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SubjectRe: [KVM PATCH v9 5/5] KVM: create irqfd-cleanup-wq on demand
Michael S. Tsirkin wrote:
> 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?
>

Guest is shutting down via vmfd->f_ops->release() and is already
guaranteed to be single-threaded, so proper locking is not really
important. Probably should document that, though ;)
>
>> +
>> + 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.
>
>
Ditto

>> +}
>> +
>> 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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