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

[Chris Wilson]

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>

(Until such a time as we have a clean way of being perfect, good
enough.)
-Chris