[Intel-gfx] [PATCH] drm/i915/pmu: Stop averaging with the previous sample

[Chris Wilson]

Quoting Chris Wilson (2017-11-24 10:42:15)
> Quoting Tvrtko Ursulin (2017-11-24 09:49:59)
> > From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > 
> > Averaging with the previous sample brings a small statistical improvement
> > to sampling counters, but can leek a little bit of state from a current
> > client to the next which mulls the border between past and present for
> > observing clients.
> > 
> > This is because on event enable clients record the current counter value
> > and use it as reference, but with rapid off-on event cycles, and due the
> > delayed nature of sampling timer self-disarm, previous sample value does
> > not get cleared under these circumstances.
> > 
> > Solution is to stop averaging with the previous sample. This has a small
> > downside of losing some precision with short and spiky signals, but the
> > alternatives look too complicated for the benefit.
> > 
> > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Sagar Arun Kamble <sagar.a.kamble at intel.com>
> 
> Nothing wrong with the patch, and I agree that chasing that little bit
> of smoothness is not worth the complexity in the edge cases. Oh well.
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

Tested-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris