[PATCH] NMI trigger switch support for debugging(updated)

I made a patch for debugging with the help of NMI trigger switch.
When kernel hangs severely, keyboard operation(e.g.Ctrl-Alt-Del)
doesn't work properly. This patch enables debugging information
to be displayed on console in this case.
I think this feature is necessary as standard functionality.
Please feel free to use this patch and let me know if you have
any comments.

Background:

When a trouble occurs in kernel, we usually begin to investigate
with following information:
 - panic >> panic message.
 - oops >> CPU registers and stack trace.
 - hang >> **NONE** no standard method established.

How it works:

Most IA32 servers have a NMI switch that fires NMI interrupt up.
The NMI interrupt can interrupt even if kernel is serious state,
for example deadlock under the interrupt disabled.
When the NMI switch is pressed after this feature is activated,
CPU registers and stack trace are displayed on console and then
panic occurs.
This feature is activated or deactivated with sysctl.

On IA32 architecture, only the following are defined as reason
of NMI interrupt:
 - memory parity error
 - I/O check error
The reason code of NMI switch is not defined, so this patch assumes
that all undefined NMI interrupts are fired by MNI switch.
However, oprofile and NMI watchdog also use undefined NMI interrupt.
Therefore this feature cannot be used at the same time with oprofile
and NMI watchdog. This feature hands NMI interrupt over to oprofile
and NMI watchdog. So, when they have been activated, this feature
doesn't work even if it is activated.

Supported architecture:

IA32

Setup:

Set up the system control parameter as follows:

# sysctl -w kernel.unknown_nmi_panic=1
kernel.unknown_nmi_panic = 1

If the NMI switch is pressed, CPU registers and stack trace will
be displayed on console and then panic occurs.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 9d626596d5b4ca..186f38c13af11c 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -1109,6 +1109,23 @@ modprobe
 The location  where  the  modprobe  binary  is  located.  The kernel uses this
 program to load modules on demand.
 
+unknown_nmi_panic
+-----------------
+
+The value in this file affects behavior of handling NMI. When the value is
+non-zero, unknown NMI is trapped and then panic occurs. At that time, kernel
+debugging information is displayed on console.
+
+NMI switch that most IA32 servers have fires unknown NMI up, for example.
+If a system hangs up, try pressing the NMI switch.
+
+[NOTE]
+   This function and oprofile share a NMI callback. Therefore this function
+   cannot be enabled when oprofile is activated.
+   And NMI watchdog will be disabled when the value in this file is set to
+   non-zero.
+
+
 2.4 /proc/sys/vm - The virtual memory subsystem
 -----------------------------------------------