Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

Rik van Riel
2014-11-25T19:00:47Z

On 11/25/2014 12:21 PM, Marcelo Tosatti wrote:
> The problem:
>
> On -RT, an emulated LAPIC timer instances has the following path:
>
> 1) hard interrupt
> 2) ksoftirqd is scheduled
> 3) ksoftirqd wakes up vcpu thread
> 4) vcpu thread is scheduled
>
> This extra context switch introduces unnecessary latency in the
> LAPIC path for a KVM guest.
>
> The solution:
>
> Allow waking up vcpu thread from hardirq context,
> thus avoiding the need for ksoftirqd to be scheduled.
>
> Normal waitqueues make use of spinlocks, which on -RT
> are sleepable locks. Therefore, waking up a waitqueue
> waiter involves locking a sleeping lock, which
> is not allowed from hard interrupt context.
>

What are the consequences for the realtime kernel of
making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so
long we need to care about it being non-preemptible?