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

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Message-Id: <20090608143913.749e19c5.akpm@linux-foundation.org>
Date:	Mon, 8 Jun 2009 14:39:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	hidave.darkstar@...il.com, 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 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?

Plus the feature becomes more general - perhaps there are use cases
where people want to slow down printks, such as: kernel goes oops, data
scrolls off, serial console/netconsole unavailable.  pause_on_oops is
supposed to help here but last time I tried it, it kinda didn't work,
plus pause_on_oops doesn't solve the data-scrolled-off problem.

Thirdly, if we do this as a general /proc/printk_delay thing, perhaps
it can be consolidated with the existing boot_delay= implementation.
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