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

[Vineet Gupta]

On 09/22/2016 12:56 AM, Peter Zijlstra wrote:
> 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.

And if we don't group them, then they _may_ not be co-scheduled (active/counting
at the same time) ? But how can this be possible.
Say we have 2 counters, both the cmds below

     perf -e cycles,instructions hackbench
     perf -e {cycles,instructions} hackbench

would assign 2 counters to the 2 conditions which keep counting until perf asks
them to stop (because the profiled application ended)

I don't understand the "scheduling" of counter - once we set them to count, there
is no real intervention/scheduling form software in terms of disabling/enabling
(assuming no multiplexing etc)

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