From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=38061 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1PKo2s-0002k2-NK
	for qemu-devel@nongnu.org; Tue, 23 Nov 2010 03:16:12 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <dlaor@redhat.com>) id 1PKo2r-0004zx-KU
	for qemu-devel@nongnu.org; Tue, 23 Nov 2010 03:16:10 -0500
Received: from mx1.redhat.com ([209.132.183.28]:51750)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <dlaor@redhat.com>) id 1PKo2r-0004zP-Dk
	for qemu-devel@nongnu.org; Tue, 23 Nov 2010 03:16:09 -0500
Message-ID: <4CEB7845.4050104@redhat.com>
Date: Tue, 23 Nov 2010 10:16:05 +0200
From: Dor Laor <dlaor@redhat.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [PATCH] qemu-kvm: introduce
	cpu_start/cpu_stop	commands
References: <1290466818-5230-1-git-send-email-aliguori@us.ibm.com>
	<4CEB6222.5050203@redhat.com>
In-Reply-To: <4CEB6222.5050203@redhat.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Reply-To: dlaor@redhat.com
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Avi Kivity <avi@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, kvm@vger.kernel.org

On 11/23/2010 08:41 AM, Avi Kivity wrote:
> On 11/23/2010 01:00 AM, Anthony Liguori wrote:
>> qemu-kvm vcpu threads don't response to SIGSTOP/SIGCONT. Instead of
>> teaching
>> them to respond to these signals, introduce monitor commands that stop
>> and start
>> individual vcpus.
>>
>> The purpose of these commands are to implement CPU hard limits using
>> an external
>> tool that watches the CPU consumption and stops the CPU as appropriate.

Why not use cgroup for that?

>>
>> The monitor commands provide a more elegant solution that signals
>> because it
>> ensures that a stopped vcpu isn't holding the qemu_mutex.
>>
>
>  From signal(7):
>
> The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.
>
> Perhaps this is a bug in kvm?
>
> If we could catch SIGSTOP, then it would be easy to unblock it only
> while running in guest context. It would then stop on exit to userspace.
>
> Using monitor commands is fairly heavyweight for something as high
> frequency as this. What control period do you see people using? Maybe we
> should define USR1 for vcpu start/stop.
>
> What happens if one vcpu is stopped while another is running? Spin
> loops, synchronous IPIs will take forever. Maybe we need to stop the
> entire process.
>