[Intel-gfx] [PATCH] drm/i915: Reduce the RPS shock

[Chris Wilson]

Quoting Chris Wilson (2019-02-15 17:00:30)
> Limit deboosting and boosting to keep ourselves at the extremes
> when in the respective power modes (i.e. slowly decrease frequencies
> while in the HIGH_POWER zone and slowly increase frequencies while
> in the LOW_POWER zone). On idle, we will hit the timeout and drop
> to the next level quickly, and conversely if busy we expect to
> hit a waitboost and rapidly switch into max power.
> 
> This should improve the UX experience by keeping the GPU clocks higher
> than they ostensibly should be (based on simple busyness) by switching
> into the INTERACTIVE mode (due to waiting for pageflips) and increasing
> clocks via waitboosting. This will incur some additional power, our
> saving grace should be rc6 and powergating to keep the extra current
> draw in check.
> 
> Food for future thought would be deadline scheduling? If we know certain
> contexts (high priority compositors) absolutely must hit the next vblank
> then we can raise the frequencies ahead of time. Part of this is covered
> by per-context frequencies, where userspace is given control over the
> frequency range they want the GPU to execute at (for largely the same
> problem as this, where the workload is very latency sensitive but at the
> EI level appears mostly idle). Indeed, the per-context series does
> extend the modeset boosting to include a frequency range tweak which
> seems applicable to solving this jittery UX behaviour.
> 
> Reported-by: Lyude Paul <lyude at redhat.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109408
> References: 0d55babc8392 ("drm/i915: Drop stray clearing of rps->last_adj")
> References: 60548c554be2 ("drm/i915: Interactive RPS mode")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Lyude Paul <lyude at redhat.com>
> Cc: Eero Tamminen <eero.t.tamminen at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Michel Thierry <michel.thierry at intel.com>

Cribbed from Lyude on irc

before reverting 0d55babc8392754352f1058866dd4182ae587d11: [4.20]

35 measurements
Average: 33.65657142857143 FPS
FPS observed: 20.8 - 46.87 FPS
Percentage under 60 FPS: 100.0%
Percentage under 55 FPS: 100.0%
Percentage under 50 FPS: 100.0%
Percentage under 45 FPS: 97.14285714285714%
Percentage under 40 FPS: 97.14285714285714%
Percentage under 35 FPS: 45.714285714285715%
Percentage under 30 FPS: 11.428571428571429%
Percentage under 25 FPS: 2.857142857142857%

After reverting: [4.19 behaviour]

30 measurements
Average: 49.833666666666666 FPS
FPS observed: 33.85 - 60.0 FPS
Percentage under 60 FPS: 86.66666666666667%
Percentage under 55 FPS: 70.0%
Percentage under 50 FPS: 53.333333333333336%
Percentage under 45 FPS: 20.0%
Percentage under 40 FPS: 6.666666666666667%
Percentage under 35 FPS: 6.666666666666667%
Percentage under 30 FPS: 0%
Percentage under 25 FPS: 0%

Patched:
42 measurements
Average: 46.05428571428571 FPS
FPS observed: 1.82 - 59.98 FPS
Percentage under 60 FPS: 88.09523809523809%
Percentage under 55 FPS: 61.904761904761905%
Percentage under 50 FPS: 45.23809523809524%
Percentage under 45 FPS: 35.714285714285715%
Percentage under 40 FPS: 33.33333333333333%
Percentage under 35 FPS: 19.047619047619047%
Percentage under 30 FPS: 7.142857142857142%
Percentage under 25 FPS: 4.761904761904762%

Tested-by: Lyude Paul <lyude at redhat.com>
-Chris