[Bug 109408] [Kabylake] RPS waitboost regression since v4.20

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109408

--- Comment #7 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to Lakshmi from comment #6)
> Reporter, can you verify the issue with latest drmtip?

Yes, Lyude already has. And we've found so far that it isn't missing
waitboosts, as we've unconditionally applied the boost to all request waits and
still the performance is jittery on -tip. My suspicion lies towards something
else inducing latency causing the GPU to idle enough for rps downclocking to
take hold; that's likely to be vblanks (nothing else matters?). Optimistically
Lyude might be able to bisect, but it'll be slow, tedious and prone to
mistaking good/bad results. And there may well be more than one change leading
to this effect.

But for now the investigation mostly says it's not waitboosting per-se that's
changed between the versions.

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