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

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Date:	Tue, 06 Jan 2015 10:53:35 +1100
From:	Cyril Bur <cyrilbur@...il.com>
To:	Don Zickus <dzickus@...hat.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 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.

> 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?

> 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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