessary especially given that the granularity
of the target resource is very coarse.

I can see that how cpuset would seem to invite this sort of usage but
cpuset itself is more of an arbitrary outgrowth (regardless of
history) in terms of resource control and most things controlled by
cpuset already have countepart interface which is readily accessible
to the normal applications.

Given that what the feature allows is restricting usage rather than
granting anything exclusively, a programmable interface wouldn't need
to worry about complications around priviledges while being able to
reap most of the benefits in an a lot easier way.  Am I missing
something?

Thanks.

-- 
tejun