Re: [LUG] Kernel drivers - printk()

[John Daragon <john@xxxxxxxxxx>]

On Wednesday 11 August 2004 14:48, Andrew Rogers wrote:
> I have a kernel module that must output a lot of info, a few megabytes.
>
> As I can't write to a file from a kernel module I write the output to
> /var/log/messages using printk(). The problem is that after I have
> called printk() a few thousand times it stops printting to
> /var/log/messages and restarts printting some time later. It is very
> likely that I am calling printk too often in a short time interval.
>
> How can I ensure that printk() has output to /var/log/messages before
> calling printk() again?

I don't think you can.

I've just had a look at printk().  You'll be disappointed at the size of the 
log buffer(s) :

#if defined(CONFIG_MULTIQUAD) || defined(CONFIG_IA64)
#define LOG_BUF_LEN     (65536)
#elif defined(CONFIG_ARCH_S390)
#define LOG_BUF_LEN     (131072)
#elif defined(CONFIG_SMP)
#define LOG_BUF_LEN     (32768)
#else
#define LOG_BUF_LEN     (16384)              /* This must be a power of two */
#endif

Told you so.

You'll be doubly disappointed that the code that actually puts bytes into the 
log buffer looks like this :

static void emit_log_char(char c)
{
       LOG_BUF(log_end) = c;
       log_end++;
       if (log_end - log_start > LOG_BUF_LEN)
               log_start = log_end - LOG_BUF_LEN;
       if (log_end - con_start > LOG_BUF_LEN)
               con_start = log_end - LOG_BUF_LEN;
       if (logged_chars < LOG_BUF_LEN)
               logged_chars++;
}

i.e it's a circular buffer, and the ends are not protected.

So - you can either :

1) write a new version of printk with sensible buffer handling :-),
2) (if this is quick-and-dirty and internal to your organisation...) redefine 
LOG_BUF_LEN to something sufficiently big to make the problem go away and 
recompile the kernel,
3) (I don't know if this is a possible solution for you, 'cause I don't know 
what you're doing ...) implement a /proc filesystem interface and write some 
user code to query it.
4) enter a world of pain. :-(

(this is usually where someone comes along and offers a brilliantly simple 
solution that I haven't thought of ...)

jd


-- 
John Daragon           argv[0] limited               john@xxxxxxxxxx
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