Re: [PATCH 7/9] perf/x86/intel: Disable sampling read slots and topdown

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

On 5/28/2019 9:52 AM, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 02:40:53PM -0700, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> To get correct PERF_METRICS value, the fixed counter 3 must start from
>> 0. It would bring problems when sampling read slots and topdown events.
>> For example,
>>          perf record -e '{slots, topdown-retiring}:S'
>> The slots would not overflow if it starts from 0.
>>
>> Add specific validate_group() support to reject the case and error out
>> for Icelake.
>>
>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
>> ---
>>   arch/x86/events/core.c       |  2 ++
>>   arch/x86/events/intel/core.c | 20 ++++++++++++++++++++
>>   arch/x86/events/perf_event.h |  2 ++
>>   3 files changed, 24 insertions(+)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 07ecfe75f0e6..a7eb842f8651 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -2065,6 +2065,8 @@ static int validate_group(struct perf_event *event)
>>   	fake_cpuc->n_events = 0;
>>   	ret = x86_pmu.schedule_events(fake_cpuc, n, NULL);
>>   
>> +	if (x86_pmu.validate_group)
>> +		ret = x86_pmu.validate_group(fake_cpuc, n);
>>   out:
>>   	free_fake_cpuc(fake_cpuc);
>>   	return ret;
>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>> index 79e9d05e047d..2bb90d652a35 100644
>> --- a/arch/x86/events/intel/core.c
>> +++ b/arch/x86/events/intel/core.c
>> @@ -4410,6 +4410,25 @@ static int icl_set_period(struct perf_event *event)
>>   	return 1;
>>   }
>>   
>> +static int icl_validate_group(struct cpu_hw_events *cpuc, int n)
>> +{
>> +	bool has_sampling_slots = false, has_metrics = false;
>> +	struct perf_event *e;
>> +	int i;
>> +
>> +	for (i = 0; i < n; i++) {
>> +		e = cpuc->event_list[i];
>> +		if (is_slots_event(e) && is_sampling_event(e))
>> +			has_sampling_slots = true;
>> +
>> +		if (is_perf_metrics_event(e))
>> +			has_metrics = true;
>> +	}
>> +	if (unlikely(has_sampling_slots && has_metrics))
>> +		return -EINVAL;
>> +	return 0;
>> +}
> 
> Why this special hack, why not disallow sampling on SLOTS on creation?

You mean unconditionally disable SLOTS sampling?

The SLOTS doesn't have to be with Topdown metrics event.
I think users may want to only sampling slot events. We should allow 
this usage.

Thanks,
Kan