From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v4) Date: Wed, 21 Jan 2015 18:36:00 -0200 Message-ID: <20150121203600.745949083@redhat.com> Cc: Luiz Capitulino , Rik van Riel , Steven Rostedt , Thomas Gleixner , kvm@vger.kernel.org, Paolo Bonzini To: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org 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) v4: fix typo (Steven Rostedt)