linux-kernel - Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by host

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Message-ID: <20111219175047.GA17347@amt.cnet>
Date:	Mon, 19 Dec 2011 15:50:47 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Eric B Munson <emunson@...bm.net>, mingo@...hat.com, hpa@...or.com,
	arnd@...db.de, ryanh@...ux.vnet.ibm.com, aliguori@...ibm.com,
	jeremy.fitzhardinge@...rix.com, levinsasha928@...il.com,
	Jan Kiszka <jan.kiszka@...mens.com>, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5 V5] Avoid soft lockup message when KVM is stopped by
 host

On Mon, Dec 19, 2011 at 02:11:08PM -0200, Marcelo Tosatti wrote:
> On Mon, Dec 19, 2011 at 02:59:10PM +0200, Avi Kivity wrote:
> > On 12/16/2011 11:31 AM, Marcelo Tosatti wrote:
> > > > >
> > > > > Slow enough that progress of the watchdog thread is unable to keep up
> > > > > with timer interrupt processing. This is considered a hang and
> > > > > should be reported.
> > > > 
> > > > It's not a guest hang though!
> > >
> > > No, but your host system is in such a load state that for the sake of
> > > system usability you better print out a warning message.
> > 
> > What's the point in printing it in the guest?  The guest can't observe
> > host conditions.
> >
> > > I don't see the advantage of preempt notifiers over the simple, paravirt
> > > solution proposed? Note kvmclock is already paravirt.
> > 
> > Right.
> > 
> > > What do you want to be done in preempt notifiers? Measure what to
> > > consider setting this flag?
> > 
> > Preemption while TASK_RUNNING or TASK_UNINTERRUPTIBLE.
> 
> Maybe it is good (not sure), need to look into schedstats and think of
> cases that would break legitimate guest hangs. And it probably also
> affects the position of clearing the flag on the guest side as its 
> currently done in Eric's patchset.

Is the task going to be TASK_UNINTERRUPTIBLE with SIGSTOP? Don't think
so. 

Note "Preemption while TASK_RUNNING or TASK_UNINTERRUPTIBLE" is not
functional for guest-paused-via-QEMU-monitor case.

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