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

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Date:	Mon, 5 Jan 2015 11:50:57 -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

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).

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.

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).

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.

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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