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@xxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
