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

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Date:	Tue, 9 Jun 2009 09:35:42 +0800
From:	Dave Young <hidave.darkstar@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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, Jun 9, 2009 at 9:01 AM, 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?
> without system_state testing we will have to consider the NMI watchdog
> and softlockup issue.
>
>>
>> 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.
>
> Seems make sense

And, if making it a general feature, I think maybe delay per screen
(ie. 25 lines) is a good way.

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



-- 
Regards
dave
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