Re: [PATCH 1/2] printk delay for each line break instead of callback

Dave Young
2010-02-09T02:44:11Z

On Tue, Feb 9, 2010 at 6:06 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Sat, 6 Feb 2010 21:34:25 +0800
> Dave Young <hidave.darkstar@gmail.com> wrote:
>
>> printk delay for every callback does not make sense, change to delay every line
>>
>> Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
>> ---
>>
>>  kernel/printk.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> --- linux-2.6.orig/kernel/printk.c    2010-02-02 13:38:47.646659531 +0800
>> +++ linux-2.6/kernel/printk.c 2010-02-02 13:39:19.446657319 +0800
>> @@ -678,7 +678,6 @@ 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 */
>> @@ -746,6 +745,7 @@ asmlinkage int vprintk(const char *fmt,
>>        */
>>       for ( ; *p; p++) {
>>               if (new_text_line) {
>> +                     printk_delay();
>>                       /* Always output the token */
>>                       emit_log_char('<');
>>                       emit_log_char(current_log_level + '0');
>
> This moves the printk_delay() so that it is now inside
> spin_lock_irqsave(logbuf_lock).
>
> This fixes the race I mentioned in the previous email, but it seems a
> bad idea.  If the delay is long enough, it could even cause other CPUs
> to get hit by the NMI watchdog when trying to acquire logbuf_lock.
>
> A better approach would be to perform the calculation of "how long must
> I delay" at this site, but perform the actual delay later, after the
> raw_local_irq_restore().  This means that if the printk contains
> "a\nb\nc\n" then we won't delay until the final \n, but that seems a
> fairly small problem.
>

Ok, agree. But need another variable to track '\n', if there's no line
break then
it will not delay at the end of printk.

--
Regards
dave