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SubjectRe: [KVM PATCH v9 5/5] KVM: create irqfd-cleanup-wq on demand
On Thu, Jul 02, 2009 at 11:38:22AM -0400, Gregory Haskins wrote:
> 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 8e04a34..cd1a0f3 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -149,6 +149,7 @@ struct kvm {
> struct {
> spinlock_t lock;
> struct list_head items;
> + bool init;
> } irqfds;
> #endif
> struct kvm_vm_stat stat;
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 4092b8d..fcc3469 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;

init is checked outside the lock here.
Why?

> +
> + mutex_lock(&irqfd_cleanup.lock);
> +
> + if (!(--irqfd_cleanup.refs))
> + destroy_workqueue(irqfd_cleanup.wq);
> +
> + mutex_unlock(&irqfd_cleanup.lock);
> +
> + kvm->irqfds.init = false;

... and cleaned outside the lock as well.

> +}
> +
> 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);


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