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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Message-Id: <04f6dcd2-35c6-6e28-2dcf-bc5f0bb446dc@us.ibm.com>
Date:   Wed, 21 Sep 2016 19:43:28 -0500
From:   Paul Clarke <pc@...ibm.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "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 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.)

>> You can do it like:
>>
>> 	perf stat -e '{cycles,instructions}'
>>
>> Which will place the cycles event and the instructions event in a group
>> and thereby guarantee they're co-scheduled.
>
> 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.

If you have a single counter, I don't believe you can support perf event groups, by definition.

Regards,
Paul Clarke, IBM