[PATCH 59/59] drm/i915: Reduce spinlock hold time during notify_ring() interrupt

[Chris Wilson]

By taking advantage of the RCU protection of the task struct, we can find
the appropriate signaler under the spinlock and then release the spinlock
before waking the task and signaling the fence.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_irq.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index edb84cb6dd5c..ea0796a6d79d 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -887,21 +887,23 @@ int intel_get_crtc_scanline(struct intel_crtc *crtc)
 
 static void notify_ring(struct intel_engine_cs *engine)
 {
+	const u32 seqno = intel_engine_get_seqno(engine);
 	struct i915_request *rq = NULL;
+	struct task_struct *tsk = NULL;
 	struct intel_wait *wait;
 
-	if (!engine->breadcrumbs.irq_armed)
+	if (unlikely(!engine->breadcrumbs.irq_armed))
 		return;
 
 	atomic_inc(&engine->irq_count);
-	set_bit(ENGINE_IRQ_BREADCRUMB, &engine->irq_posted);
+
+	rcu_read_lock();
 
 	spin_lock(&engine->breadcrumbs.irq_lock);
 	wait = engine->breadcrumbs.irq_wait;
 	if (wait) {
-		bool wakeup = engine->irq_seqno_barrier;
-
-		/* We use a callback from the dma-fence to submit
+		/*
+		 * We use a callback from the dma-fence to submit
 		 * requests after waiting on our own requests. To
 		 * ensure minimum delay in queuing the next request to
 		 * hardware, signal the fence now rather than wait for
@@ -912,19 +914,23 @@ static void notify_ring(struct intel_engine_cs *engine)
 		 * and to handle coalescing of multiple seqno updates
 		 * and many waiters.
 		 */
-		if (i915_seqno_passed(intel_engine_get_seqno(engine),
-				      wait->seqno)) {
+		if (i915_seqno_passed(seqno, wait->seqno)) {
 			struct i915_request *waiter = wait->request;
 
-			wakeup = true;
 			if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
 				      &waiter->fence.flags) &&
 			    intel_wait_check_request(wait, waiter))
 				rq = i915_request_get(waiter);
-		}
 
-		if (wakeup)
-			wake_up_process(wait->tsk);
+			tsk = wait->tsk;
+		} else {
+			if (engine->irq_seqno_barrier &&
+			    i915_seqno_passed(seqno, wait->seqno - 1)) {
+				set_bit(ENGINE_IRQ_BREADCRUMB,
+					&engine->irq_posted);
+				tsk = wait->tsk;
+			}
+		}
 	} else {
 		if (engine->breadcrumbs.irq_armed)
 			__intel_engine_disarm_breadcrumbs(engine);
@@ -937,6 +943,11 @@ static void notify_ring(struct intel_engine_cs *engine)
 		i915_request_put(rq);
 	}
 
+	if (tsk && tsk->state & TASK_NORMAL)
+		wake_up_process(tsk);
+
+	rcu_read_unlock();
+
 	trace_intel_engine_notify(engine, wait);
 }
 
-- 
2.17.0