On Wed, Jul 18, 2012 at 02:22:19PM +0300, Michael S. Tsirkin wrote: > > > > > > > So as was discussed kvm_set_irq under spinlock is bad for > > > > > > > scalability > > > > > > > with multiple VCPUs. Why do we need a spinlock simply to protect > > > > > > > level_asserted? Let's use an atomic test and set/test and clear > > > > > > > and the > > > > > > > problem goes away. > > > > > > > > > > > > > That sad reality is that for level interrupt we already scan all > > > > > > vcpus > > > > > > under spinlock. > > > > > > > > > > Where? > > > > > > > > > ioapic > > > > > > $ grep kvm_for_each_vcpu virt/kvm/ioapic.c > > > $ > > > > > > ? > > > > > > > Come on Michael. You can do better than grep and actually look at what > > code does. The code that loops over all vcpus while delivering an irq is > > in kvm_irq_delivery_to_apic(). Now grep for that. > > Hmm, I see, it's actually done for edge if injected from ioapic too, > right? > > So set_irq does a linear scan, and for each matching CPU it calls > kvm_irq_delivery_to_apic which is another scan? > So it's actually N^2 worst case for a broadcast?
No it isn't, I misread the code. Anyway, maybe not trivially but this looks fixable to me: we could drop the ioapic lock before calling kvm_irq_delivery_to_apic. > > -- > > Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
