printk: add printk_delay to make messages readable for some scenarios

When syslog is not possible, at the same time there's no serial/net
console available, it will be hard to read the printk messages.  For
example oops/panic/warning messages in shutdown phase.

Add a printk delay feature, we can make each printk message delay some
milliseconds.

Setting the delay by proc/sysctl interface: /proc/sys/kernel/printk_delay

The value range from 0 - 10000, default value is 0

[akpm@linux-foundation.org: fix a few things]
Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/kernel.txt

@@ -313,6 +313,14 @@ send before ratelimiting kicks in.
 
 ==============================================================
 
+printk_delay:
+
+Delay each printk message in printk_delay milliseconds
+
+Value from 0 - 10000 is allowed.
+
+==============================================================
+
 randomize-va-space:
 
 This option can be used to select the type of process address

include/linux/kernel.h

@@ -246,6 +246,8 @@ extern int printk_ratelimit(void);
 extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
 				   unsigned int interval_msec);
 
+extern int printk_delay_msec;
+
 /*
  * Print a one-time message (analogous to WARN_ONCE() et al):
  */

kernel/printk.c

@@ -653,6 +653,20 @@ static int recursion_bug;
 static int new_text_line = 1;
 static char printk_buf[1024];
 
+int printk_delay_msec __read_mostly;
+
+static inline void printk_delay(void)
+{
+	if (unlikely(printk_delay_msec)) {
+		int m = printk_delay_msec;
+
+		while (m--) {
+			mdelay(1);
+			touch_nmi_watchdog();
+		}
+	}
+}
+
 asmlinkage int vprintk(const char *fmt, va_list args)
 {
 	int printed_len = 0;
@@ -662,6 +676,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	char *p;
 
 	boot_delay_msec();
+	printk_delay();
 
 	preempt_disable();
 	/* This stops the holder of console_sem just where we want him */

kernel/sysctl.c

@@ -106,6 +106,9 @@ static int __maybe_unused one = 1;
 static int __maybe_unused two = 2;
 static unsigned long one_ul = 1;
 static int one_hundred = 100;
+#ifdef CONFIG_PRINTK
+static int ten_thousand = 10000;
+#endif
 
 /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
 static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
@@ -722,6 +725,17 @@ static struct ctl_table kern_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "printk_delay",
+		.data		= &printk_delay_msec,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec_minmax,
+		.strategy	= &sysctl_intvec,
+		.extra1		= &zero,
+		.extra2		= &ten_thousand,
+	},
 #endif
 	{
 		.ctl_name	= KERN_NGROUPS_MAX,