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, 59 insertions, 0 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 8451dfc..b2b5c3a 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -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 @@ out:
 
 __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,