drm/i915: Reduce the RPS shock

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 <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109408
References: 0d55bab ("drm/i915: Drop stray clearing of rps->last_adj")
References: 60548c5 ("drm/i915: Interactive RPS mode")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Eero Tamminen <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Michel Thierry <[email protected]>

Quoting Lyude Paul:
> Before reverting 0d55bab: [4.20]
>
> 35 measurements [of gnome-shell animations]
> 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 <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: ickle

drivers/gpu/drm/i915/i915_irq.c

@@ -1288,6 +1288,18 @@ static void gen6_pm_rps_work(struct work_struct *work)
 
 	rps->last_adj = adj;
 
+	/*
+	 * 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.
+	 */
+	if ((adj < 0 && rps->power.mode == HIGH_POWER) ||
+	    (adj > 0 && rps->power.mode == LOW_POWER))
+		rps->last_adj = 0;
+
 	/* sysfs frequency interfaces may have snuck in while servicing the
 	 * interrupt
 	 */