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>

2 files changed, 29 insertions, 0 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 932ea21feb18..f38b07f78a4e 100644
--- a/kernel/printk.c
+++ b/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 */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6ba49c7cb128..0dfaa47d7cb6 100644
--- a/kernel/sysctl.c
+++ b/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,