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

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Message-ID: <54B6A8DF.3070806@redhat.com>
Date:	Wed, 14 Jan 2015 18:35:27 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>,
	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
Subject: Re: [patch -rt 0/2] use simple waitqueue for kvm vcpu waitqueue (v3)



On 14/01/2015 18:12, Marcelo Tosatti wrote:
> 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)
> 
> 

Looks good.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
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