[KVM PATCH v8 3/3] KVM: create irqfd-cleanup-wq on demand

[Gregory Haskins <ghaskins@novell.com>]

We currently create this wq on module_init, which may be wasteful if the
host never creates a guest that uses irqfd.  This patch changes the
algorithm so that the workqueue is only created when at least one guest
is using irqfd.  The queue is cleaned up when the last guest using irqfd
is shutdown.

To keep things simple, we only check whether the guest has tried to create
an irqfd, not whether there are actually irqfds active.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

 include/linux/kvm_host.h |    1 
 virt/kvm/eventfd.c       |  100 ++++++++++++++++++++++++++++++++++------------
 2 files changed, 75 insertions(+), 26 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 7605bc4..0b0b6ac 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -144,6 +144,7 @@ struct kvm {
 	struct {
 		spinlock_t        lock;
 		struct list_head  items;
+		int               init:1;
 	} irqfds;
 #endif
 	struct kvm_vm_stat stat;
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 0fd200c..87f615b 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -49,7 +49,16 @@ struct _irqfd {
 	struct work_struct        shutdown;
 };
 
-static struct workqueue_struct *irqfd_cleanup_wq;
+struct _irqfd_cleanup {
+	struct mutex             lock;
+	int                      refs;
+	struct workqueue_struct *wq;
+};
+
+static struct _irqfd_cleanup irqfd_cleanup = {
+	.lock = __MUTEX_INITIALIZER(irqfd_cleanup.lock),
+	.refs = 0,
+};
 
 static void
 irqfd_inject(struct work_struct *work)
@@ -110,7 +119,7 @@ irqfd_deactivate(struct _irqfd *irqfd)
 
 	list_del_init(&irqfd->list);
 
-	queue_work(irqfd_cleanup_wq, &irqfd->shutdown);
+	queue_work(irqfd_cleanup.wq, &irqfd->shutdown);
 }
 
 /*
@@ -161,6 +170,62 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
 	add_wait_queue(wqh, &irqfd->wait);
 }
 
+/*
+ * create a host-wide workqueue for issuing deferred shutdown requests
+ * aggregated from all vm* instances. We need our own isolated single-thread
+ * queue to prevent deadlock against flushing the normal work-queue.
+ */
+static int
+irqfd_cleanup_init(struct kvm *kvm)
+{
+	int ret = 0;
+
+	mutex_lock(&irqfd_cleanup.lock);
+
+	/*
+	 * Check the current init state from within the lock so that we
+	 * sync all users to the thread creation.
+	 */
+	if (kvm->irqfds.init)
+		goto out;
+
+	if (!irqfd_cleanup.refs) {
+		struct workqueue_struct *wq;
+
+		wq = create_singlethread_workqueue("kvm-irqfd-cleanup");
+		if (!wq) {
+			ret = -ENOMEM;
+			goto out;
+		}
+
+		irqfd_cleanup.wq = wq;
+	}
+
+	irqfd_cleanup.refs++;
+	kvm->irqfds.init = true;
+
+out:
+	mutex_unlock(&irqfd_cleanup.lock);
+
+	return ret;
+}
+
+static void
+irqfd_cleanup_release(struct kvm *kvm)
+{
+	if (!kvm->irqfds.init)
+		return;
+
+	mutex_lock(&irqfd_cleanup.lock);
+
+	if (!(--irqfd_cleanup.refs))
+		destroy_workqueue(irqfd_cleanup.wq);
+
+	mutex_unlock(&irqfd_cleanup.lock);
+
+	kvm->irqfds.init = false;
+}
+
 static int
 kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
 {
@@ -170,6 +235,10 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
 	int ret;
 	unsigned int events;
 
+	ret = irqfd_cleanup_init(kvm);
+	if (ret < 0)
+		return ret;
+
 	irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL);
 	if (!irqfd)
 		return -ENOMEM;
@@ -268,7 +337,7 @@ kvm_irqfd_deassign(struct kvm *kvm, int fd, int gsi)
 	 * so that we guarantee there will not be any more interrupts on this
 	 * gsi once this deassign function returns.
 	 */
-	flush_workqueue(irqfd_cleanup_wq);
+	flush_workqueue(irqfd_cleanup.wq);
 
 	return 0;
 }
@@ -302,28 +371,7 @@ kvm_irqfd_release(struct kvm *kvm)
 	 * Block until we know all outstanding shutdown jobs have completed
 	 * since we do not take a kvm* reference.
 	 */
-	flush_workqueue(irqfd_cleanup_wq);
-
-}
-
-/*
- * create a host-wide workqueue for issuing deferred shutdown requests
- * aggregated from all vm* instances. We need our own isolated single-thread
- * queue to prevent deadlock against flushing the normal work-queue.
- */
-static int __init irqfd_module_init(void)
-{
-	irqfd_cleanup_wq = create_singlethread_workqueue("kvm-irqfd-cleanup");
-	if (!irqfd_cleanup_wq)
-		return -ENOMEM;
-
-	return 0;
-}
+	flush_workqueue(irqfd_cleanup.wq);
+	irqfd_cleanup_release(kvm);
 
-static void __exit irqfd_module_exit(void)
-{
-	destroy_workqueue(irqfd_cleanup_wq);
 }
-
-module_init(irqfd_module_init);
-module_exit(irqfd_module_exit);