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

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Message-ID: <a8e1da1002081831j6fdea8a7v41cc07b635ab7dda@mail.gmail.com>
Date:	Tue, 9 Feb 2010 10:31:49 +0800
From:	Dave Young <hidave.darkstar@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/2] allow printk delay after multi lines

On Tue, Feb 9, 2010 at 5:56 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> 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.

Will fix.

>
> - 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).

I think it's the similar "boot_delay". It's a little different as
calibration is not done on the early booting phase, so busy loop
is used instead to delay. It's hard to make boot delay accurate.

Maybe delay_per_lines can be done while booting as well,
but it should be another patch,  isn't it?

>
> - 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.

Fine to me as well. Thanks for your comments.

-- 
Regards
dave
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