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

From: Peter Zijlstra
Date: Mon Jan 06 2014 - 13:07:00 EST

On Mon, Jan 06, 2014 at 04:47:57PM +0000, Waskiewicz Jr, Peter P wrote:
> > As is I don't really see a good use for RMIDs and I would simply not use
> > them.
> If you want to use CQM in the hardware, then the RMID is how you get the
> cache usage data from the CPU. If you don't want to use CQM, then you
> can ignore RMIDs.

I think you can make do with a single RMID (per cpu). When you program
the counter (be it for a task, cpu or cgroup context) you set the 1 RMID
and EVSEL and read the CTR.

What I'm not entirely clear on is if the EVSEL and CTR MSR are per
logical CPU or per L3 (package); /me prays they're per logical CPU.

> One of the best use cases for using RMIDs is in virtualization.

*groan*.. /me plugs wax in ears and goes la-la-la-la

> A VM
> may be a heavy cache user, or a light cache user. Tracing different VMs
> on different RMIDs can allow an admin to identify which VM may be
> causing high levels of eviction, and either migrate it to another host,
> or move other tasks/VMs to other hosts. Without CQM, it's much harder
> to find which process is eating the cache up.

Not necessarily VMs, there's plenty large processes that exhibit similar
problems.. why must people always do VMs :-(

That said, even with a single RMID you can get that information by
simply running it against all competing processes one at a time. Since
there's limited RMID space you need to rotate at some point anyway.

The cgroup interface you propose wouldn't allow for rotation; other than
manual by creating different cgroups one after another.

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