[PATCH] detect soft lockups

This patch adds a new kernel debug feature: CONFIG_DETECT_SOFTLOCKUP.

When enabled then per-CPU watchdog threads are started, which try to run
once per second.  If they get delayed for more than 10 seconds then a
callback from the timer interrupt detects this condition and prints out a
warning message and a stack dump (once per lockup incident).  The feature
is otherwise non-intrusive, it doesnt try to unlock the box in any way, it
only gets the debug info out, automatically, and on all CPUs affected by
the lockup.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-Off-By: Matthias Urlichs <smurf@smurf.noris.de>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

lib/Kconfig.debug

@@ -46,6 +46,25 @@ config LOG_BUF_SHIFT
 		     13 =>  8 KB
 		     12 =>  4 KB
 
+config DETECT_SOFTLOCKUP
+	bool "Detect Soft Lockups"
+	depends on DEBUG_KERNEL
+	default y
+	help
+	  Say Y here to enable the kernel to detect "soft lockups",
+	  which are bugs that cause the kernel to loop in kernel
+	  mode for more than 10 seconds, without giving other tasks a
+	  chance to run.
+
+	  When a soft-lockup is detected, the kernel will print the
+	  current stack trace (which you should report), but the
+	  system will stay locked up. This feature has negligible
+	  overhead.
+
+	  (Note that "hard lockups" are separate type of bugs that
+	   can be detected via the NMI-watchdog, on platforms that
+	   support it.)
+
 config SCHEDSTATS
 	bool "Collect scheduler statistics"
 	depends on DEBUG_KERNEL && PROC_FS