Re: perf: is it possible to userspace rdpmc but only on a certain core type

[Liang, Kan]

On 2025-01-17 5:04 p.m., Vince Weaver wrote:
> Hello
> 
> so we've been working on PAPI support for Intel Top-Down events, which
> let's say does "exciting" things involving the rdpmc instruction.
> 
> One issue we are having is that on a hybrid machine (Raptor Lake in this 
> case with performance/efficiency cores) there is no top-down support
> for the E-cores, and it will gpf/segfault if you try to rdpmc the top-down 
> events.
> 
> Obviously PAPI would like to avoid this, and somehow only run the rdpmc 
> from userspace if scheduled on a P-core.
> 
> Is there any way to atomically do this?  Somehow detect what core we are 
> on and atomically execute a userspace instruction before a core-reschedule 
> can happen?
> 
> Or barring that, any other way to handle this in a way that won't crash 
> without having to have the users have to bind to a core any time they want 
> to run PAPI?

Can the PAPI rely on the event_idx(), similar to what Andi's pmu-tools
do? For a stopped event, the index is always 0.
https://github.com/andikleen/pmu-tools/blob/master/jevents/rdpmc.c#L117

Thanks,
Kan