Gleb Natapov wrote: > On Wed, Oct 21, 2009 at 10:34:53AM -0400, Gregory Haskins wrote: >> IRQFD currently uses a deferred workqueue item to execute the injection >> operation. It was originally designed this way because kvm_set_irq() >> required the caller to hold the irq_lock mutex, and the eventfd callback >> is invoked from within a non-preemptible critical section. >> >> With the advent of lockless injection support in kvm_set_irq, the deferment >> mechanism is no longer technically needed. Since context switching to the >> workqueue is a source of interrupt latency, lets switch to a direct >> method. >> > kvm_set_irq is fully lockless only in MSI case. IOAPIC/PIC has mutexes.
Right, but irqfd by design only works with MSI (or MSI like edge triggers) anyway. Legacy-type injections follow a different path. In any case, I didn't change the locking (you did ;). You recently patched the irqfd code to remove the irq_lock, but we still had the deferment mechanism in place to avoid the mutex_lock from within the POLLIN callback. Since the mutex_lock is now no longer acquired in this path, the deferment technique is not needed either. Its only adding overhead for no purpose. So I am simply cleaning that up to improve interrupt performance. HTH, -Greg
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