Commit bfe8df3d authored by Randy Dunlap's avatar Randy Dunlap Committed by Linus Torvalds
Browse files

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
......@@ -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.
......
......@@ -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
......
......@@ -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);
......
......@@ -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,
......
......@@ -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
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment