[patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v2)

[Marcelo Tosatti]

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.

cyclictest command line:
# cyclictest -m -n -q -p99 -l 1000000 -h60  -D 1m

This patch reduces the average latency in my tests from 14us to 11us.

v2: improve changelog (Rik van Riel)




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