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Commit bfe8df3d authored by Randy Dunlap's avatar Randy Dunlap Committed by Linus Torvalds
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slow down printk during boot 



Optionally add a boot delay after each kernel printk() call, crudely
measured in milliseconds, with a maximum delay of 10 seconds per printk.

Enable CONFIG_BOOT_PRINTK_DELAY=y and then add (e.g.):
"lpj=loops_per_jiffy boot_delay=100"
to the kernel command line.

It has been useful in cases like "during boot, my machine just reboots or the
screen goes black" by slowing down printk, (and adding initcall_debug), we can
usually see the last thing that happened before the lights went out which is
usually a valuable clue.

[akpm@linux-foundation.org: not all architectures implement CONFIG_HZ]
[akpm@linux-foundation.org: fix lots of stuff]
[bunk@stusta.de: kernel/printk.c: make 2 variables static]
[heiko.carstens@de.ibm.com: fix slow down printk on boot compile error]
Signed-off-by: default avatarRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: default avatarDave Jones <davej@redhat.com>
Signed-off-by: default avatarAdrian Bunk <bunk@stusta.de>
Signed-off-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1bcf5482

Documentation/kernel-parameters.txt

+5 −0
Original line number Diff line number Diff line
@@ -349,6 +349,11 @@ and is between 256 and 4096 characters. It is defined in the file 
	blkmtd_bs=

	blkmtd_count=



	boot_delay=	Milliseconds to delay each printk during boot.

			Values larger than 10 seconds (10000) are changed to

			no delay (0).

			Format: integer



	bttv.card=	[HW,V4L] bttv (bt848 + bt878 based grabber cards)

	bttv.radio=	Most important insmod options are available as

			kernel args too.

include/linux/jiffies.h

+2 −0
Original line number Diff line number Diff line
@@ -148,6 +148,8 @@ static inline u64 get_jiffies_64(void) 
 */

#define MAX_JIFFY_OFFSET ((LONG_MAX >> 1)-1)



extern unsigned long preset_lpj;



/*

 * We want to do realistic conversions of time so we need to use the same

 * values the update wall clock code uses as the jiffies size.  This value

init/calibrate.c

+1 −1
Original line number Diff line number Diff line
@@ -10,7 +10,7 @@ 


#include <asm/timex.h>



static unsigned long preset_lpj;

unsigned long preset_lpj;

static int __init lpj_setup(char *str)

{

	preset_lpj = simple_strtoul(str,NULL,0);

kernel/printk.c

+59 −0
Original line number Diff line number Diff line
@@ -22,6 +22,8 @@ 
#include <linux/tty_driver.h>

#include <linux/console.h>

#include <linux/init.h>

#include <linux/jiffies.h>

#include <linux/nmi.h>

#include <linux/module.h>

#include <linux/moduleparam.h>

#include <linux/interrupt.h>			/* For in_interrupt() */
@@ -162,6 +164,61 @@ static int __init log_buf_len_setup(char *str) 


__setup("log_buf_len=", log_buf_len_setup);



#ifdef CONFIG_BOOT_PRINTK_DELAY



static unsigned int boot_delay; /* msecs delay after each printk during bootup */

static unsigned long long printk_delay_msec; /* per msec, based on boot_delay */



static int __init boot_delay_setup(char *str)

{

	unsigned long lpj;

	unsigned long long loops_per_msec;



	lpj = preset_lpj ? preset_lpj : 1000000;	/* some guess */

	loops_per_msec = (unsigned long long)lpj / 1000 * HZ;



	get_option(&str, &boot_delay);

	if (boot_delay > 10 * 1000)

		boot_delay = 0;



	printk_delay_msec = loops_per_msec;

	printk(KERN_DEBUG "boot_delay: %u, preset_lpj: %ld, lpj: %lu, "

		"HZ: %d, printk_delay_msec: %llu\n",

		boot_delay, preset_lpj, lpj, HZ, printk_delay_msec);

	return 1;

}

__setup("boot_delay=", boot_delay_setup);



static void boot_delay_msec(void)

{

	unsigned long long k;

	unsigned long timeout;



	if (boot_delay == 0 || system_state != SYSTEM_BOOTING)

		return;



	k = (unsigned long long)printk_delay_msec * boot_delay;



	timeout = jiffies + msecs_to_jiffies(boot_delay);

	while (k) {

		k--;

		cpu_relax();

		/*

		 * use (volatile) jiffies to prevent

		 * compiler reduction; loop termination via jiffies

		 * is secondary and may or may not happen.

		 */

		if (time_after(jiffies, timeout))

			break;

		touch_nmi_watchdog();

	}

}

#else

static inline void boot_delay_msec(void)

{

}

#endif



/*

 * Commands to do_syslog:

 *
@@ -527,6 +584,8 @@ asmlinkage int vprintk(const char *fmt, va_list args) 
	static char printk_buf[1024];

	static int log_level_unknown = 1;



	boot_delay_msec();



	preempt_disable();

	if (unlikely(oops_in_progress) && printk_cpu == smp_processor_id())

		/* If a crash is occurring during printk() on this CPU,

lib/Kconfig.debug

+18 −0
Original line number Diff line number Diff line
@@ -413,6 +413,24 @@ config FORCED_INLINING 
	  become the default in the future, until then this option is there to

	  test gcc for this.



config BOOT_PRINTK_DELAY

	bool "Delay each boot printk message by N milliseconds"

	depends on DEBUG_KERNEL && PRINTK && GENERIC_CALIBRATE_DELAY

	help

	  This build option allows you to read kernel boot messages

	  by inserting a short delay after each one.  The delay is

	  specified in milliseconds on the kernel command line,

	  using "boot_delay=N".



	  It is likely that you would also need to use "lpj=M" to preset

	  the "loops per jiffie" value.

	  See a previous boot log for the "lpj" value to use for your

	  system, and then set "lpj=M" before setting "boot_delay=N".

	  NOTE:  Using this option may adversely affect SMP systems.

	  I.e., processors other than the first one may not boot up.

	  BOOT_PRINTK_DELAY also may cause DETECT_SOFTLOCKUP to detect

	  what it believes to be lockup conditions.



config RCU_TORTURE_TEST

	tristate "torture tests for RCU"

	depends on DEBUG_KERNEL