Re: [PATCH 0/4] x86: Add Cache QoS Monitoring (CQM) support

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jan 06, 2014 at 04:34:04PM +0000, Waskiewicz Jr, Peter P wrote:
> On Mon, 2014-01-06 at 12:16 +0100, Peter Zijlstra wrote:
> > On Sun, Jan 05, 2014 at 05:23:07AM +0000, Waskiewicz Jr, Peter P wrote:
> > > The CPU side is easy and clean.  When something in the software wants to
> > > monitor when a particular task is scheduled and started, write whatever
> > > RMID that task is assigned to (through some mechanism) to the proper MSR
> > > in the CPU.  When that task is swapped out, clear the MSR to stop
> > > monitoring of that RMID.  When that RMID's statistics are requested by
> > > the software (through some mechanism), then the CPU's MSRs are written
> > > with the RMID in question, and the value is read of what has been
> > > collected so far.  In my case, I decided to use a cgroup for this
> > > "mechanism" since so much of the grouping and task/group association
> > > already exists and doesn't need to be rebuilt or re-invented.
> > 
> > This still doesn't explain why you can't use perf-cgroup for this.
> 
> I'm not completely familiar with perf-cgroup, so I looked for some
> documentation for it to better understand it.  Are you referring to perf
> -G to monitor an existing cgroup/all cgroups?  Or something else?  If
> it's the former, I'm not following you how this would fit.

All the bits under CONFIG_CGROUP_PERF, I've no idea how userspace looks.

> > > > In general, I'm quite strongly opposed against using cgroup as
> > > > arbitrary grouping mechanism for anything other than resource control,
> > > > especially given that we're moving away from multiple hierarchies.
> > > 
> > > Just to clarify then, would the mechanism in the cpuacct cgroup to
> > > create a group off the root subsystem be considered multi-hierarchical?
> > > If not, then the intent for this new cacheqos subsystem is to be
> > > identical in that regard to cpuacct in the behavior.
> > > 
> > > This is a resource controller, it just happens to be tied to a hardware
> > > resource instead of an OS resource.
> > 
> > No, cpuacct and perf-cgroup aren't actually controllers at all. They're
> > resource monitors at best. Same with your Cache QoS Monitor, it doesn't
> > control anything.
> 
> I may be using controller in a different way than you are.  Yes, the
> Cache QoS Monitor is monitoring cache data.  But it is also controlling
> the allocation and deallocation of RMIDs to tasks/task groups as
> monitoring is enabled and disabled for those groups.  That's why I
> called it a controller.  If that's not accurate, I apologize.

Yeah that's not accurate, nor desired I think, because you get into
horrible problems with hierarchies, do child groups belong to your RMID
or not?

As is I don't really see a good use for RMIDs and I would simply not use
them.