From: Randy Dunlap 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] Signed-off-by: Randy Dunlap Signed-off-by: Dave Jones Signed-off-by: Andrew Morton --- Documentation/kernel-parameters.txt | 5 ++ include/linux/jiffies.h | 2 + init/calibrate.c | 2 - kernel/printk.c | 51 ++++++++++++++++++++++++++ lib/Kconfig.debug | 18 +++++++++ 5 files changed, 77 insertions(+), 1 deletion(-) diff -puN Documentation/kernel-parameters.txt~slow-down-printk-during-boot Documentation/kernel-parameters.txt --- a/Documentation/kernel-parameters.txt~slow-down-printk-during-boot +++ a/Documentation/kernel-parameters.txt @@ -345,6 +345,11 @@ and is between 256 and 4096 characters. 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. diff -puN include/linux/jiffies.h~slow-down-printk-during-boot include/linux/jiffies.h --- a/include/linux/jiffies.h~slow-down-printk-during-boot +++ a/include/linux/jiffies.h @@ -150,6 +150,8 @@ 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 diff -puN init/calibrate.c~slow-down-printk-during-boot init/calibrate.c --- a/init/calibrate.c~slow-down-printk-during-boot +++ a/init/calibrate.c @@ -10,7 +10,7 @@ #include -static unsigned long preset_lpj; +unsigned long preset_lpj; static int __init lpj_setup(char *str) { preset_lpj = simple_strtoul(str,NULL,0); diff -puN kernel/printk.c~slow-down-printk-during-boot kernel/printk.c --- a/kernel/printk.c~slow-down-printk-during-boot +++ a/kernel/printk.c @@ -22,6 +22,8 @@ #include #include #include +#include +#include #include #include #include /* For in_interrupt() */ @@ -162,6 +164,50 @@ out: __setup("log_buf_len=", log_buf_len_setup); +#ifdef CONFIG_BOOT_PRINTK_DELAY + +unsigned int boot_delay; /* msecs delay after each printk during bootup */ +unsigned long long printk_delay_msec; /* per msec, based on boot_delay */ + +static int __init boot_delay_setup(char *str) +{ + unsigned long lpj = preset_lpj ? preset_lpj : 1000000; /* some guess */ + unsigned long long loops_per_msec = 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(int millisecs) +{ + unsigned long long k = printk_delay_msec * millisecs; + unsigned long timeout; + + timeout = jiffies + msecs_to_jiffies(millisecs); + 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(); + } +} + +#endif + /* * Commands to do_syslog: * @@ -513,6 +559,11 @@ asmlinkage int printk(const char *fmt, . r = vprintk(fmt, args); va_end(args); +#ifdef CONFIG_BOOT_PRINTK_DELAY + if (boot_delay && system_state == SYSTEM_BOOTING) + boot_delay_msec(boot_delay); +#endif + return r; } diff -puN lib/Kconfig.debug~slow-down-printk-during-boot lib/Kconfig.debug --- a/lib/Kconfig.debug~slow-down-printk-during-boot +++ a/lib/Kconfig.debug @@ -411,6 +411,24 @@ 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 + 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 _