linux-kernel - Re: [PATCH 0/2] Quieten softlockup detector on virtualised kernels

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Message-ID: <1420512220.2910.39.camel@cyril>
Date:	Tue, 06 Jan 2015 13:43:40 +1100
From:	Cyril Bur <cyrilbur@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, mpe@...erman.id.au,
	drjones@...hat.com, dzickus@...hat.com, mingo@...nel.org,
	uobergfe@...hat.com, chaiw.fnst@...fujitsu.com, cl@...u.com,
	fabf@...net.be, atomlin@...hat.com, benzh@...omium.org
Subject: Re: [PATCH 0/2] Quieten softlockup detector on virtualised kernels

On Mon, 2015-01-05 at 14:09 -0800, Andrew Morton wrote:
> On Mon, 22 Dec 2014 16:06:02 +1100 Cyril Bur <cyrilbur@...il.com> wrote:
> 
> > When the hypervisor pauses a virtualised kernel the kernel will observe a jump
> > in timebase, this can cause spurious messages from the softlockup detector.
> > 
> > Whilst these messages are harmless, they are accompanied with a stack trace
> > which causes undue concern and more problematically the stack trace in the
> > guest has nothing to do with the observed problem and can only be misleading.
> > 
> > Futhermore, on POWER8 this is completely avoidable with the introduction of
> > the Virtual Time Base (VTB) register.
> 
> Does this problem apply to other KVM implementations and to Xen?  If
> so, what would implementations of running_clock() for those look like? 
> If not, why not?
Yes the problem should appear on other KVM implementations, not really
sure about Xen but I don't see why the problem wouldn't crop up.

x86 do have a method for dealing with it in the softlockup detector,
they've added a check in the softlockup using a paravirtualised clock
where the guest can discover if it had been paused, Xen could be using
too.
It doesn't appear s390 do anything.

Thanks,

Cyril
> 
> 


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