Date: Fri, 10 Oct 2014 13:02:02 +0100
From: Matt Fleming <matt@console-pimps.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>, Jiri Olsa <jolsa@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Matt Fleming <matt.fleming@intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [PATCH 10/11] perf/x86/intel: Support task events with Intel CQM
Message-ID: <20141010120202.GY14343@console-pimps.org>
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On Wed, 08 Oct, at 04:49:57PM, Peter Zijlstra wrote:
> 
> The thing is, with multiplexing you cannot fail at event creation time
> anyhow. The only time where you can 'fail' is when programming the PMU,
> when its full its full.
> 
> Those that don't fit, get to wait their turn.

For CQM it's not about "fitting" everything in the PMU but more about
monitoring the same thing (task, cgroup) with different events, i.e. one
thing with two RMIDs. We have the RMID recycling algorithm to make
things fit, but that doesn't help us out here.

An example scenario that isn't supported by this patch series is
monitoring a cgroup while simultaneously monitoring a task that's part
of that cgroup. Whichever event is created second will fail at event
init time.

And that seemed like a fair approach to me. But the more I think about
it, the more I begin to agree that maybe we should allow users the
flexibility to create conflicting events, particularly because there
appears to be precedent in other parts of perf.

Hmm... "rotation" is starting to become my least favourite word.

-- 
Matt Fleming, Intel Open Source Technology Center
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