Re: [PATCH] printk: add printk_delay to make messages readable for some scenarios

[Dave Young <hidave.darkstar@gmail.com>]

On Mon, Jun 22, 2009 at 10:51:54PM -0700, Andrew Morton wrote:
> On Tue, 16 Jun 2009 16:54:12 +0800 Dave Young <hidave.darkstar@gmail.com> wrote:
> 
> > When syslog is not possible, at the same time there's no serial/net console
> > available, it will be hard to read the printk messages.
> > For example oops/panic/warning messages in shutdown phase.
> > 
> > Here add a printk delay feature, we can make each printk message
> > delay some milliseconds.
> > 
> > Setting the delay by proc/sysctl interface: /proc/sys/kernel/printk_delay
> > 
> > The value range from 0 - 10000, default value is 0
> > 
> 
> Seems OK(ish).
> 
> > ---
> > Documentation/sysctl/kernel.txt |    8 ++++++++
> > include/linux/kernel.h          |    2 ++
> > include/linux/sysctl.h          |    1 +
> > kernel/printk.c                 |   14 ++++++++++++++
> > kernel/sysctl.c                 |   12 ++++++++++++
> > 5 files changed, 37 insertions(+)
> > 
> > --- linux-2.6.orig/kernel/printk.c	2009-06-16 11:07:15.000000000 +0800
> > +++ linux-2.6/kernel/printk.c	2009-06-16 11:31:01.000000000 +0800
> > @@ -638,6 +638,19 @@ static int recursion_bug;
> >  static int new_text_line = 1;
> >  static char printk_buf[1024];
> > 
> > +int printk_delay_msec;
> > +
> > +static inline void printk_delay(void)
> > +{
> > +	if (unlikely(printk_delay_msec)) {
> > +		int m = printk_delay_msec;
> > +		while (m--) {
> > +			mdelay(1);
> > +			touch_nmi_watchdog();
> > +		}
> > +	}
> > +}
> > +
> >  asmlinkage int vprintk(const char *fmt, va_list args)
> >  {
> >  	int printed_len = 0;
> > @@ -647,6 +660,7 @@ asmlinkage int vprintk(const char *fmt,
> >  	char *p;
> > 
> >  	boot_delay_msec();
> > +	printk_delay();
> > 
> >  	preempt_disable();
> >  	/* This stops the holder of console_sem just where we want him */
> > --- linux-2.6.orig/include/linux/kernel.h	2009-04-18 19:48:06.000000000 +0800
> > +++ linux-2.6/include/linux/kernel.h	2009-06-16 11:24:43.000000000 +0800
> > @@ -243,6 +243,8 @@ extern int printk_ratelimit(void);
> >  extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
> >  				   unsigned int interval_msec);
> > 
> > +extern int printk_delay_msec;
> > +
> 
> Sigh.
> 
> It's pretty dumb to clutter up kernel.h just because we want to share a
> declaration with sysctl.c.  And it was pretty dumb to fill sysctl.c with
> extern declarations.
> 
> What we should do is to create a new header file which contains all the
> declarations which sysctl.c needs.  Include that from within sysctl.c
> and all .c files which which to share declarations with sysctl.c.
> 
> But that's all a matter for another patchset.
> 

Should do, thanks for pointing out.

> >  /*
> >   * Print a one-time message (analogous to WARN_ONCE() et al):
> >   */
> > --- linux-2.6.orig/kernel/sysctl.c	2009-06-15 10:33:11.000000000 +0800
> > +++ linux-2.6/kernel/sysctl.c	2009-06-16 11:29:52.000000000 +0800
> > @@ -102,6 +102,7 @@ static int __maybe_unused one = 1;
> >  static int __maybe_unused two = 2;
> >  static unsigned long one_ul = 1;
> >  static int one_hundred = 100;
> > +static int ten_thousand = 10000;
> > 
> >  /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
> >  static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
> > @@ -699,6 +700,17 @@ static struct ctl_table kern_table[] = {
> >  		.mode		= 0644,
> >  		.proc_handler	= &proc_dointvec,
> >  	},
> > +	{
> > +		.ctl_name	= KERN_PRINTK_DELAY,
> > +		.procname	= "printk_delay",
> > +		.data		= &printk_delay_msec,
> > +		.maxlen		= sizeof(int),
> > +		.mode		= 0644,
> > +		.proc_handler	= &proc_dointvec_minmax,
> > +		.strategy	= &sysctl_intvec,
> > +		.extra1		= &zero,
> > +		.extra2		= &ten_thousand,
> > +	},
> >  #endif
> 
> See that endif there?  If CONFIG_PRINTK=n, we'll get an unused-var
> warning for `ten_thousand'.  Another sysctl.c wart.  I'll fix it.

Yes, thanks very much.

> 
> >  	{
> >  		.ctl_name	= KERN_NGROUPS_MAX,
> > --- linux-2.6.orig/include/linux/sysctl.h	2009-04-09 16:23:01.000000000 +0800
> > +++ linux-2.6/include/linux/sysctl.h	2009-06-16 12:30:14.000000000 +0800
> > @@ -163,6 +163,7 @@ enum
> >  	KERN_MAX_LOCK_DEPTH=74,
> >  	KERN_NMI_WATCHDOG=75, /* int: enable/disable nmi watchdog */
> >  	KERN_PANIC_ON_NMI=76, /* int: whether we will panic on an unrecovered */
> > +	KERN_PRINTK_DELAY = 77,	/* int: tune printk delay*/
> 
> No, we don't do this any more.  I'll fix that too.
> 
> Please retest the result.

Tested, thanks.

> 
> 
> diff -puN include/linux/sysctl.h~printk-add-printk_delay-to-make-messages-readable-for-some-scenarios-fix include/linux/sysctl.h
> --- a/include/linux/sysctl.h~printk-add-printk_delay-to-make-messages-readable-for-some-scenarios-fix
> +++ a/include/linux/sysctl.h
> @@ -163,7 +163,6 @@ enum
>  	KERN_MAX_LOCK_DEPTH=74,
>  	KERN_NMI_WATCHDOG=75, /* int: enable/disable nmi watchdog */
>  	KERN_PANIC_ON_NMI=76, /* int: whether we will panic on an unrecovered */
> -	KERN_PRINTK_DELAY = 77,	/* int: tune printk delay*/
>  };
>  
>  
> --- a/kernel/sysctl.c~printk-add-printk_delay-to-make-messages-readable-for-some-scenarios-fix
> +++ a/kernel/sysctl.c
> @@ -103,7 +103,9 @@ static int __maybe_unused one = 1;
>  static int __maybe_unused two = 2;
>  static unsigned long one_ul = 1;
>  static int one_hundred = 100;
> +#ifdef CONFIG_PRINTK
>  static int ten_thousand = 10000;
> +#endif
>  
>  /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
>  static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
> @@ -710,7 +712,7 @@ static struct ctl_table kern_table[] = {
>  		.proc_handler	= &proc_dointvec,
>  	},
>  	{
> -		.ctl_name	= KERN_PRINTK_DELAY,
> +		.ctl_name	= CTL_UNNUMBERED,
>  		.procname	= "printk_delay",
>  		.data		= &printk_delay_msec,
>  		.maxlen		= sizeof(int),
> _
>