Re: [REGRESSION] x86, perf: counter freezing breaks rr

[Kyle Huey]

On Tue, Nov 20, 2018 at 1:18 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>
> > I suppose that's fair that it's better for some use cases. The flip
> > side is that it's no longer possible to get exactly accurate counts
> > from user space if you're using the PMI (because any events between
> > the overflow itself and the transition to the PMI handler are
> > permanently lost) which is catastrophically bad for us :)
>
> Yes that's a fair point. For most usages it doesn't matter.
>
> I suspect that's a case for supporting opt-out for freezing
> per perf event, and rr using that.

I don't see how you could easily opt-out on a per perf event basis. If
I'm reading the SDM correctly the Freeze_PerfMon_On_PMI setting is
global and affects all counters on that CPU. Even counters that don't
use the PMI at all will still be frozen if another counter overflows
and counter freezing is enabled. It would seem that a counter that
wants to use counter freezing and a counter that wants the behavior we
want would be mutually exclusive. I suppose the kernel could handle
all of that but it's a bit involved.

- Kyle