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: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 396c38b3cb69..7d16e6433302 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -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