From: Marcelo Tosatti <mtosatti@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Rik van Riel <riel@redhat.com>,
Luiz Capitulino <lcapitulino@redhat.com>,
linux-rt-users@vger.kernel.org, kvm@vger.kernel.org
Subject: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v5)
Date: Wed, 08 Apr 2015 20:33:23 -0300 [thread overview]
Message-ID: <20150408233323.842124241@redhat.com> (raw)
Sebastian,
rebased against v3.18.7-rt2 as requested.
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)
v5: rebase against v3.18.7-rt2.
next reply other threads:[~2015-04-08 23:35 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-08 23:33 Marcelo Tosatti [this message]
2015-04-08 23:33 ` [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq Marcelo Tosatti
2015-04-08 23:33 ` [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe Marcelo Tosatti
2015-04-09 13:18 ` [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v5) Sebastian Andrzej Siewior
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