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

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Date:	Mon, 8 Jun 2009 09:26:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Dave Young <hidave.darkstar@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] printk: add halt_delay parameter for printk delay in
 halt phase

On Mon, 8 Jun 2009 10:48:07 +0200 Ingo Molnar <mingo@...e.hu> wrote:

> 
> * Dave Young <hidave.darkstar@...il.com> wrote:
> 
> > On Mon, Jun 8, 2009 at 4:28 PM, Andrew Morton<akpm@...ux-foundation.org> wrote:
> > > On Mon, 8 Jun 2009 10:14:39 +0200 Ingo Molnar <mingo@...e.hu> wrote:
> > >
> > >>
> > >> * Dave Young <hidave.darkstar@...il.com> wrote:
> > >>
> > >> > Add a halt_delay module parameter in printk.c used to read the
> > >> > printk messages in halt/poweroff/restart phase, delay each printk
> > >> > messages by halt_delay milliseconds. It is useful for debugging if
> > >> > there's no other way to dump kernel messages that time.
> > >> >
> > >> > The halt_delay max value is 65535, default value is 0, change it
> > >> > by:
> > >> >
> > >> > echo xxx > /sys/module/printk/parameters/halt_delay
> > >> >
> > >> > Signed-off-by: Dave Young <hidave.darkstar@...il.com>
> > >> > ---
> > >> > Documentation/kernel-parameters.txt | __ __5 +++++
> > >> > kernel/printk.c __ __ __ __ __ __ __ __ __ __ | __ 17 +++++++++++++++++
> > >> > 2 files changed, 22 insertions(+)
> > >> >
> > >> > --- linux-2.6.orig/kernel/printk.c __2009-06-08 13:55:35.000000000 +0800
> > >> > +++ linux-2.6/kernel/printk.c __ __ __ 2009-06-08 13:56:23.000000000 +0800
> > >> > @@ -250,6 +250,22 @@ static inline void boot_delay_msec(void)
> > >> > __}
> > >> > __#endif
> > >> >
> > >> > +/* msecs delay after each halt/poweroff/restart phase printk,
> > >> > + unsigned short is enough for delay in milliseconds */
> > >> > +static unsigned short halt_delay;
> > >> > +
> > >> > +static inline void halt_delay_msec(void)
> > >> > +{
> > >> > + __ if (unlikely(halt_delay == 0 || !(system_state == SYSTEM_HALT
> > >> > + __ __ __ __ __ __ __ __ __ __ __ __ __ || system_state == SYSTEM_POWER_OFF
> > >> > + __ __ __ __ __ __ __ __ __ __ __ __ __ || system_state == SYSTEM_RESTART)))
> > >> > + __ __ __ __ __ return;
> > >>
> > >> This is a tiny bit ugly (and goes into the vprintf path) but i can
> > >> see no other way either - a system_state > SYSTEM_RUNNING check
> > >> would needlessly include the suspend-to-disk state (which we dont
> > >> want to include here).
> > >>
> > >> In theory we could turn system_state into a bitmask and have a
> > >> print_delay_mask check instead of these flags ... but that is a far
> > >> wider change and i'm not sure it's a net step forwards.
> > >>
> > >> I've applied your patch to tip:core/printk with small edits to the
> > >> changelog - Linus & Andrew is Cc:ed, should they have any
> > >> objections.
> > >
> > > Could we not put just a single delay in there, immediately prior to halting,
> > > restarting or poweroffing?
> > 
> > But, then prink messages will still flush too fast for us to see 
> > the detail.

Only if there are so many unlogged messages that they scroll of the
screen.  Is that the case?

> Plus often there's a loop in architecture code that tries various 
> methods of reboot. We dont know which one works - and any of them 
> could produce warning messages. (and this happened a number of times 
> in the past)
> 
> So this would mean having to find up to a hundred of 'reboot now' 
> places in a two dozen architectures (and keeping them all maintained 
> ongoing as well). Does not seem like a good choice to me.
> 

hm.  If we need to actually capture all of those.

questions: is it possible for interrupts to be disabled at this time? 
If so, can we get an NMI watchdog hit?

Is the softlockup detector still running and if so, can it trigger?

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