[Intel-gfx] [PATCH] drm/i915: Optimistically spin for the request completion

[Chris Wilson]

This provides a nice boost to mesa in swap bound scenarios (as mesa
throttles itself to the previous frame and given the scenario that will
complete shortly). It will also provide a good boost to systems running
with semaphores disabled and so frequently waiting on the GPU as it
switches rings.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 43 +++++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index de15bd319bd0..2aaa3cab2be4 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1191,6 +1191,28 @@ static bool missed_irq(struct drm_i915_private *dev_priv,
 	return test_bit(ring->id, &dev_priv->gpu_error.missed_irq_rings);
 }
 
+static int __i915_spin_request(struct drm_i915_gem_request *req)
+{
+	struct intel_engine_cs *ring = i915_gem_request_get_ring(req);
+	struct drm_i915_private *dev_priv = to_i915(ring->dev);
+	int ret = -EBUSY;
+
+	intel_uncore_forcewake_get(dev_priv, FORCEWAKE_ALL);
+	while (!need_resched()) {
+		if (i915_gem_request_completed(req, true)) {
+			ret = 0;
+			goto out;
+		}
+
+		cpu_relax_lowlatency();
+	}
+	if (i915_gem_request_completed(req, false))
+		ret = 0;
+out:
+	intel_uncore_forcewake_put(dev_priv, FORCEWAKE_ALL);
+	return ret;
+}
+
 /**
  * __i915_wait_request - wait until execution of request has finished
  * @req: duh!
@@ -1235,12 +1257,20 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
 	if (ring->id == RCS && INTEL_INFO(dev)->gen >= 6)
 		gen6_rps_boost(dev_priv, file_priv);
 
-	if (!irq_test_in_progress && WARN_ON(!ring->irq_get(ring)))
-		return -ENODEV;
-
 	/* Record current time in case interrupted by signal, or wedged */
 	trace_i915_gem_request_wait_begin(req);
 	before = ktime_get_raw_ns();
+
+	/* Optimistic spin before touching IRQs */
+	ret = __i915_spin_request(req);
+	if (ret == 0)
+		goto out;
+
+	if (!irq_test_in_progress && WARN_ON(!ring->irq_get(ring))) {
+		ret = -ENODEV;
+		goto out;
+	}
+
 	for (;;) {
 		struct timer_list timer;
 
@@ -1289,14 +1319,15 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
 			destroy_timer_on_stack(&timer);
 		}
 	}
-	now = ktime_get_raw_ns();
-	trace_i915_gem_request_wait_end(req);
-
 	if (!irq_test_in_progress)
 		ring->irq_put(ring);
 
 	finish_wait(&ring->irq_queue, &wait);
 
+out:
+	now = ktime_get_raw_ns();
+	trace_i915_gem_request_wait_end(req);
+
 	if (timeout) {
 		s64 tres = *timeout - (now - before);
 
-- 
2.1.4