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

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Date:	Tue, 6 Jan 2015 10:01:57 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Cyril Bur <cyrilbur@...il.com>
Cc:	linux-kernel@...r.kernel.org, mpe@...erman.id.au,
	drjones@...hat.com, akpm@...ux-foundation.org, mingo@...nel.org,
	uobergfe@...hat.com, chaiw.fnst@...fujitsu.com, cl@...u.com,
	fabf@...net.be, atomlin@...hat.com, benzh@...omium.org,
	mtosatti@...hat.com
Subject: Re: [PATCH 0/2] Quieten softlockup detector on virtualised kernels

On Tue, Jan 06, 2015 at 10:53:35AM +1100, Cyril Bur wrote:
> On Mon, 2015-01-05 at 11:50 -0500, Don Zickus wrote:
> > cc'ing Marcelo
> > 
> > On Mon, Dec 22, 2014 at 04:06:02PM +1100, Cyril Bur 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.
> > 
> > Hi Cyril,
> > 
> > Your solution seems simple and doesn't disturb the softlockup code as much
> > as the x86 solution does.  The only small issue I had was the use of
> > sched_clock instead of local_clock.  I keep forgetting the difference
> > (unstable clock is the biggest reason I think).
> My apologies there it appears I stuffed up, local_clock was used
> initially in the softlockup code, I'll send a v2.

Thanks!

> 
> > Other than that, I am not the biggest fan of putting multiple virtual
> > guest solutions for the same problem into the watchdog code.  I would
> > prefer a common solution/framework to leverage.
> Agreed.
> 
> > I have the x86 folks focusing on the steal_time stuff.  It started with
> > KVM and I believe VMWare is working on utilizing it too (and maybe Xen).
> I'm not sure I've ever seen this, could you please point me towards
> something I can look at?

I am not too familar with it, but the kernel/watchdog.c code has calls to
kvm_check_and_clear_guest_paused(), which is probably a good place to
start.

Cheers,
Don

> 
> > Not sure if that is useful or could be incoporated into the power8 code.
> > Though to be honest I am curious if the steal_time code could be ported to
> > your solution as it seems the watchdog code could remove all the
> > steal_time warts.
> Happy to help sus out the situation here, again, if you could pass on
> what the x86 guys are working on, thanks.
> 
> 
> Thanks,
> 
> Cyril
> > I have cc'd Marcelo into this discussion as he was the last person I
> > remember talking with about this problem.
> > 
> > Cheers,
> > Don
> 
> 
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