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

Kyle Huey
2018-11-20T20:53:55Z

On Tue, Nov 20, 2018 at 12:11 PM Andi Kleen <ak@linux.intel.com> wrote:
>
> > > > Given that we're already at rc3, and that this renders rr unusable,
> > > > we'd ask that counter freezing be disabled for the 4.20 release.
> > >
> > > The boot option should be good enough for the release?
> >
> > I'm not entirely sure what you mean here. We want you to flip the
> > default boot option so this feature is off for this release. i.e. rr
> > should work by default on 4.20 and people should have to opt into the
> > inaccurate behavior if they want faster PMI servicing.
>
> I don't think it's inaccurate, it's just different
> than what you are used to.
>
> For profiling including the kernel it's actually far more accurate
> because the count is stopped much earlier near the sampling
> point. Otherwise there is a considerable over count into
> the PMI handler.
>
> In your case you limit the count to ring 3 so it's always cut off
> at the transition point into the kernel, while with freezing
> it's at the overflow point.

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 :)

- Kyle