linux-kernel - Re: [PATCH 2/2] allow printk delay after multi lines

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Message-Id: <20100208135654.04b79bbc.akpm@linux-foundation.org>
Date:	Mon, 8 Feb 2010 13:56:54 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Young <hidave.darkstar@...il.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/2] allow printk delay after multi lines

On Sat, 6 Feb 2010 21:40:56 +0800
Dave Young <hidave.darkstar@...il.com> wrote:

> printk delay help us to capture printk messages on some unconvenient senarios,
> but it is still not easy to read.
> 
> Add another sysctl variable printk_delay_per_lines to make it more readable.
> We can set the lines according to screen height, then take pictures by camera.
> 
> kmesg will delay printk_delay_per_lines * printk_delay_msecs milliseconds
> after every printk_delay_per_lines lines when printk_delay is enabled.
> 
> Setting the lines by proc/sysctl interface:
> /proc/sys/kernel/printk_delay_per_lines
> 
> Andrew, sorry, I have not find time to cleanup the kernel.h sysctl variables.
> If I'm free I will try to do it.
> 
> The value range from 1 - 100, default value is 1
> 
> ...
>
> --- linux-2.6.orig/include/linux/kernel.h	2010-02-02 13:38:09.537495564 +0800
> +++ linux-2.6/include/linux/kernel.h	2010-02-02 13:40:47.657480122 +0800
> @@ -246,6 +246,7 @@ extern bool printk_timed_ratelimit(unsig
>  				   unsigned int interval_msec);
>  
>  extern int printk_delay_msec;
> +extern int printk_delay_per_lines;
>  
>  /*
>   * Print a one-time message (analogous to WARN_ONCE() et al):
> --- linux-2.6.orig/kernel/printk.c	2010-02-02 13:39:19.446657319 +0800
> +++ linux-2.6/kernel/printk.c	2010-02-02 13:40:47.660813615 +0800
> @@ -656,16 +656,26 @@ static int new_text_line = 1;
>  static char printk_buf[1024];
>  
>  int printk_delay_msec __read_mostly;
> +int printk_delay_per_lines __read_mostly;
>  
>  static inline void printk_delay(void)
>  {
>  	if (unlikely(printk_delay_msec)) {
> -		int m = printk_delay_msec;
> +		static int m, l;
>  
> +		if (!l)
> +			l = printk_delay_per_lines;
> +
> +		if (--l) {
> +			m += printk_delay_msec;
> +			return;
> +		}
> +		m += printk_delay_msec;
>  		while (m--) {
>  			mdelay(1);
>  			touch_nmi_watchdog();
>  		}
> +		m = 0;
>  	}
>  }

- The default value is zero, not 1.  And zero will be treated as 4G. 
  That's a bug.

- This feature would be a lot more useful if the user could specify
  printk_delay_per_lines on the boot command line.  Ditto
  printk_delay_msec.  So you can stop the important mesages from
  scrolling off.  (I think there's already a way to do that, but I'm
  too lazy to go remember what it was).

- The permitted range of 1-100 for printk_delay_per_lines seems
  arbitrary and unneeded.  Why shouldn't I be able to set it to 10,000?
  I see no harm in permitting that.

- If the user sets printk_delay_per_lines=N, the kernel will pause
  for N*printk_delay_msec every N lines.  This is odd, and unintuitive.
  And it'll really hurt if I set printk_delay_per_lines=10000!

  I'd expect the kernel to pause for printk_delay_msec every N lines,
  and I think that would be better.


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