Date:	Fri, 09 Jan 2015 14:15:00 +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 Tue, 2015-01-06 at 10:01 -0500, Don Zickus wrote:
> 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.
> 
Ah yes that, I did initially have a look at what it does when I
undertook to solve the problem on power and I suppose the two solutions
are similar in that they both just use a virtualised time source. The
similarities stop there though, the paravirtualised clock that x86 uses
provides (as the name of the function implies) a 'was paused' flag.
Obviously the flag isn't something the vtb register on power8 can
provide and since we have a vtb, its preferable to use that.
Perhaps x86 can do something with running_clock?

Regards,

Cyril

> 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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