From: Marcelo Tosatti <mtosatti@redhat.com>
To: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Cc: Luiz Capitulino <lcapitulino@redhat.com>,
Rik van Riel <riel@redhat.com>,
Steven Rostedt <srostedt@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>
Subject: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v4)
Date: Wed, 21 Jan 2015 18:36:00 -0200 [thread overview]
Message-ID: <20150121203600.745949083@redhat.com> (raw)
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)
next reply other threads:[~2015-01-21 20:38 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-21 20:36 Marcelo Tosatti [this message]
2015-01-21 20:36 ` [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq Marcelo Tosatti
2015-01-21 20:36 ` [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe Marcelo Tosatti
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