>On Tue, Nov 26, 2013 at 06:14:27PM +0200, Gleb Natapov wrote: >> On Tue, Nov 26, 2013 at 06:05:37PM +0200, Michael S. Tsirkin wrote: >> > On Tue, Nov 26, 2013 at 02:56:10PM +0200, Gleb Natapov wrote: >> > > On Tue, Nov 26, 2013 at 01:47:03PM +0100, Paolo Bonzini wrote: >> > > > Il 26/11/2013 13:40, Zhanghaoyu (A) ha scritto: >> > > > > When guest set irq smp_affinity, VMEXIT occurs, then the vcpu >> > > > > thread will IOCTL return to QEMU from hypervisor, then vcpu >> > > > > thread ask the hypervisor to update the irq routing table, in >> > > > > kvm_set_irq_routing, synchronize_rcu is called, current vcpu thread >> > > > > is blocked for so much time to wait RCU grace period, and during >> > > > > this period, this vcpu cannot provide service to VM, so those >> > > > > interrupts delivered to this vcpu cannot be handled in time, and the >> > > > > apps running on this vcpu cannot be serviced too. >> > > > > It's unacceptable in some real-time scenario, e.g. telecom. >> > > > > >> > > > > So, I want to create a single workqueue for each VM, to >> > > > > asynchronously performing the RCU synchronization for irq routing >> > > > > table, and let the vcpu thread return and VMENTRY to service VM >> > > > > immediately, no more need to blocked to wait RCU grace period. >> > > > > And, I have implemented a raw patch, took a test in our telecom >> > > > > environment, above problem disappeared. >> > > > >> > > > I don't think a workqueue is even needed. You just need to use >> > > > call_rcu to free "old" after releasing kvm->irq_lock. >> > > > >> > > > What do you think? >> > > > >> > > It should be rate limited somehow. Since it guest triggarable >> > > guest may cause host to allocate a lot of memory this way. >> > >> > The checks in __call_rcu(), should handle this I think. These keep >> > a per-CPU counter, which can be adjusted via rcutree.blimit, which >> > defaults to taking evasive action if more than 10K callbacks are >> > waiting on a given CPU. >> > >> > >> Documentation/RCU/checklist.txt has: >> >> An especially important property of the synchronize_rcu() >> primitive is that it automatically self-limits: if grace periods >> are delayed for whatever reason, then the synchronize_rcu() >> primitive will correspondingly delay updates. In contrast, >> code using call_rcu() should explicitly limit update rate in >> cases where grace periods are delayed, as failing to do so can >> result in excessive realtime latencies or even OOM conditions. > >I just asked Paul what this means.
My understanding shown as blow, The synchronous grace period API synchronize_rcu() can prevent current thread from generating a large number of rcu-update subsequently, just as the "self-limits" described above in Documentation/RCU/checklist.txt, can avoid memory exhaustion, but the asynchronous API call_rcu() cannot limit the update rate, need explicitly rate limit. Thanks, Zhang Haoyu > >> -- >> Gleb.