From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rik van Riel <riel@redhat.com>
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq
Date: Tue, 25 Nov 2014 13:57:37 -0500
Message-ID: <5474D121.4070905@redhat.com>
References: <20141125172108.992070524@redhat.com> <20141125172329.435080075@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Cc: Luiz Capitulino <lcapitulino@redhat.com>,
	Steven Rostedt <srostedt@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>, kvm@vger.kernel.org
To: Marcelo Tosatti <mtosatti@redhat.com>, linux-kernel@vger.kernel.org
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx1.redhat.com ([209.132.183.28]:54623 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751152AbaKYTAr (ORCPT <rfc822;kvm@vger.kernel.org>);
	Tue, 25 Nov 2014 14:00:47 -0500
In-Reply-To: <20141125172329.435080075@redhat.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On 11/25/2014 12:21 PM, Marcelo Tosatti wrote:
> 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.
> 

What are the consequences for the realtime kernel of
making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so
long we need to care about it being non-preemptible?