linux-kernel - Re: [PATCH v2 0/2] Quieten softlockup detector on virtualised kernels

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Date:	Thu, 5 Feb 2015 15:48:49 -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, fabf@...net.be,
	atomlin@...hat.com, benzh@...omium.org, schwidefsky@...ibm.com
Subject: Re: [PATCH v2 0/2] Quieten softlockup detector on virtualised kernels

On Fri, Jan 09, 2015 at 02:34:36PM +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.

Originally I was suggesting to talk with the x86/kvm folks about coming up
with a common solution.  But I didn't hear a response from those cc'd at
the time.  This solution looks a lot cleaner than what x86 is doing.

I am willing to see how this works out on ppc64's kvm and see if they still
see any issues after awhile.  If not, maybe I can poke the x86 folks to
migrate to something similar.

Acked-by: Don Zickus <dzickus@...hat.com>

> 
> Futhermore, on POWER8 this is completely avoidable with the introduction of
> the Virtual Time Base (VTB) register.
> 
> V2:
> 	Remove the export of running_clock
> 	Added #ifdef CONFIG_PPC_PSERIES and optimised the non lpar + vtb cases.
> 	Replaced the use of sched_clock_with local_clock it was used originally in
> the softlockup detector.
> 
> Cyril Bur (2):
>   Add another clock for use with the soft lockup watchdog.
>   powerpc: add running_clock for powerpc to prevent spurious softlockup
>     warnings
> 
>  arch/powerpc/kernel/time.c | 32 ++++++++++++++++++++++++++++++++
>  include/linux/sched.h      |  1 +
>  kernel/sched/clock.c       | 13 +++++++++++++
>  kernel/watchdog.c          |  2 +-
>  4 files changed, 47 insertions(+), 1 deletion(-)
> 
> -- 
> 1.9.1
> 
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