Re: Perf event to counter mapping question

[Atish Patra]

On Thu, Feb 23, 2023 at 12:27 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Feb 22, 2023 at 04:28:36PM -0800, Atish Patra wrote:
>
> > AFAIK, ARM64 allows all-to-all mapping in pmuv3[1]. That makes life
> > much easier. It just needs to pick the next available counter.
> > On the other hand, x86 allows selective counter mapping which is
> > discovered from the json file and maintained in per event
> > constraints[4].
>
> All the contraint management is done in kernel, and yes, it's a giant
> pain in the rear side.
>
> From what I understand the reason for these contraints is complexity of
> implementation, less constraints is more 'wires' in the hardware.
>
> With PMU use being ever more popular, we're seeing the x86 PMU move
> towards less constraints -- although I don't think we'll ever get rid of
> them :/
>
> > 2. Mandate all-to-all mapping similar to ARM64.
>
> If at all possible, I would strongly recommend taking this route. Yes,
> the hardware people will complain, but newer x86 hardware having less,
> or simpler, constraints might be sufficient to convince them.
>

Yeah. That's where folks want to go in order to provide flexibility
for future platform vendors by
allowing constraints.

Can you provide some examples or some pointers that describe these
simpler constraints ?

Finding a middle path would certainly keep everyone happy :). Thanks a
lot for your input.

> (and if you do have to do contraints, please take a lesson from x86 and
>  *never* allow overlapping contraints as AMD had, solving those
>  constraints is not fun)
>
> As you note, this is *much* simpler to program and virtualize.
>
> > Note: This is only for programmable counters. If the platform supports
> > any fixed counters (i.e. can monitor
> > only a specific event), that needs to be provisioned via some other
> > method. IIRC the fixed counters(apart from cycle) in ARM64 are part of
> > AMU not PMU.
>
> So free running counters are ideal and fairly simple to multiplex/use.
>
> The moment you start adding overflow interrupts / filters and any other
> complexities to fixed function counters it becomes a mess (look at the
> x86 PMU again).



--
Regards,
Atish