[Intel-gfx] [PATCH 09/10] drm/i915/gt: Split the breadcrumb spinlock between global and contexts

[Chris Wilson <chris@chris-wilson.co.uk>]

As we funnel more and more contexts into the breadcrumbs on an engine,
the hold time of b->irq_lock grows. As we may then contend with the
b->irq_lock during request submission, this increases the burden upon
the engine->active.lock and so directly impacts both our execution
latency and client latency. If we split the b->irq_lock by introducing a
per-context spinlock to manage the signalers within a context, we then
only need the b->irq_lock for enabling/disabling the interrupt and can
avoid taking the lock for walking the list of contexts within the signal
worker. Even with the current setup, this greatly reduces the number of
times we have to take and fight for b->irq_lock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_breadcrumbs.c   | 128 ++++++++++--------
 drivers/gpu/drm/i915/gt/intel_context.c       |   1 +
 drivers/gpu/drm/i915/gt/intel_context_types.h |   1 +
 3 files changed, 72 insertions(+), 58 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index 2b3ad17c63b9..221e7801d760 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -105,15 +105,15 @@ 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))
+	list_add_rcu(&ce->signal_link, &b->signalers);
+	if (list_is_last(&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);
+	list_del_rcu(&ce->signal_link);
 	intel_context_put(ce);
 }
 
@@ -180,20 +180,27 @@ 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;
+	struct intel_context *ce;
 
 	signal = NULL;
 	if (!llist_empty(&b->signaled_requests))
 		signal = llist_del_all(&b->signaled_requests);
 
-	spin_lock(&b->irq_lock);
+	if (!signal && READ_ONCE(b->irq_armed) && list_empty(&b->signalers)) {
+		spin_lock(&b->irq_lock);
+		if (b->irq_armed && list_empty(&b->signalers))
+			__intel_breadcrumbs_disarm_irq(b);
+		spin_unlock(&b->irq_lock);
+	}
 
-	if (!signal && b->irq_armed && list_empty(&b->signalers))
-		__intel_breadcrumbs_disarm_irq(b);
+	rcu_read_lock();
+	list_for_each_entry_rcu(ce, &b->signalers, signal_link) {
+		struct list_head *pos, *next;
 
-	list_for_each_entry_safe(ce, cn, &b->signalers, signal_link) {
-		GEM_BUG_ON(list_empty(&ce->signals));
+		spin_lock(&ce->signal_lock);
+
+		if (list_empty(&ce->signals))
+			goto unlock;
 
 		list_for_each_safe(pos, next, &ce->signals) {
 			struct i915_request *rq =
@@ -226,13 +233,18 @@ 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 */
+				spin_lock(&b->irq_lock);
 				remove_signaling_context(b, ce);
+				spin_unlock(&b->irq_lock);
+
 				add_retire(b, ce->timeline);
 			}
 		}
-	}
 
-	spin_unlock(&b->irq_lock);
+unlock:
+		spin_unlock(&ce->signal_lock);
+	}
+	rcu_read_unlock();
 
 	llist_for_each_safe(signal, sn, signal) {
 		struct i915_request *rq =
@@ -308,9 +320,9 @@ void intel_breadcrumbs_free(struct intel_breadcrumbs *b)
 	kfree(b);
 }
 
-static void insert_breadcrumb(struct i915_request *rq,
-			      struct intel_breadcrumbs *b)
+static void insert_breadcrumb(struct i915_request *rq)
 {
+	struct intel_breadcrumbs *b = READ_ONCE(rq->engine)->breadcrumbs;
 	struct intel_context *ce = rq->context;
 	struct list_head *pos;
 
@@ -349,8 +361,33 @@ static void insert_breadcrumb(struct i915_request *rq,
 	 * the list.
 	 */
 	if (list_empty(&ce->signals)) {
+		/*
+		 * rq->engine is locked by rq->engine->active.lock. That
+		 * however is not known until after rq->engine has been
+		 * dereferenced and the lock acquired. Hence we acquire the
+		 * lock and then validate that rq->engine still matches the
+		 * lock we hold for it.
+		 *
+		 * Here, we are using the breadcrumb lock as a proxy for the
+		 * rq->engine->active.lock, and we know that since the
+		 * breadcrumb will be serialised within i915_request_submit
+		 * the engine cannot change while active as long as we hold
+		 * the breadcrumb lock on that engine.
+		 *
+		 * From the dma_fence_enable_signaling() path, we are outside
+		 * of the request submit/unsubmit path, and so we must be more
+		 * careful to acquire the right lock.
+		 */
+		spin_lock(&b->irq_lock);
+		while (unlikely(b != READ_ONCE(rq->engine)->breadcrumbs)) {
+			spin_unlock(&b->irq_lock);
+			b = READ_ONCE(rq->engine)->breadcrumbs;
+			spin_lock(&b->irq_lock);
+		}
 		add_signaling_context(b, ce);
 		GEM_BUG_ON(!b->irq_armed);
+		spin_unlock(&b->irq_lock);
+
 		pos = &ce->signals;
 	} else {
 		list_for_each_prev(pos, &ce->signals) {
@@ -372,7 +409,7 @@ static void insert_breadcrumb(struct i915_request *rq,
 
 bool i915_request_enable_breadcrumb(struct i915_request *rq)
 {
-	struct intel_breadcrumbs *b;
+	struct intel_context *ce = rq->context;
 
 	/* Serialises with i915_request_retire() using rq->lock */
 	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &rq->fence.flags))
@@ -387,48 +424,17 @@ bool i915_request_enable_breadcrumb(struct i915_request *rq)
 	if (!test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
 		return true;
 
-	/*
-	 * rq->engine is locked by rq->engine->active.lock. That however
-	 * is not known until after rq->engine has been dereferenced and
-	 * the lock acquired. Hence we acquire the lock and then validate
-	 * that rq->engine still matches the lock we hold for it.
-	 *
-	 * Here, we are using the breadcrumb lock as a proxy for the
-	 * rq->engine->active.lock, and we know that since the breadcrumb
-	 * will be serialised within i915_request_submit/i915_request_unsubmit,
-	 * the engine cannot change while active as long as we hold the
-	 * breadcrumb lock on that engine.
-	 *
-	 * From the dma_fence_enable_signaling() path, we are outside of the
-	 * request submit/unsubmit path, and so we must be more careful to
-	 * acquire the right lock.
-	 */
-	b = READ_ONCE(rq->engine)->breadcrumbs;
-	spin_lock(&b->irq_lock);
-	while (unlikely(b != READ_ONCE(rq->engine)->breadcrumbs)) {
-		spin_unlock(&b->irq_lock);
-		b = READ_ONCE(rq->engine)->breadcrumbs;
-		spin_lock(&b->irq_lock);
-	}
-
-	/*
-	 * Now that we are finally serialised with request submit/unsubmit,
-	 * [with b->irq_lock] and with i915_request_retire() [via checking
-	 * SIGNALED with rq->lock] confirm the request is indeed active. If
-	 * it is no longer active, the breadcrumb will be attached upon
-	 * i915_request_submit().
-	 */
+	spin_lock(&ce->signal_lock);
 	if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
-		insert_breadcrumb(rq, b);
-
-	spin_unlock(&b->irq_lock);
+		insert_breadcrumb(rq);
+	spin_unlock(&ce->signal_lock);
 
 	return true;
 }
 
 void i915_request_cancel_breadcrumb(struct i915_request *rq)
 {
-	struct intel_breadcrumbs *b = rq->engine->breadcrumbs;
+	struct intel_context *ce = rq->context;
 
 	/*
 	 * We must wait for b->irq_lock so that we know the interrupt handler
@@ -436,18 +442,22 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 	 * the DMA_FENCE_FLAG_SIGNALED_BIT/I915_FENCE_FLAG_SIGNAL dance (if
 	 * required).
 	 */
-	spin_lock(&b->irq_lock);
+	spin_lock(&ce->signal_lock);
 	if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags)) {
-		struct intel_context *ce = rq->context;
-
 		list_del(&rq->signal_link);
-		if (list_empty(&ce->signals))
+
+		if (list_empty(&ce->signals)) {
+			struct intel_breadcrumbs *b = rq->engine->breadcrumbs;
+
+			spin_lock(&b->irq_lock);
 			remove_signaling_context(b, ce);
+			spin_unlock(&b->irq_lock);
+		}
 
 		clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
 		i915_request_put(rq);
 	}
-	spin_unlock(&b->irq_lock);
+	spin_unlock(&ce->signal_lock);
 }
 
 void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
@@ -463,8 +473,9 @@ void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
 	drm_printf(p, "IRQ: %s\n", enableddisabled(b->irq_armed));
 	drm_printf(p, "Signals:\n");
 
-	spin_lock_irq(&b->irq_lock);
-	list_for_each_entry(ce, &b->signalers, signal_link) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(ce, &b->signalers, signal_link) {
+		spin_lock_irq(&ce->signal_lock);
 		list_for_each_entry(rq, &ce->signals, signal_link) {
 			drm_printf(p, "\t[%llx:%llx%s] @ %dms\n",
 				   rq->fence.context, rq->fence.seqno,
@@ -473,6 +484,7 @@ void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
 				   "",
 				   jiffies_to_msecs(jiffies - rq->emitted_jiffies));
 		}
+		spin_unlock_irq(&ce->signal_lock);
 	}
-	spin_unlock_irq(&b->irq_lock);
+	rcu_read_unlock();
 }
diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index 52db2bde44a3..c3010c8eb966 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -297,6 +297,7 @@ intel_context_init(struct intel_context *ce,
 
 	ce->vm = i915_vm_get(engine->gt->vm);
 
+	spin_lock_init(&ce->signal_lock);
 	INIT_LIST_HEAD(&ce->signal_link);
 	INIT_LIST_HEAD(&ce->signals);
 
diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
index 4954b0df4864..a78c1c225ce3 100644
--- a/drivers/gpu/drm/i915/gt/intel_context_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
@@ -51,6 +51,7 @@ struct intel_context {
 	struct i915_address_space *vm;
 	struct i915_gem_context __rcu *gem_context;
 
+	spinlock_t signal_lock;
 	struct list_head signal_link;
 	struct list_head signals;
 
-- 
2.20.1

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