[PATCH] drm/i915: Reduce the RPS shock

* [PATCH] drm/i915: Reduce the RPS shock
@ 2019-02-15 17:00 Chris Wilson
  2019-02-15 18:55 ` Chris Wilson
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: Chris Wilson @ 2019-02-15 17:00 UTC (permalink / raw)
  To: intel-gfx; +Cc: Michel Thierry, Eero Tamminen

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@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@chris-wilson.co.uk>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
---
 drivers/gpu/drm/i915/i915_irq.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 441d2674b272..d26e3d19c993 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/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
 	 */
-- 
2.20.1

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