linux-kernel - Re: [RFC PATCH 0/3] Avoid soft lockup message when KVM is stopped by host

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Message-ID: <4E3AE912.7050402@us.ibm.com>
Date:	Thu, 04 Aug 2011 13:46:42 -0500
From:	Anthony Liguori <aliguori@...ibm.com>
To:	dlaor@...hat.com
CC:	Eric B Munson <emunson@...bm.net>, kvm@...r.kernel.org,
	kvm-ia64@...r.kernel.org, kvm-ppc@...r.kernel.org, avi@...hat.com,
	riel@...hat.com, glommer@...hat.com, linux-kernel@...r.kernel.org,
	Ryan A Harper <raharper@...ibm.com>,
	Glauber Costa <glommer@...il.com>
Subject: Re: [RFC PATCH 0/3] Avoid soft lockup message when KVM is stopped
 by host

On 08/04/2011 03:37 AM, Dor Laor wrote:
> On 08/03/2011 05:24 PM, Eric B Munson wrote:
>> This set is just a rough first pass at avoiding soft lockup warnings
>> when a host
>> pauses the execution of a guest. A flag is set by the host in the
>> shared page
>> used for the pvclock when the host goes to stop the guest. When the guest
>> resumes and detects a soft lockup, this flag is checked and cleared
>> and the soft
>> lockup message is skipped.
>
> While this will cover the case were the host stops a guest, there will
> be other plain cases where the host is just over committed and will
> cause a softlockup false positive on the guest.
>
> Softlockup should use stolen time that makes use of the guest running
> info would cover both cases

At least in the current steal time implementation, there are numerous 
cases where steal time is not accounted but you'd hit a soft lockup.

Pausing an idle guest via (qemu) stop is an example.  Likewise, a guest 
that is descheduled while idle but then not scheduled for prolonged 
periods of time would also not be accounted as steal time.

Regards,

Anthony Liguori
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