[PATCH 2/3] sysrq documentation: document why the command header only is shown

Andy Whitcroft
2009-01-07T21:50:42Z

Document the interactions between loglevel and the sysrq output. Also
document how to work round it should output on the console be required.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
---
Documentation/sysrq.txt | 18 ++++++++++++++++++
1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt
index 265f637..a262ffb 100644
--- a/Documentation/sysrq.txt
+++ b/Documentation/sysrq.txt
@@ -210,6 +210,24 @@ within a function called by handle_sysrq, you must be aware that you are in
a lock (you are also in an interrupt handler, which means don't sleep!), so
you must call __handle_sysrq_nolock instead.

+* When I hit a sysrq combination only the header appears on the console, why?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Sysrq output is subject to the same console loglevel control as all
+other console output. This means that if the kernel was booted 'quiet'
+as is common on distro kernels the output may not appear on the actual
+console, even though it will appear in the dmesg buffer, and be accessible
+via the dmesg command and to the consumers of /proc/kmsg. As a specific
+exception the header line from the sysrq command is passed to all console
+consumers as if the current loglevel was maximum. If only the header
+is emitted it is almost cirtain that the kernel loglevel is too low.
+Should you require the output on the console channel then you will need
+to temporarily up the console loglevel using alt-sysrq-8 or:
+
+ echo 8 > /proc/sysrq-trigger
+
+Remember to return the loglevel to normal after triggering the sysrq
+command you are interested in.
+
* I have more questions, who can I ask?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
And I'll answer any questions about the registration system you got, also
--
1.6.0.4.911.gc990