[PATCH 7/7] irqbypass: Use xarray to track producers and consumers

[Sean Christopherson <seanjc@google.com>]

Track IRQ bypass produsers and consumers using an xarray to avoid the O(2n)
insertion time associated with walking a list to check for duplicate
entries, and to search for an partner.

At low (tens or few hundreds) total producer/consumer counts, using a list
is faster due to the need to allocate backing storage for xarray.  But as
count creeps into the thousands, xarray wins easily, and can provide
several orders of magnitude better latency at high counts.  E.g. hundreds
of nanoseconds vs. hundreds of milliseconds.

Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: David Matlack <dmatlack@google.com>
Cc: Like Xu <like.xu.linux@gmail.com>
Reported-by: Yong He <alexyonghe@tencent.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217379
Link: https://lore.kernel.org/all/20230801115646.33990-1-likexu@tencent.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/irqbypass.h |  2 --
 virt/lib/irqbypass.c      | 68 +++++++++++++++++++--------------------
 2 files changed, 34 insertions(+), 36 deletions(-)

diff --git a/include/linux/irqbypass.h b/include/linux/irqbypass.h
index 6d4e4882843c..e9d9485eaf99 100644
--- a/include/linux/irqbypass.h
+++ b/include/linux/irqbypass.h
@@ -46,7 +46,6 @@ struct irq_bypass_consumer;
  * for a physical device assigned to a VM.
  */
 struct irq_bypass_producer {
-	struct list_head node;
 	void *token;
 	struct irq_bypass_consumer *consumer;
 	int irq;
@@ -73,7 +72,6 @@ struct irq_bypass_producer {
  * portions of the interrupt handling to the VM.
  */
 struct irq_bypass_consumer {
-	struct list_head node;
 	void *token;
 	struct irq_bypass_producer *producer;
 
diff --git a/virt/lib/irqbypass.c b/virt/lib/irqbypass.c
index 261ef77f6364..3f7734e63d0f 100644
--- a/virt/lib/irqbypass.c
+++ b/virt/lib/irqbypass.c
@@ -22,8 +22,8 @@
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("IRQ bypass manager utility module");
 
-static LIST_HEAD(producers);
-static LIST_HEAD(consumers);
+static DEFINE_XARRAY(producers);
+static DEFINE_XARRAY(consumers);
 static DEFINE_MUTEX(lock);
 
 /* @lock must be held when calling connect */
@@ -86,13 +86,13 @@ static void __disconnect(struct irq_bypass_producer *prod,
  * @producer: pointer to producer structure
  * @eventfd: pointer to the eventfd context associated with the producer
  *
- * Add the provided IRQ producer to the list of producers and connect
- * with any matching token found on the IRQ consumers list.
+ * Add the provided IRQ producer to the set of producers and connect with the
+ * consumer with a matching token, if one exists.
  */
 int irq_bypass_register_producer(struct irq_bypass_producer *producer,
 				 struct eventfd_ctx *eventfd)
 {
-	struct irq_bypass_producer *tmp;
+	unsigned long token = (unsigned long)eventfd;
 	struct irq_bypass_consumer *consumer;
 	int ret;
 
@@ -101,22 +101,20 @@ int irq_bypass_register_producer(struct irq_bypass_producer *producer,
 
 	guard(mutex)(&lock);
 
-	list_for_each_entry(tmp, &producers, node) {
-		if (tmp->token == eventfd)
-			return -EBUSY;
-	}
+	ret = xa_insert(&producers, token, producer, GFP_KERNEL);
+	if (ret)
+		return ret;
 
-	list_for_each_entry(consumer, &consumers, node) {
-		if (consumer->token == eventfd) {
-			ret = __connect(producer, consumer);
-			if (ret)
-				return ret;
-			break;
+	consumer = xa_load(&consumers, token);
+	if (consumer) {
+		ret = __connect(producer, consumer);
+		if (ret) {
+			WARN_ON_ONCE(xa_erase(&producers, token) != producer);
+			return ret;
 		}
 	}
 
 	producer->token = eventfd;
-	list_add(&producer->node, &producers);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
@@ -125,8 +123,9 @@ EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
  * irq_bypass_unregister_producer - unregister IRQ bypass producer
  * @producer: pointer to producer structure
  *
- * Remove a previously registered IRQ producer from the list of producers
- * and disconnect it from any connected IRQ consumer.
+ * Remove a previously registered IRQ producer (note, it's safe to call this
+ * even if registration was unsuccessful).  Disconnect from the associated
+ * consumer, if one exists.
  */
 void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
 {
@@ -138,8 +137,8 @@ void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
 	if (producer->consumer)
 		__disconnect(producer, producer->consumer);
 
+	WARN_ON_ONCE(xa_erase(&producers, (unsigned long)producer->token) != producer);
 	producer->token = NULL;
-	list_del(&producer->node);
 }
 EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
 
@@ -148,11 +147,13 @@ EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
  * @consumer: pointer to consumer structure
  * @eventfd: pointer to the eventfd context associated with the consumer
  *
+ * Add the provided IRQ consumer to the set of consumer and connect with the
+ * producer with a matching token, if one exists.
  */
 int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer,
 				 struct eventfd_ctx *eventfd)
 {
-	struct irq_bypass_consumer *tmp;
+	unsigned long token = (unsigned long)eventfd;
 	struct irq_bypass_producer *producer;
 	int ret;
 
@@ -164,22 +165,20 @@ int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer,
 
 	guard(mutex)(&lock);
 
-	list_for_each_entry(tmp, &consumers, node) {
-		if (tmp->token == eventfd || tmp == consumer)
-			return -EBUSY;
-	}
+	ret = xa_insert(&consumers, token, consumer, GFP_KERNEL);
+	if (ret)
+		return ret;
 
-	list_for_each_entry(producer, &producers, node) {
-		if (producer->token == eventfd) {
-			ret = __connect(producer, consumer);
-			if (ret)
-				return ret;
-			break;
+	producer = xa_load(&producers, token);
+	if (producer) {
+		ret = __connect(producer, consumer);
+		if (ret) {
+			WARN_ON_ONCE(xa_erase(&consumers, token) != consumer);
+			return ret;
 		}
 	}
 
 	consumer->token = eventfd;
-	list_add(&consumer->node, &consumers);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
@@ -188,8 +187,9 @@ EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
  * irq_bypass_unregister_consumer - unregister IRQ bypass consumer
  * @consumer: pointer to consumer structure
  *
- * Remove a previously registered IRQ consumer from the list of consumers
- * and disconnect it from any connected IRQ producer.
+ * Remove a previously registered IRQ consumer (note, it's safe to call this
+ * even if registration was unsuccessful).  Disconnect from the associated
+ * producer, if one exists.
  */
 void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
 {
@@ -201,7 +201,7 @@ void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
 	if (consumer->producer)
 		__disconnect(consumer->producer, consumer);
 
+	WARN_ON_ONCE(xa_erase(&consumers, (unsigned long)consumer->token) != consumer);
 	consumer->token = NULL;
-	list_del(&consumer->node);
 }
 EXPORT_SYMBOL_GPL(irq_bypass_unregister_consumer);
-- 
2.49.0.504.g3bcea36a83-goog