Re: [KVM PATCH v9 5/5] KVM: create irqfd-cleanup-wq on demand

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 06, 2009 at 12:03:28PM -0400, Gregory Haskins wrote:
> 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 ;)

Is this an impirtant optimization? Simple locking is better..

> >> +
> >> +	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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> 
>