linux-kernel - Re: [PATCH v3] printk: add halt_delay parameter for printk delay in halt phase

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Message-Id: <20090608193341.07e94b79.akpm@linux-foundation.org>
Date:	Mon, 8 Jun 2009 19:33:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Young <hidave.darkstar@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v3] printk: add halt_delay parameter for printk delay in
  halt phase

On Tue, 9 Jun 2009 09:01:19 +0800 Dave Young <hidave.darkstar@...il.com> wrote:

> On Tue, Jun 9, 2009 at 5:39 AM, Andrew Morton<akpm@...ux-foundation.org> wrote:
> > On Mon, 8 Jun 2009 19:15:01 +0200
> > Ingo Molnar <mingo@...e.hu> wrote:
> >
> >> > questions: is it possible for interrupts to be disabled at this
> >> > time? If so, can we get an NMI watchdog hit?
> >>
> >> no, we generally turn off the nmi watchdog during shutdown, disable
> >> the lapic and io-apic, etc.
> >
> > Is x86 the only architecture which implements an NMI watchdog?
> >
> >> > Is the softlockup detector still running and if so, can it
> >> > trigger?
> >>
> >> in (non-emergency) reboot, last i checked, we stopped all other CPUs
> >> first, and then killed the current one. There's no chance for the
> >> watchdog thread to run.
> >
> > OK, but... __See below.
> >
> >> Anyway ... you seem to be uncomfortable about this patch - should i
> >> delay it for now to let it all play out? We are close to the merge
> >> window.
> >
> > I'm OK - I'm just bouncing ideas and questions off you guys, to make sure
> > that we've thought this through all the way.
> >
> > Here's another: why is it a boot option rather than a runtime-tunable?
> > A /proc tweakable is generally preferable because it avoids the
> > oh-crap-i-forgot-to-edit-grub.conf thing. __And we could perhaps then
> > remove all those system_state tests: userspace sets printk_delay
> > immediately prior to running halt/reboot/etc?
> 
> Andrew, thanks your comments.
> I original intention is to use not boot options but sysfs interface.
> Do you perfer proc?

sysfs is OK, if there's a logical place for it.  /sys/kernel/, I suppose.

> without system_state testing we will have to consider the NMI watchdog
> and softlockup issue.

Yup.


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