On 14/01/2015 18:12, Marcelo Tosatti wrote:

Against v3.14-rt branch of 
git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git

The problem:

On -RT, an emulated LAPIC timer instance 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)
v3: limit (once) guest triggered printk and WARN_ON (Paolo Bonzini)

Looks good.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>