[PATCH 18/19] drm/i915: Use a preemption timeout to enforce interactivity

[Chris Wilson]

Use a liberal timeout of 20ms to ensure that the rendering for an
interactive pageflip is started in a timely fashion, and that
user interaction is not blocked by GPU, or CPU, hogs. This is at the cost
of resetting whoever was blocking the preemption, likely leading to that
context/process being banned from submitting future requests.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_drv.h      |  3 ++-
 drivers/gpu/drm/i915/i915_gem.c      | 18 ++++++++++--------
 drivers/gpu/drm/i915/intel_display.c | 18 +++++++++++++++++-
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 800230ba1c3b..87388feb973d 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3150,8 +3150,9 @@ int i915_gem_object_wait(struct drm_i915_gem_object *obj,
 			 struct intel_rps_client *rps);
 int i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
 				  unsigned int flags,
-				  int priority);
+				  int priority, unsigned int timeout);
 #define I915_PRIORITY_DISPLAY I915_PRIORITY_MAX
+#define I915_PREEMPTION_TIMEOUT_DISPLAY (20 * 1000 * 1000) /* 20 ms */
 
 int __must_check
 i915_gem_object_set_to_wc_domain(struct drm_i915_gem_object *obj, bool write);
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index a860d501134e..3760133ca08e 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -469,7 +469,8 @@ i915_gem_object_wait_reservation(struct reservation_object *resv,
 	return timeout;
 }
 
-static void __fence_set_priority(struct dma_fence *fence, int prio)
+static void __fence_set_priority(struct dma_fence *fence,
+				 int prio, unsigned int timeout)
 {
 	struct i915_request *rq;
 	struct intel_engine_cs *engine;
@@ -482,11 +483,12 @@ static void __fence_set_priority(struct dma_fence *fence, int prio)
 
 	rcu_read_lock();
 	if (engine->schedule)
-		engine->schedule(rq, prio, 0);
+		engine->schedule(rq, prio, timeout);
 	rcu_read_unlock();
 }
 
-static void fence_set_priority(struct dma_fence *fence, int prio)
+static void fence_set_priority(struct dma_fence *fence,
+			       int prio, unsigned int timeout)
 {
 	/* Recurse once into a fence-array */
 	if (dma_fence_is_array(fence)) {
@@ -494,16 +496,16 @@ static void fence_set_priority(struct dma_fence *fence, int prio)
 		int i;
 
 		for (i = 0; i < array->num_fences; i++)
-			__fence_set_priority(array->fences[i], prio);
+			__fence_set_priority(array->fences[i], prio, timeout);
 	} else {
-		__fence_set_priority(fence, prio);
+		__fence_set_priority(fence, prio, timeout);
 	}
 }
 
 int
 i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
 			      unsigned int flags,
-			      int prio)
+			      int prio, unsigned int timeout)
 {
 	struct dma_fence *excl;
 
@@ -518,7 +520,7 @@ i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
 			return ret;
 
 		for (i = 0; i < count; i++) {
-			fence_set_priority(shared[i], prio);
+			fence_set_priority(shared[i], prio, timeout);
 			dma_fence_put(shared[i]);
 		}
 
@@ -528,7 +530,7 @@ i915_gem_object_wait_priority(struct drm_i915_gem_object *obj,
 	}
 
 	if (excl) {
-		fence_set_priority(excl, prio);
+		fence_set_priority(excl, prio, timeout);
 		dma_fence_put(excl);
 	}
 	return 0;
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 3acd75753a31..b7992a427e8d 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12786,7 +12786,23 @@ intel_prepare_plane_fb(struct drm_plane *plane,
 
 	ret = intel_plane_pin_fb(to_intel_plane_state(new_state));
 
-	i915_gem_object_wait_priority(obj, 0, I915_PRIORITY_DISPLAY);
+	/*
+	 * Reschedule our dependencies, and ensure we run within a timeout.
+	 *
+	 * Note that if the timeout is exceeded, then whoever was running that
+	 * prevented us from acquiring the GPU is declared rogue and reset. An
+	 * unresponsive process will then be banned in order to preserve
+	 * interactivity. Since this can be seen as a bit heavy-handed, we
+	 * select a timeout for when the dropped frames start to become a
+	 * noticeable nuisance for the user (20 ms, i.e. preemption was blocked
+	 * for more than a frame). Note, this is only a timeout for a delay in
+	 * preempting the current request in order to run our dependency chain,
+	 * our dependency chain may itself take a long time to run to completion
+	 * before we can present the framebuffer.
+	 */
+	i915_gem_object_wait_priority(obj, 0,
+				      I915_PRIORITY_DISPLAY,
+				      I915_PREEMPTION_TIMEOUT_DISPLAY);
 
 	mutex_unlock(&dev_priv->drm.struct_mutex);
 	i915_gem_object_unpin_pages(obj);
-- 
2.16.3