[Intel-gfx] [PATCH v2] 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. In the most favourable of microbenchmarks, this can
increase performance by around 15% - though in practice improvements
will be marginal and rarely noticeable.

v2: Account for user timeouts

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_gem.c | 47 +++++++++++++++++++++++++++++++++++------
 1 file changed, 41 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index de15bd319bd0..f8895ed368cb 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1191,6 +1191,32 @@ 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,
+			       unsigned long timeout)
+{
+	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;
+		}
+
+		if (timeout && time_after_eq(jiffies, timeout))
+			break;
+
+		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 +1261,20 @@ 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, timeout_expire);
+	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 +1323,15 @@ 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