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

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Message-ID: <4EE8BBF4.8070108@redhat.com>
Date:	Wed, 14 Dec 2011 17:08:36 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Eric B Munson <emunson@...bm.net>
CC:	mingo@...hat.com, hpa@...or.com, arnd@...db.de,
	ryanh@...ux.vnet.ibm.com, aliguori@...ibm.com, mtosatti@...hat.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 12/08/2011 05:19 PM, Eric B Munson wrote:
> > error compiling committee.c: too many arguments to function
> > 
> My concern for preempt notifiers is masking real soft lockup warnings.  If the
> flag is set every time the vm is preempted, it becomes more likely that we will
> mask real warnings.  The ioctl was choosen because it sets the flag only when
> the guest is being paused deliberately.
>
> AFAIK, SIGSTOP is not a supported way to stop a qemu vm so a soft lockup

This is a kvm patch, not a qemu patch.  In general we try to address
general requirements, not just those that are specific to one userspace,
even if it is near to our hearts.

> warning would be working as designed there.  If that isn't the case, or if it
> ever changes, we could always install a signal handler for SIGCONT that set the
> flag before resuming the vm.
>
> Scheduling delays are also beyond the scope of this problem and I see the soft
> lockup warning as appropriate in that case.

I don't think you should see a guest softlockup if the host is
overloaded.  Nor should you see it due to a long live migration pause,
or STOP/CONT.  You should see a guest softlockup if it is spinning due
to a guest bug, and not for any other reason.

I think we need a mix of the ioctl (for STOP/CONT and voluntary pauses,
like live migration or qemu stop/cont) and preempt notifiers (together
with a check for TASK_RUNNING/TASK_UNINTERRUPTIBLE, for scheduling or
swap delays)

-- 
error compiling committee.c: too many arguments to function

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