Re: [PATCH 00/12] Cqm2: Intel Cache quality monitoring fixes

[David Carrillo-Cisneros]

On Wed, Feb 1, 2017 at 4:35 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> "Luck, Tony" <tony.luck@xxxxxxxxx> writes:
>> 9)    Measure per logical CPU (pick active RMID in same precedence for task/cpu as CAT picks CLOSID)
>> 10)   Put multiple CPUs into a group
>
> I'm not sure this is a real requirement. It's just an optimization,
> right? If you can assign policies to threads, you can implicitly set it
> per CPU through affinity (or the other way around).

That's difficult when distinct users/systems do monitoring and system
management.  What if the cluster manager decides to change affinity
for a task after the monitoring service has initiated monitoring a CPU
in the way you describe?

> The only benefit would be possibly less context switch overhead,
> but if all the thread (including idle) assigned to a CPU have the
> same policy it would have the same results.

I think another of the reasons for the CPU monitoring requirement is
to monitor interruptions in CPUs running the idle thread. In CAT,
those interruptions use the CPU's CLOSID. Here they'd use the CPU's
RMID. Since RMID's are scarce, CPUs can be aggregated into groups to
save many.

Also, if perf's like monitoring is supported, it'd allow something like

  perf stat -e LLC-load,LLC-prefetches,intel_cqm/total_bytes -C 2

Thanks,
David