panic: add option to dump blocked tasks in panic_print

For debugging kernel panics and other bugs, there is already an option of
panic_print to dump all tasks' call stacks.  On today's large servers
running many containers, there could be thousands of tasks or more, and
this will print out huge amount of call stacks, taking a lot of time (for
serial console which is main target user case of panic_print).

And in many cases, only those several tasks being blocked are key for the
panic, so add an option to only dump blocked tasks' call stacks.

[akpm@linux-foundation.org: clarify documentation a little]
Link: https://lkml.kernel.org/r/20240202132042.3609657-1-feng.tang@intel.comSigned-off-by: Feng Tang <feng.tang@intel.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4182,6 +4182,7 @@
 			bit 4: print ftrace buffer
 			bit 5: print all printk messages in buffer
 			bit 6: print all CPUs backtrace (if available in the arch)
+			bit 7: print only tasks in uninterruptible (blocked) state
 			*Be aware* that this option may print a _lot_ of lines,
 			so there are risks of losing older messages in the log.
 			Use this option carefully, maybe worth to setup a

Documentation/admin-guide/sysctl/kernel.rst

@@ -853,6 +853,7 @@ bit 3  print locks info if ``CONFIG_LOCKDEP`` is on
 bit 4  print ftrace buffer
 bit 5  print all printk messages in buffer
 bit 6  print all CPUs backtrace (if available in the arch)
+bit 7  print only tasks in uninterruptible (blocked) state
 =====  ============================================
 
 So for example to print tasks and memory info on panic, user can::

kernel/panic.c

@@ -73,6 +73,7 @@ EXPORT_SYMBOL_GPL(panic_timeout);
 #define PANIC_PRINT_FTRACE_INFO		0x00000010
 #define PANIC_PRINT_ALL_PRINTK_MSG	0x00000020
 #define PANIC_PRINT_ALL_CPU_BT		0x00000040
+#define PANIC_PRINT_BLOCKED_TASKS	0x00000080
 unsigned long panic_print;
 
 ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
@@ -227,6 +228,9 @@ static void panic_print_sys_info(bool console_flush)
 
 	if (panic_print & PANIC_PRINT_FTRACE_INFO)
 		ftrace_dump(DUMP_ALL);
+
+	if (panic_print & PANIC_PRINT_BLOCKED_TASKS)
+		show_state_filter(TASK_UNINTERRUPTIBLE);
 }
 
 void check_panic_on_warn(const char *origin)