linux-kernel - [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v3)

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Date:	Wed, 14 Jan 2015 15:12:51 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Cc:	Luiz Capitulino <lcapitulino@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v3)

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)

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