drm/i915/gt: Track signaled breadcrumbs outside of the breadcrumb spinlock

Make b->signaled_requests a lockless-list so that we can manipulate it
outside of the b->irq_lock.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201123113717.20500-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_breadcrumbs.c

@@ -173,26 +173,34 @@ static void add_retire(struct intel_breadcrumbs *b, struct intel_timeline *tl)
 		intel_engine_add_retire(b->irq_engine, tl);
 }
 
-static bool __signal_request(struct i915_request *rq, struct list_head *signals)
+static bool __signal_request(struct i915_request *rq)
 {
-	clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
-
 	if (!__dma_fence_signal(&rq->fence)) {
 		i915_request_put(rq);
 		return false;
 	}
 
-	list_add_tail(&rq->signal_link, signals);
 	return true;
 }
 
+static struct llist_node *
+slist_add(struct llist_node *node, struct llist_node *head)
+{
+	node->next = head;
+	return node;
+}
+
 static void signal_irq_work(struct irq_work *work)
 {
 	struct intel_breadcrumbs *b = container_of(work, typeof(*b), irq_work);
 	const ktime_t timestamp = ktime_get();
+	struct llist_node *signal, *sn;
 	struct intel_context *ce, *cn;
 	struct list_head *pos, *next;
-	LIST_HEAD(signal);
+
+	signal = NULL;
+	if (unlikely(!llist_empty(&b->signaled_requests)))
+		signal = llist_del_all(&b->signaled_requests);
 
 	spin_lock(&b->irq_lock);
 
@@ -224,8 +232,6 @@ static void signal_irq_work(struct irq_work *work)
 	if (b->irq_armed && list_empty(&b->signalers))
 		__intel_breadcrumbs_disarm_irq(b);
 
-	list_splice_init(&b->signaled_requests, &signal);
-
 	list_for_each_entry_safe(ce, cn, &b->signalers, signal_link) {
 		GEM_BUG_ON(list_empty(&ce->signals));
 
@@ -242,7 +248,10 @@ 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);
+			clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
+			if (__signal_request(rq))
+				/* We own signal_node now, xfer to local list */
+				signal = slist_add(&rq->signal_node, signal);
 		}
 
 		/*
@@ -262,9 +271,9 @@ static void signal_irq_work(struct irq_work *work)
 
 	spin_unlock(&b->irq_lock);
 
-	list_for_each_safe(pos, next, &signal) {
+	llist_for_each_safe(signal, sn, signal) {
 		struct i915_request *rq =
-			list_entry(pos, typeof(*rq), signal_link);
+			llist_entry(signal, typeof(*rq), signal_node);
 		struct list_head cb_list;
 
 		spin_lock(&rq->lock);
@@ -291,7 +300,7 @@ intel_breadcrumbs_create(struct intel_engine_cs *irq_engine)
 
 	spin_lock_init(&b->irq_lock);
 	INIT_LIST_HEAD(&b->signalers);
-	INIT_LIST_HEAD(&b->signaled_requests);
+	init_llist_head(&b->signaled_requests);
 
 	init_irq_work(&b->irq_work, signal_irq_work);
 
@@ -355,7 +364,8 @@ static void insert_breadcrumb(struct i915_request *rq,
 	 * its signal completion.
 	 */
 	if (__request_completed(rq)) {
-		if (__signal_request(rq, &b->signaled_requests))
+		if (__signal_request(rq) &&
+		    llist_add(&rq->signal_node, &b->signaled_requests))
 			irq_work_queue(&b->irq_work);
 		return;
 	}

drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h

@@ -35,7 +35,7 @@ struct intel_breadcrumbs {
 	struct intel_engine_cs *irq_engine;
 
 	struct list_head signalers;
-	struct list_head signaled_requests;
+	struct llist_head signaled_requests;
 
 	struct irq_work irq_work; /* for use from inside irq_lock */
 

drivers/gpu/drm/i915/i915_request.h

@@ -177,7 +177,11 @@ struct i915_request {
 	struct intel_context *context;
 	struct intel_ring *ring;
 	struct intel_timeline __rcu *timeline;
-	struct list_head signal_link;
+
+	union {
+		struct list_head signal_link;
+		struct llist_node signal_node;
+	};
 
 	/*
 	 * The rcu epoch of when this request was allocated. Used to judiciously