[PATCH 7/9] drm/i915/gt: Hold context/request reference while breadcrumbs are active

Chris Wilson chris at chris-wilson.co.uk
Sun Jul 19 00:03:10 UTC 2020


Currently we hold no actual reference to the request nor context while
they are attached to a breadcrumb. To avoid freeing the request/context
too early, we serialise with cancel-breadcrumbs by taking the irq
spinlock in i915_request_retire(). The alternative is to take a
reference for a new breadcrumb and release it upon signaling; the
question is which is more noticeable, as it is more or less the same
number of atomic operations overall, just along different paths.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 52 +++++++++++++++------
 drivers/gpu/drm/i915/i915_request.c         |  9 ++--
 2 files changed, 42 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index 70ea3eeb25d3..1e9022b0bee7 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -29,6 +29,7 @@
 #include "i915_drv.h"
 #include "i915_trace.h"
 #include "intel_breadcrumbs.h"
+#include "intel_context.h"
 #include "intel_gt_pm.h"
 #include "intel_gt_requests.h"
 
@@ -100,6 +101,22 @@ static void __intel_breadcrumbs_disarm_irq(struct intel_breadcrumbs *b)
 	intel_gt_pm_put_async(b->irq_engine->gt);
 }
 
+static void add_signaling_context(struct intel_breadcrumbs *b,
+				  struct intel_context *ce)
+{
+	intel_context_get(ce);
+	list_add_tail(&ce->signal_link, &b->signalers);
+	if (list_is_first(&ce->signal_link, &b->signalers))
+		__intel_breadcrumbs_arm_irq(b);
+}
+
+static void remove_signaling_context(struct intel_breadcrumbs *b,
+				     struct intel_context *ce)
+{
+	list_del(&ce->signal_link);
+	intel_context_put(ce);
+}
+
 static inline bool __request_completed(const struct i915_request *rq)
 {
 	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
@@ -162,7 +179,6 @@ static bool __signal_request(struct i915_request *rq, struct list_head *signals)
 	if (!__dma_fence_signal(&rq->fence))
 		return false;
 
-	i915_request_get(rq);
 	list_add_tail(&rq->signal_link, signals);
 	return true;
 }
@@ -189,6 +205,7 @@ static void signal_irq_work(struct irq_work *work)
 			struct i915_request *rq =
 				list_entry(pos, typeof(*rq), signal_link);
 
+			GEM_BUG_ON(rq->context != ce);
 			GEM_BUG_ON(!check_signal_order(ce, rq));
 			if (!__request_completed(rq))
 				break;
@@ -198,7 +215,8 @@ static void signal_irq_work(struct irq_work *work)
 			 * spinlock as the callback chain may end up adding
 			 * more signalers to the same context or engine.
 			 */
-			__signal_request(rq, &signal);
+			if (!__signal_request(rq, &signal))
+				i915_request_put(rq);
 		}
 
 		/*
@@ -210,7 +228,7 @@ static void signal_irq_work(struct irq_work *work)
 			/* Advance the list to the first incomplete request */
 			__list_del_many(&ce->signals, pos);
 			if (&ce->signals == pos) { /* now empty */
-				list_del_init(&ce->signal_link);
+				remove_signaling_context(b, ce);
 				add_retire(b, ce->timeline);
 			}
 		}
@@ -299,6 +317,8 @@ static void insert_breadcrumb(struct i915_request *rq,
 	if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
 		return;
 
+	i915_request_get(rq);
+
 	/*
 	 * If the request is already completed, we can transfer it
 	 * straight onto a signaled list, and queue the irq worker for
@@ -307,11 +327,11 @@ static void insert_breadcrumb(struct i915_request *rq,
 	if (__request_completed(rq)) {
 		if (__signal_request(rq, &b->signaled_requests))
 			irq_work_queue(&b->irq_work);
+		else
+			i915_request_put(rq);
 		return;
 	}
 
-	__intel_breadcrumbs_arm_irq(b);
-
 	/*
 	 * We keep the seqno in retirement order, so we can break
 	 * inside intel_engine_signal_breadcrumbs as soon as we've
@@ -326,16 +346,19 @@ static void insert_breadcrumb(struct i915_request *rq,
 	 * start looking for our insertion point from the tail of
 	 * the list.
 	 */
-	list_for_each_prev(pos, &ce->signals) {
-		struct i915_request *it =
-			list_entry(pos, typeof(*it), signal_link);
-
-		if (i915_seqno_passed(rq->fence.seqno, it->fence.seqno))
-			break;
+	if (list_empty(&ce->signals)) {
+		add_signaling_context(b, ce);
+		pos = &ce->signals;
+	} else {
+		list_for_each_prev(pos, &ce->signals) {
+			struct i915_request *it =
+				list_entry(pos, typeof(*it), signal_link);
+
+			if (i915_seqno_passed(rq->fence.seqno, it->fence.seqno))
+				break;
+		}
 	}
 	list_add(&rq->signal_link, pos);
-	if (pos == &ce->signals) /* catch transitions from empty list */
-		list_move_tail(&ce->signal_link, &b->signalers);
 	GEM_BUG_ON(!check_signal_order(ce, rq));
 	set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
 
@@ -416,9 +439,10 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 
 		list_del(&rq->signal_link);
 		if (list_empty(&ce->signals))
-			list_del_init(&ce->signal_link);
+			remove_signaling_context(b, ce);
 
 		clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
+		i915_request_put(rq);
 	}
 	spin_unlock(&b->irq_lock);
 }
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 394587e70a2d..4ebb0f144ac4 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -300,12 +300,11 @@ bool i915_request_retire(struct i915_request *rq)
 		__i915_request_fill(rq, POISON_FREE);
 	rq->ring->head = rq->postfix;
 
-	spin_lock_irq(&rq->lock);
-	if (!i915_request_signaled(rq))
+	if (!i915_request_signaled(rq)) {
+		spin_lock_irq(&rq->lock);
 		dma_fence_signal_locked(&rq->fence);
-	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags))
-		i915_request_cancel_breadcrumb(rq);
-	spin_unlock_irq(&rq->lock);
+		spin_unlock_irq(&rq->lock);
+	}
 
 	if (i915_request_has_waitboost(rq)) {
 		GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));
-- 
2.20.1



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