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

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Date:	Mon, 6 Jan 2014 16:34:04 +0000
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Li Zefan <lizefan@...wei.com>,
	"containers@...ts.linux-foundation.org" 
	<containers@...ts.linux-foundation.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] x86: Add Cache QoS Monitoring (CQM) support

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.

> > > 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.

Cheers,
-PJ

-- 
PJ Waskiewicz				Open Source Technology Center
peter.p.waskiewicz.jr@...el.com		Intel Corp.