[Intel-gfx] [PATCH v3] 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
v3: Limit the spinning to a single jiffie (~1us) at most. On an
otherwise idle system, there is no scheduler contention and so without a
limit we would spin until the GPU is ready.

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 | 48 +++++++++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 1cc46558bf2a..df96f9704cf2 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1191,6 +1191,33 @@ 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);
+	unsigned long timeout;
+	int ret = -EBUSY;
+
+	intel_uncore_forcewake_get(dev_priv, FORCEWAKE_ALL);
+	timeout = jiffies + 1;
+	while (!need_resched()) {
+		if (i915_gem_request_completed(req, true)) {
+			ret = 0;
+			goto out;
+		}
+
+		if (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 +1262,20 @@ 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 for the next jiffie 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 +1324,15 @@ 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