linux-kernel - Re: [PATCH 4/5] watchdog: control hard lockup detection default

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Date:	Mon, 18 Aug 2014 14:53:32 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	akpm@...ux-foundation.org, kvm@...r.kernel.org,
	pbonzini@...hat.com, mingo@...hat.com,
	LKML <linux-kernel@...r.kernel.org>,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH 4/5] watchdog: control hard lockup detection default

On Mon, Aug 18, 2014 at 08:07:35PM +0200, Ingo Molnar wrote:
> 
> * Don Zickus <dzickus@...hat.com> wrote:
> 
> > On Mon, Aug 18, 2014 at 11:16:44AM +0200, Ingo Molnar wrote:
> > > 
> > > * Don Zickus <dzickus@...hat.com> wrote:
> > > 
> > > > The running kernel still has the ability to enable/disable at any
> > > > time with /proc/sys/kernel/nmi_watchdog us usual. However even
> > > > when the default has been overridden /proc/sys/kernel/nmi_watchdog
> > > > will initially show '1'. To truly turn it on one must disable/enable
> > > > it, i.e.
> > > >   echo 0 > /proc/sys/kernel/nmi_watchdog
> > > >   echo 1 > /proc/sys/kernel/nmi_watchdog
> > > 
> > > This looks like a bug, why is this so?
> > 
> > It is, but it always has been there in the case of the PMU 
> > not being able to provide a resource for the hardlockup.  
> > This change just exposes it more.
> 
> There seems to be two issues:
> 
> 1)
> 
> When it's impossible to enable the hardlockup detector, it 
> should default to -1 or so, and attempts to set it should 
> return a -EINVAL or so.

Ok, it didn't because I set the knob to mean both hard and soft lockup.
But the code knows the failures and can set to -1 if it had to.

> 
> Bootup messages should also indicate when it's not possible to 
> enable it but a user requests it.

It does today.

> 
> 2)
> 
> The softlockup and hardlockup detection control variables 
> should be in separate flags, inside and outside the kernel - 
> they (should) not relate to each other.

They did because years ago I thought we wanted to keep them as one entity
instead of two.  I would have to re-work the code to do this (and add more
knobs).

I presume you would want those changes done before taking this patchset?

Cheers,
Don
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