Re: [UNTESTED][PATCH] perf/x86: Fix limit_period() for 'freq mode events'

[Ravi Bangoria]

On 16-Jan-25 9:51 AM, Ravi Bangoria wrote:
>>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>>> index c75c482d4c52..924aa35676d3 100644
>>> --- a/arch/x86/events/core.c
>>> +++ b/arch/x86/events/core.c
>>> @@ -629,10 +629,22 @@ int x86_pmu_hw_config(struct perf_event *event)
>>>  		event->hw.config |= x86_pmu_get_event_config(event);
>>>  
>>>  	if (event->attr.sample_period && x86_pmu.limit_period) {
>>> -		s64 left = event->attr.sample_period;
>>> -		x86_pmu.limit_period(event, &left);
>>> -		if (left > event->attr.sample_period)
>>> -			return -EINVAL;
>>> +		if (event->attr.freq) {
>>> +			s64 left = event->hw.sample_period;
>>> +
>>> +			x86_pmu.limit_period(event, &left);
>>> +			if (left != event->hw.sample_period) {
>>> +				event->hw.sample_period = left;
>>> +				event->hw.last_period = left;
>>> +				local64_set(&event->hw.period_left, left);
>>> +			}
>>
>> For a better start period, I'd prefer the below patch.
>> https://lore.kernel.org/lkml/20241022130414.2493923-1-kan.liang@xxxxxxxxxxxxxxx/
>>
>> The limit_period() check was introduced in the c46e665f0377 ("perf/x86:
>> Add INST_RETIRED.ALL workarounds"). For my understanding, it's to check
>> the !freq case. If so, I'm thinking something as below.
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 79a4aad5a0a3..6467ecc65486 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -630,7 +630,7 @@ int x86_pmu_hw_config(struct perf_event *event)
>>  	if (event->attr.type == event->pmu->type)
>>  		event->hw.config |= x86_pmu_get_event_config(event);
>>
>> -	if (event->attr.sample_period && x86_pmu.limit_period) {
>> +	if (!event->attr.freq && x86_pmu.limit_period) {
>>  		s64 left = event->attr.sample_period;
>>  		x86_pmu.limit_period(event, &left);
>>  		if (left > event->attr.sample_period)
> 
> LGTM.
> 
>>> +		} else {
>>> +			s64 left = event->attr.sample_period;
>>> +
>>> +			x86_pmu.limit_period(event, &left);
>>> +			if (left > event->attr.sample_period)
>>> +				return -EINVAL;
>>> +		}
>>>  	}
>>>  
>>>  	/* sample_regs_user never support XMM registers */
>>> @@ -2648,6 +2660,9 @@ static int x86_pmu_check_period(struct perf_event *event, u64 value)
>>>  	if (x86_pmu.check_period && x86_pmu.check_period(event, value))
>>>  		return -EINVAL;
>>>  
>>> +	if (event->attr.freq)
>>> +		return 0;
>>> +
>>
>> The ioctl(PERF_EVENT_IOC_PERIOD) can be used to set both freq and
>> period. But according to the implementation, yes, the
>> perf_event_check_period() should be only for the !freq mode.
>>
>> If so, we may change the generic code.
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index f91ba29048ce..a9a04d4f3619 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -5960,14 +5960,15 @@ static int _perf_event_period(struct perf_event
>> *event, u64 value)
>>  	if (!value)
>>  		return -EINVAL;
>>
>> -	if (event->attr.freq && value > sysctl_perf_event_sample_rate)
>> -		return -EINVAL;
>> -
>> -	if (perf_event_check_period(event, value))
>> -		return -EINVAL;
>> -
>> -	if (!event->attr.freq && (value & (1ULL << 63)))
>> -		return -EINVAL;
>> +	if (event->attr.freq) {
>> +		if (value > sysctl_perf_event_sample_rate)
>> +			return -EINVAL;
>> +	} else {
>> +		if (perf_event_check_period(event, value))
>> +			return -EINVAL;
>> +		if (value & (1ULL << 63))
>> +			return -EINVAL;
>> +	}
>>
>>  	event_function_call(event, __perf_event_period, &value);
> 
> LGTM. And you can remove the if (event->attr.freq) check from
> intel_pmu_has_bts_period().

Hmm, There are other code paths from which intel_pmu_has_bts_period()
gets called. So you need that check. Sorry for the noise!