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    SubjectRe: [PATCH 2/2] allow printk delay after multi lines
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    On Tue, Feb 9, 2010 at 6:01 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
    > On Mon, 8 Feb 2010 13:56:54 -0800
    > Andrew Morton <akpm@linux-foundation.org> wrote:
    >
    >> On Sat, 6 Feb 2010 21:40:56 +0800
    >> Dave Young <hidave.darkstar@gmail.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;
    >> >     }
    >> >  }
    >>
    >
    > Also...
    >
    > - The above code is racy: if multiple CPUs run printk_delay()
    >  concurrently, `m' and `l' will get mucked up.
    >
    >  We should verify that the effects of this race are benign.  This
    >  will be hard.
    >
    >  Or we fix the race.  One could add locking (irq-safe locking), but
    >  I suspect a better approach would be to use atomic ops.
    >

    will change to use atomic ops

    --
    Regards
    dave
    --
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