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

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Date:   Tue, 20 Sep 2016 13:56:20 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     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: perf event grouping for dummies (was Re: [PATCH] arc: perf: Enable
 generic "cache-references" and "cache-misses" events)

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. How does perf user make use of this
info - I tried googling around but can't seem to find anything which explains the
semantics.

I can see that group events to work on ARC (although in our case a counter can
cont one condition at a time only) and the results seem to be similar whther we
group or not.

------------->8------------
[ARCLinux]# perf stat -e {cycles,instructions} hackbench
Running with 10*40 (== 400) tasks.
Time: 37.430

 Performance counter stats for 'hackbench':

        3487777173    cycles
        1351709784    instructions            #    0.39  insn per cycle

      38.957481536 seconds time elapsed

[ARCLinux]# perf stat -e cycles hackbench
Running with 10*40 (== 400) tasks.
Time: 36.735

 Performance counter stats for 'hackbench':

        3426151391    cycles

      38.247235981 seconds time elapsed

[ARCLinux]#
[ARCLinux]# perf stat -e instructions hackbench
Running with 10*40 (== 400) tasks.
Time: 37.537

 Performance counter stats for 'hackbench':

        1355421559    instructions

      39.061784281 seconds time elapsed
------------->8------------

...
> 
> 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 ?

-Vineet