Re: [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@xxxxxxx 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;