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 9:09 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.

static ssize_t
__perf_read(struct perf_event *event, char __user *buf, size_t count)
{
	u64 read_format = event->attr.read_format;
	int ret;

	/*
	 * Return end-of-file for a read on an event that is in
	 * error state (i.e. because it was pinned but it couldn't be
	 * scheduled on to the CPU at some point).
	 */
	if (event->state == PERF_EVENT_STATE_ERROR)
		return 0;

	if (count < event->read_size)
		return -ENOSPC;

	WARN_ON_ONCE(event->ctx->parent_ctx);
	if (read_format & PERF_FORMAT_GROUP)
		ret = perf_read_group(event, read_format, buf);
	else
		ret = perf_read_one(event, read_format, buf);

	return ret;
}


Thanks,
Kan