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

[Rik van Riel]

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?


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/