linux-kernel - Re: [PATCH V7 07/14] perf/core: Add a new PERF_EV_CAP

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Message-ID: <af274c58-6935-22cd-2830-519c3977e136@linux.intel.com>
Date:   Fri, 24 Jul 2020 13:00:28 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     peterz@...radead.org, Andi Kleen <ak@...ux.intel.com>
Cc:     acme@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, like.xu@...ux.intel.com
Subject: Re: [PATCH V7 07/14] perf/core: Add a new PERF_EV_CAP_COEXIST event
 capability



On 7/24/2020 12:43 PM, peterz@...radead.org wrote:
> On Fri, Jul 24, 2020 at 04:59:34PM +0200, Peter Zijlstra wrote:
>> On Fri, Jul 24, 2020 at 07:46:32AM -0700, Andi Kleen wrote:
>>>> Something that seems to 'work' is:
>>>> '{cycles,cpu/instructions,period=50000/}', so maybe you can make the
>>>> group modifier :S use any sampling event if there is one, and otherwise
>>>> designate the leader.
>>>>
>>>> Then you can write things like:
>>>>
>>>>    '{slots, metric1, metric2, cpu/cycles,freq=50000/}:S'
>>>>
>>>> and then since cycles is specified as a sampling event, it will use
>>>> that.
>>>
>>> Okay possible, but it makes things more complicated
>>> for the user to understand and requires special documentation.
>>> Hopefully it's worth it the internal simplification.
>>
>> You already require special documentation for this metrics stuff. We
>> already need to state that SLOTS cannot be a sampling event, so you
>> already need to pay attention to this anyway.
>>
>> A shortcut could be a :s event modifier, then you can write:
>>
>>   '{slots, metric1, metric2, cycles:s}:S'
>>
>> and have the tool select the :s tagged one.

It looks like PT encountered a similar issue as us.
They use the 2nd event of the group as the
"leader". I think we can simply extend the function to check the slots 
event in perf tool.
https://lore.kernel.org/lkml/20200401101613.6201-17-adrian.hunter@intel.com/


> 
> Having slots as leader also would allow doing something like
> FORMAT_METRIC, where we return sibling/leader in some fashion.
> 
> That also makes sense for instructions, because, IIRC,
> instructions/slots is the better IPC.
> 
> And we should probably consider FORMAT_RESET.

What's FORMAT_RESET for?

Thanks,
Kan