Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table

[Gleb Natapov]

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.

--
                        Gleb.