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

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Date:   Wed, 1 Feb 2017 17:19:18 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     David Carrillo-Cisneros <davidcc@...gle.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        "Luck, Tony" <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        "Shivappa, Vikas" <vikas.shivappa@...el.com>,
        Stephane Eranian <eranian@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        "Anvin, H Peter" <h.peter.anvin@...el.com>
Subject: Re: [PATCH 00/12] Cqm2: Intel Cache quality monitoring fixes

> > 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?

Why would you want to monitor a CPU if you don't know what it is
running?  The results would be meaningless. So you really want
to integrate those two services.

> 
> > 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,

idle threads are just threads, so they could be just exposed
to perf (e.g. combination of pid 0 + cpu filter)


> 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

This would work without a special API.

-Andi