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

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Message-ID: <20090608085819.GH6372@elte.hu>
Date:	Mon, 8 Jun 2009 10:58:19 +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 4:46 PM, Ingo Molnar<mingo@...e.hu> wrote:
> >
> > * 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?
> 
> I'm glad to check the system_state usage, then write a followup patch.

I'd also suggest to add a comment to the constants, to make sure 
that when new system states are added, the ordering assumptions are 
not broken.

	Ingo
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