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

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Message-ID: <a8e1da1002081844j7f91ee3fgf69d71bf235a09ed@mail.gmail.com>
Date:	Tue, 9 Feb 2010 10:44:07 +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 1/2] printk delay for each line break instead of callback

On Tue, Feb 9, 2010 at 6:06 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Sat, 6 Feb 2010 21:34:25 +0800
> Dave Young <hidave.darkstar@...il.com> wrote:
>
>> printk delay for every callback does not make sense, change to delay every line
>>
>> Signed-off-by: Dave Young <hidave.darkstar@...il.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
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