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 10:46:11 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dave Young <hidave.darkstar@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] printk: add halt_delay parameter for printk delay
	in halt phase


* Dave Young <hidave.darkstar@...il.com> wrote:

> On Mon, Jun 8, 2009 at 4:14 PM, 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).
> 
> Can we move suspend-to-disk before halt state?

Yes, we could do that - if all system_state uses are checked for 
side-effects - in particular comparisons. We have a few places that 
do 'if (system_state > SYSTEM_RUNNING)' - to designate 'shutdown 
state'. Now, if we have any use of > SYSTEM_SHUTDOWN that might 
break from such a reordering.

I wouldnt expect such usage really, but it has to be checked. That 
would make this patch quite a bit cleaner.

Mind sending a followup delta patch with this cleanup?

	Ingo
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