linux-kernel - Re: perf event grouping for dummies (was Re: [PATCH] arc: perf: Enable generic "cache-references" and "cache-misses" events)

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Date:   Thu, 22 Sep 2016 09:56:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paul Clarke <pc@...ibm.com>
Cc:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Will Deacon <Will.Deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>, Jiri Olsa <jolsa@...hat.com>
Subject: Re: perf event grouping for dummies (was Re: [PATCH] arc: perf:
 Enable generic "cache-references" and "cache-misses" events)

On Wed, Sep 21, 2016 at 07:43:28PM -0500, Paul Clarke wrote:
> On 09/20/2016 03:56 PM, Vineet Gupta wrote:
> >On 09/01/2016 01:33 AM, Peter Zijlstra wrote:
> >>>- is that what perf event grouping is ?
> >>
> >>Again, nope. Perf event groups are single counter (so no implicit
> >>addition) that are co-scheduled on the PMU.
> >
> >I'm not sure I understand - does this require specific PMU/arch support - as in
> >multiple conditions feeding to same counter.
> 
> My read is that is that what Peter meant was that each event in the
> perf event group is a single counter, so all the events in the group
> are counted simultaneously.  (No multiplexing.)

Right, sorry for the poor wording.

> >Again when you say co-scheduled what do you mean - why would anyone use the event
> >grouping - is it when they only have 1 counter and they want to count 2
> >conditions/events at the same time - isn't this same as event multiplexing ?
> 
> I'd say it's the converse of multiplexing.  Instead of mapping
> multiple events to a single counter, perf event groups map a set of
> events each to their own counter, and they are active simultaneously.
> I suppose it's possible for the _groups_ to be multiplexed with other
> events or groups, but the group as a whole will be scheduled together,
> as a group.

Correct.

Each events get their own hardware counter. Grouped events are
co-scheduled on the hardware.

You can multiplex groups. But if one event in a group is schedule, they
all must be.