Re: [PATCH V7 08/14] perf/x86/intel: Generic support for hardware TopDown metrics

["Liang, Kan" <kan.liang@linux.intel.com>]

On 7/28/2020 9:44 AM, peterz@infradead.org wrote:
> On Tue, Jul 28, 2020 at 09:28:39AM -0400, Liang, Kan wrote:
>>
>>
>> On 7/28/2020 9:09 AM, Peter Zijlstra wrote:
>>> On Fri, Jul 24, 2020 at 03:10:52PM -0400, Liang, Kan wrote:
>>>
>>>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>>>> index 6cb079e0c9d9..010ac74afc09 100644
>>>> --- a/arch/x86/events/intel/core.c
>>>> +++ b/arch/x86/events/intel/core.c
>>>> @@ -2405,27 +2405,18 @@ static u64 icl_update_topdown_event(struct
>>>> perf_event *event)
>>>>    	return slots;
>>>>    }
>>>>
>>>> -static void intel_pmu_read_topdown_event(struct perf_event *event)
>>>> +static void intel_pmu_read_event(struct perf_event *event)
>>>>    {
>>>> -	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>>>> -
>>>> -	/* Only need to call update_topdown_event() once for group read. */
>>>> -	if ((cpuc->txn_flags & PERF_PMU_TXN_READ) &&
>>>> -	    !is_slots_event(event))
>>>>    		return;
>>>>
>>>> -	perf_pmu_disable(event->pmu);
>>>> -	x86_pmu.update_topdown_event(event);
>>>> -	perf_pmu_enable(event->pmu);
>>>> -}
>>>> -
>>>> -static void intel_pmu_read_event(struct perf_event *event)
>>>> -{
>>>>    	if (event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD)
>>>>    		intel_pmu_auto_reload_read(event);
>>>> -	else if (is_topdown_count(event) && x86_pmu.update_topdown_event)
>>>> -		intel_pmu_read_topdown_event(event);
>>>> -	else
>>>> +	else if (is_slots_count(event) && x86_pmu.update_topdown_event) {
>>>> +		perf_pmu_disable(event->pmu);
>>>> +		x86_pmu.update_topdown_event(event);
>>>> +		perf_pmu_enable(event->pmu);
>>>> +	} else
>>>>    		x86_perf_event_update(event);
>>>>    }
>>>
>>> I'm a little puzzled by this; what happens if you:
>>>
>>> 	fd = sys_perf_event_open(&attr_slots);
>>> 	fd1 = sys_perf_event_open(&attr_metric, .group_fd=fd);
>>>
>>> 	read(fd1);
>>>
>>> ?
>>>
>>
>> I did a quick test. It depends on the .read_format of attr_metric.
>> If PERF_FORMAT_GROUP is applied for attr_metric, perf_read_group() will be
>> invoked. The value of fd1 is updated correctly.
>> If the flag is not applied, 0 will be returned.
> 
> Exactly :-), was that intentional?

Kind of, because a metric event must be in a group with the leader slots 
event. If a user reads (treats) the metric event as a singleton event, 
an invalid value should be expected.

> Because prior to this change that
> would've worked just fine.
> 
> Now, I agree it's a daft thing, because that value is pretty useless
> without also having the slots value, but I feel we should be explicit in
> our choices here.
> 
> If for example, we would've had hardware provide us the raw metric
> counters, instead of us having to reconstruct them, this would've been a
> semi useful thing.
> 
> So I'm tempted to leave things as it, and keep this 'working'.

I will update the perf tool document to force the PERF_FORMAT_GROUP flag 
for each metric events.

Thanks,
Kan