From: Marcelo Tosatti <mtosatti@redhat.com>
To: linux-kernel@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
Subject: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v2)
Date: Tue, 25 Nov 2014 15:21:08 -0200 [thread overview]
Message-ID: <20141125172108.992070524@redhat.com> (raw)
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)
next reply other threads:[~2014-11-25 17:24 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-25 17:21 Marcelo Tosatti [this message]
2014-11-25 17:21 ` [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq Marcelo Tosatti
2014-11-25 18:57 ` Rik van Riel
2014-11-25 19:08 ` Rik van Riel
2014-11-25 19:30 ` Marcelo Tosatti
2014-11-25 20:23 ` Thomas Gleixner
2014-11-25 17:21 ` [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe Marcelo Tosatti
2014-11-25 17:38 ` Paolo Bonzini
2014-11-25 19:01 ` Jan Kiszka
2014-12-04 13:43 ` Marcelo Tosatti
2014-11-25 19:11 ` Rik van Riel
2014-11-25 20:24 ` Thomas Gleixner
2014-12-04 13:53 ` Paolo Bonzini
2014-12-05 16:17 ` Marcelo Tosatti
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