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 16:42:29 +0800
From:	Dave Young <hidave.darkstar@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	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, 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.

-- 
Regards
dave
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