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Subject: Re: [PATCH V7 07/14] perf/core: Add a new PERF_EV_CAP_COEXIST event
 capability
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     acme@redhat.com, mingo@kernel.org, linux-kernel@vger.kernel.org,
        jolsa@kernel.org, eranian@google.com,
        alexander.shishkin@linux.intel.com, ak@linux.intel.com,
        like.xu@linux.intel.com
References: <20200723171117.9918-1-kan.liang@linux.intel.com>
 <20200723171117.9918-8-kan.liang@linux.intel.com>
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 <20200724114628.GJ43129@hirez.programming.kicks-ass.net>
 <0d33a25c-cfe4-af7f-c915-a98dba17b53b@linux.intel.com>
 <20200724135412.GA10769@hirez.programming.kicks-ass.net>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <e04f1708-e8c0-3787-1572-443f03ab73ca@linux.intel.com>
Date:   Fri, 24 Jul 2020 10:19:25 -0400
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On 7/24/2020 9:54 AM, Peter Zijlstra wrote:
> On Fri, Jul 24, 2020 at 09:43:44AM -0400, Liang, Kan wrote:
>>
>>
>> On 7/24/2020 7:46 AM, peterz@infradead.org wrote:
>>> On Fri, Jul 24, 2020 at 12:55:43PM +0200, peterz@infradead.org wrote:
>>>>> +	event_sched_out(event, cpuctx, ctx);
>>>>> +	perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
>>>>> +}
>>>>
>>>> Ah, so the problem here is that ERROR is actually recoverable using
>>>> IOC_ENABLE. We don't want that either. Let me try and figure out of EXIT
>>>> would work.
>>>
>>> EXIT is difficult too.. Also, that COEXIST thing hurt my brian, can't we
>>> do a simpler SIBLING thing that simply requires the event to be a group
>>> sibling?
>>>
>>> The consequence is that SLOTS must be the leader, is that really a
>>> problem? You keep providing the {cycles, slots, metric-things} example,
>>> but afaict {slots, metric-thing, cycles} should work just fine too.
>>> PERF_SAMPLE_READ with PERF_FORMAT_GROUP doesn't need to the the leader.
>>
>> I'm not sure I get your point.
>> For the PERF_SAMPLE_READ with PERF_FORMAT_GROUP case, other events can be
>> the leader, e.g., {cycles, slots, metric-things}:S.
>> In this case, the SLOTS event is not a leader. I don't think we can assume
>> that the SLOTS event must be the leader.
> 
> You can have a sibling event with SAMPLE_READ and FORMAT_GROUP just
> fine. The sampling event doesn't need to be the leader.
> 

There will be a problem for the current perf tool, which assumes that 
the leader event is the sampling event.

I will check how can we specially handle it in the perf tool.

Thanks,
Kan