[PATCH v3 3/5] perf: Allow adding fixed random jitter to the alternate sampling period

[Mark Barnett]

On 3/10/25 12:47, Peter Zijlstra wrote:
> On Fri, Mar 07, 2025 at 08:22:45PM +0000, mark.barnett at arm.com wrote:
>> @@ -9922,7 +9923,10 @@ static int __perf_event_overflow(struct perf_event *event,
>>   	if (event->attr.alt_sample_period) {
>>   		bool using_alt = hwc->using_alt_sample_period;
>>   		u64 sample_period = (using_alt ? event->attr.sample_period
>> -					       : event->attr.alt_sample_period);
>> +					       : event->attr.alt_sample_period)
>> +				  + (event->attr.jitter_alt_period
>> +					? get_random_u32_below(2 << event->attr.jitter_alt_period)
>> +					: 0);
> 
> So, ... this here is NMI context, right? Have you looked at the guts of
> get_random_u32_below() ?
> 
> I would strongly suggest you go do so.
> 

Good catch. I think a pseudo-random generator would be fine here and it 
looks like the implementation of prandom is safe to use in an interrupt 
context. I can change to use that.

>>   
>>   		hwc->sample_period = sample_period;
>>   		hwc->using_alt_sample_period = !using_alt;