runtime debug level change

loglevel Set the default console log level.
loglevel=level
Specify the initial console log level. Any log messages with levels
less than this (that is, of higher priority) will be printed to the
console, whereas any messages with levels equal to or greater than
this will not be displayed.
The console log level can also be changed by the klogd program, or
by writing the specified level to the /proc/sys/kernel/printk file.
The kernel log levels are:
0 (KERN_EMERG)
The system is unusable.
1 (KERN_ALERT)
Actions that must be taken care of immediately.
2 (KERN_CRIT)
Critical conditions.
3 (KERN_ERR)
Noncritical error conditions.
4 (KERN_WARNING)
Warning conditions that should be taken care of.
5 (KERN_NOTICE)
Normal, but significant events.
6 (KERN_INFO)
Informational messages that require no action.
7 (KERN_DEBUG)
Kernel debugging messages, output by the kernel if the developer
enabled debugging at compile time.
log_buf_len Set the size of the kernel log buffer.
log_buf_len=n[KMG]
Set the size of the kernel’s internal log buffer. n must be a power of
2, if not, it will be rounded up to be a power of 2. This value can
also be changed by the CONFIG_LOG_BUF_SHIFT kernel configuration
value.
initcall_debug Debug the initcall functions in the kernel.